Previously published data on vertical fluxes of particulate carbon (PC), nitrogen (PN), organisms... more Previously published data on vertical fluxes of particulate carbon (PC), nitrogen (PN), organisms (MICRO), and extracted adenosine triphosphate (ATP) into screened sediment traps (335/xm) from the VERTEX 5 and ADIOS I programs are reexamined as they relate to biogeochemical cycling and oceanic productivity. The four stations discussed represent an oligotrophic to mesotrophic gradient in total primary production (PT), ranging from 245 to 1141 mg C m-2 d-1 and a gradient in PC flux from the euphotic zone, ranging from 12 to 164 mg C m-2 d-• for particles <335/xm in diameter. Vertical fluxes of PC, PN, MICRO, and ATP decreased as negative power functions of depth with significantly higher depth-dependent losses for ATP fluxes. The flux of intact biota (free, particleassociated, and some active "swimmers," measured microscopically and by extracted ATP) decreased rapidly in the upper 200 m, contributing as much as 52.4% at the most productive station and as little as 1.6% to the flux of PC at oligotrophic stations, remaining relatively constant or increasing slightly (to 3.4-9.6% PC flux) between 200 and 2000 m. Multiple regression analyses, expressing fluxes as • Now at Marine Sciences Research Center, State University of New York at Stony Brook.
The inventories and dynamics of dissolved organic matter (DOM) in the surface water at Station AL... more The inventories and dynamics of dissolved organic matter (DOM) in the surface water at Station ALOHA were analyzed from the Hawaii Ocean Time-series (HOT) data set for the period 1989-1999. Euphotic zone, depthintegrated (0-175 m) concentrations of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) were temporally variable. In particular, during the period 1993-1999, concentrations of DOC and DON increased while inventories of DOP remained unchanged. DOC inventories increased by 303 mmol C m Ϫ2 yr Ϫ1 , a value equivalent to approximately 2% of measured primary production (14 C method) at this site. DON increased at 11 mmol N m Ϫ2 yr Ϫ1 , resulting in a mean molar DOC : DON ratio of 27.5 for the accumulated DOM. Accumulation of DOC and DON without corresponding accumulation of DOP resulted in changes to the bulk organic C : N : P stoichiometry; bulk DOC : DOP ratios increased 16% and DON : DOP ratios increased by 17%. These results indicate that a small fraction of the annually produced organic matter escaped biological utilization on time scales of months to years. More importantly, the accumulated DOM inventories grew progressively enriched in C and N relative to P. Fundamental changes in the North Pacific Subtropical Gyre (NPSG) habitat appear to have altered microbial processes that regulate organic matter fluxes. Considered together, the long-term increases in DOC and DON inventories are consistent with previous observations, indicating that a recent reorganization of plankton community dynamics may have altered organic matter cycling in this ecosystem. * Range of water column temperatures (0-175 m). † Top numbers are mean integrated stocks; numbers in parentheses are standard deviations of the means. ‡ SRP, soluble reactive phosphorus. § n, annual number of observations from which mean integrated stocks were calculated. Typically 8-10 depths were sampled between the surface and 175 m for each observation.-, no data available.
This is the author manuscript accepted for publication and has undergone full peer review but has... more This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as
Scientists working on the Hawaii Ocean Time-series (HOT) program have been making repeated observ... more Scientists working on the Hawaii Ocean Time-series (HOT) program have been making repeated observations of the hydrography, chemistry and biology of the water column at a station north of Oahu, Hawaii since October 1988. The objective of this research is to provide a comprehensive description of the ocean at a site representative of the North Pacific subtropical gyre. Cruises are made approximately once per month to the deep-water Station ALOHA (A Long-term Oligotrophic Habitat Assessment; 22 45' N, 158 00' W) located 100 km north of Oahu, Hawaii. Measurements of the thermohaline structure, water column chemistry, currents, optical properties, primary production, plankton community structure, and rates of particle export are made on each cruise. This document reports the data collected in 2005. However, we have included some data from 1988-2004 to place the 2005 measurements in the context of ongoing time-series observations. The data reported here are a subset of the complete data set. Summary plots are given for CTD, biogeochemical, optical, meteorological, navigational, thermosalinograph and ADCP observations. The complete data set resides on a pair of Workstations at the University of Hawaii. These data are in ASCII format, and can easily be accessed using either anonymous file transfer protocol (FTP) ,the World Wide Web (WWW) or the Hawaii Ocean Time-series Data Organization and Graphical System (HOT-DOGS).
The marine cyanobacterium Prochlorococcus can utilize glucose as a source of carbon. However, the... more The marine cyanobacterium Prochlorococcus can utilize glucose as a source of carbon. However, the relative importance of inorganic and organic carbon assimilation and the timing of glucose assimilation are still poorly understood in these numerically dominant cyanobacteria. Here we investigated whole microbial community and group-specific primary production and glucose assimilation, using incubations with radioisotopes combined with flow cytometry cell sorting. We also studied changes in the microbial community structure in response to glucose enrichments and analyzed the transcription of Prochlorocccus genes involved in carbon metabolism and photosynthesis.Our results showed a circadian rhythm for glucose assimilation in Prochlorococcus, with maximum assimilation during the midday and minimum at midnight, which was different compared with that of the total microbial community. This suggests that rhythms in glucose assimilation have been adapted in Prochlorococcus to couple the acti...
Global analyses of satellite and modeled data suggest decreased phytoplankton abundance and prima... more Global analyses of satellite and modeled data suggest decreased phytoplankton abundance and primary productivity in oligotrophic gyres as they expand in response to increased surface temperatures, shoaling of surface mixed layers, and decreased supply of subsurface macronutrients. However, analogous changes in the phytoplankton have not been evident in situ at Hawaii Ocean Time-series (HOT) Station ALOHA (22 • 45 ′ N, 158 • 00 ′ W), suggesting that physiological or structural reorganization not observed from space, uncorrected sensor drift, or uncharacterized geographic variability may be responsible for the apparent discrepancy. To address the latter, we compared interannual patterns of in situ phytoplankton dynamics and mixed layer properties to gyre extent and boundary location based on multiple definitions including dynamic topography, a threshold of satellite surface chlorophyll (chl a) ≤0.07 mg m −3 , and multivariate biophysical seascapes using modeled or satellite data. Secular increases in gyre extent were apparent, although the rate of expansion was much slower than previously reported, whereas strong interannual oscillations were evident for all definitions of the gyre. Modeled and satellite-based multivariate seascapes agreed well in terms of expansion (surface area of seascapes) and isolation of Station ALOHA (distance to seascape boundary) resulting in a combined data record of nearly three decades. Isolation was associated positively with the North Pacific Gyre Oscillation (NPGO), and negatively with Multivariate ENSO Index (MEI), and Pacific Decadal Oscillation (PDO). The converse was true for the gyre's expansion. Expansion followed a shoaling and freshening of the surface mixed layer and declines of in situ net primary production (PP) suggesting that Station ALOHA may serve as an early indicator of gyre biogeographic patterns. Lags between geographic indicators and in situ conditions appear to partially explain past observed discrepancies between patterns from satellite remote sensing and those from in situ conditions at Station ALOHA.
Proceedings of the National Academy of Sciences, 2021
Significance The ocean’s “biological pump” exports sinking particles containing carbon, nutrients... more Significance The ocean’s “biological pump” exports sinking particles containing carbon, nutrients, and energy to the deep sea, contributing centrally to the global carbon cycle. Here, we identify key organisms and biological processes associated with elevated carbon flux to the abyss. Our analyses reveal that, during summer export, specific populations of photosynthetic algae, heterotrophic protists, and bacteria reach the abyss on sinking particles. Deep-sea bacteria respond rapidly to this elevated nutrient delivery to the abyss in summer. During other seasons, different organisms and processes appear responsible for particle export to the deep sea. Our analyses reveal key biota and biological processes that interconnect surface productivity, particle export, and the deep-sea ecosystem, thereby influencing the function and efficiency of the ocean’s biological pump.
Trichodesmium is an important dinitrogen (N2)-fixing cyanobacterium in marine ecosystems. Recent ... more Trichodesmium is an important dinitrogen (N2)-fixing cyanobacterium in marine ecosystems. Recent nucleic acid analyses indicate that Trichodesmium colonies with their diverse epibionts support various nitrogen (N) transformations beyond N2 fixation. However, rates of these transformations and concentration gradients of N compounds in Trichodesmium colonies remain largely unresolved. We combined isotope-tracer incubations, micro-profiling and numeric modelling to explore carbon fixation, N cycling processes as well as oxygen, ammonium and nitrate concentration gradients in individual field-sampled Trichodesmium colonies. Colonies were net-autotrophic, with carbon and N2 fixation occurring mostly during the day. Ten percent of the fixed N was released as ammonium after 12-h incubations. Nitrification was not detectable but nitrate consumption was high when nitrate was added. The consumed nitrate was partly reduced to ammonium, while denitrification was insignificant. Thus, the potenti...
The fluorescence and scattering properties of Prochlorococcus and Synechococcus at Station ALOHA ... more The fluorescence and scattering properties of Prochlorococcus and Synechococcus at Station ALOHA as measured by flow cytometry (termed the FCM phenotype) vary with depth and over a variety of time scales. The variation in FCM phenotypes may reflect population selection or physiological acclimation to local conditions. Observations before, during, and after a storm with deep water mixing show a short-term homogenization of the FCM phenotypes with depth, followed by a return to the stable pattern over the time span of a few days. These dynamics indicate that, within the upper mixed-layer, the FCM phenotype distribution represents acclimation to ambient light. The populations in the pycnocline (around 100 m and below), remain stable and are invariant with light conditions. In samples where both cyanobacteria coexist, fluorescence properties of Prochlorococcus and Synechococcus are tightly correlated providing further evidence that FCM phenotype variability is caused by a common environmental factor or factors. Measurements of the dynamics of FCM phenotypes provide insights into phytoplankton physiology and adaptation. Alternatively, FCM phenotype census of a water mass may provide information about its origin and illumination history.
The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligo... more The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligotrophic oceans, the foundation of marine food webs, and an important component of global biogeochemical cycles. The prominence of Prochlorococcus across the environmental gradients of the open ocean is attributed to its extensive genetic diversity and flexible chlorophyll physiology, enabling light capture over a wide range of intensities. What remains unknown is the balance between temporal dynamics of genetic diversity and chlorophyll physiology in the ability of Prochlorococcus to respond to a variety of short (approximately 1 day) and longer (months to year) changes in the environment. Previous field research established depth-dependent Prochlorococcus single cell chlorophyll distributions in the North Pacific Subtropical Gyre. Here, we examined whether the shifts in chlorophyll distributions correspond to changes in Prochlorococcus genetic diversity (i.e., ecotype-level community structure) or photoacclimation of stable communities over short time intervals. We report that community structure was relatively stable despite abrupt shifts in Prochlorococcus chlorophyll physiology, due to unexpected physiological plasticity of highlight adapted Prochlorococcus ecotypes. Through comparison with seasonalscale changes, our data suggest that variability on daily scales triggers shifts in Prochlorococcus photoacclimation, while seasonal-scale dynamics trigger shifts in community structure. Together, these data highlight the importance of incorporating the process of photoacclimation and chlorophyll dynamics into interpretations of phytoplankton population dynamics from chlorophyll data as well as the importance of daily-scale variability to Prochlorococcus ecology.
Temporal variability in dissolved inorganic, organic phosphate (Pi, DOP) and particulate phosphor... more Temporal variability in dissolved inorganic, organic phosphate (Pi, DOP) and particulate phosphorus (PPO 4) concentrations, and microbial utilization of Pi and dissolved adenosine-5 ′-triphosphate (DATP) was studied at Station ALOHA (22.75 • N, 158 • W) in the North Pacific Subtropical Gyre (NPSG) over a multi-year period. Spatial variability of the same properties was investigated along two transects, to and from Hawaii, that traversed the NPSG boundaries to the east (2014) and north (2016). Radiotracer techniques were employed to measure the turnover time of Pi and DATP pools to calculate Pi uptake rates and the Pi hydrolysis rates of DATP. Pi concentrations were more variable, both in time and space, than DOP, ranging two orders of magnitude compared to a factor of two for DOP. The DATP pool, while constituting on average <0.15% of the total DOP-P, was as dynamic as Pi (∼1-200 pmol l −1), with lowest concentrations coinciding with Pi depletion. The Pi turnover times ranged from a few hours to several weeks, and were correlated with measured Pi concentrations (r = 0.9; Station ALOHA, n = 28; 2014, n = 14; 2016, n = 12). Pi uptake rates averaged 3.6 ± 1.3 nmol-P l −1 d −1 (n = 28: Station ALOHA), 9.2 ± 4.7 nmol-P l −1 d −1 , (n = 15; 2014) and 5.1 ± 2.5 nmol-P l −1 d −1 , (n = 12; 2016). The turnover time of the DATP pool was typically substantially shorter (0.4-5 days) than for the Pi-pool, and uptake rates ranged from 1 to 115 pmol l −1 d −1. However, at very low Pi and ATP concentrations, ATP turnover was longer than Pi turnover and ATP uptake rates lower. Total ATP hydrolysis was high along both transects, exceeding the ATP taken up by the microbial community, resulting in a net release of Pi into the ambient seawater. This net release was positively correlated to Pi concentration. The relative contribution by microbial size classes to total P-uptake depended on whether P was derived from ambient Pi or from DATP, with the <0.6>0.2 µm size class dominating the DATP uptake. Our results indicate that during Pi limiting conditions, regenerated P is rapidly consumed, and that Pi limitation occurs locally and transiently but does not appear to be the predominant condition in the upper water column of the NPSG.
The southeast subtropical Pacific Ocean was sampled along a zonal transect between the coasts of ... more The southeast subtropical Pacific Ocean was sampled along a zonal transect between the coasts of Chile and Easter Island. This remote area of the world's ocean presents strong gradients in physical (e.g., temperature, density and light), chemical (e.g., salinity and nutrient concentrations) and microbiological (e.g., cell abundances, biomass and specific growth rates) properties. The goal of this study was to describe the phosphorus (P) dynamics in three main ecosystems along this transect: the upwelling regime off the northern Chilean coast, the oligotrophic area associated with the southeast subtropical Pacific gyre and the transitional area in between these two biomes. We found that inorganic phosphate (Pi) concentrations were high and turnover times were long (>210 nmol l À1 and >31 d, respectively) in the upper water column, along the entire transect. Pi uptake rates in the gyre were low (euphotic layer integrated rates were 0.26 mmol m À2 d À1 in the gyre and 1.28 mmol m À2 d À1 in the upwelling region), yet not only driven by decreases in particle mass or cell abundance (particulate P-and cell-normalized Pi uptake rates in the euphotic layer were $1-4 times and $3-15 times lower in the gyre than in the upwelling, respectively). However these Pi uptake rates were at or near the maximum Pi uptake velocity (i.e., uptake rates in Pi amended samples were not significantly different from those at ambient concentration: 1.5 and 23.7 nmol l À1 d À1 at 50% PAR in the gyre and upwelling, respectively). Despite the apparent Pi replete conditions, selected dissolved organic P (DOP) compounds were readily hydrolyzed. Nucleotides were the most bioavailable of the DOP substrates tested. Microbes actively assimilated adenosine-5 0triphosphate (ATP) leading to Pi and adenosine incorporation as well as Pi release to the environment. The southeast subtropical Pacific Ocean is a Pi-sufficient environment, yet DOP hydrolytic processes are maintained and contribute to P-cycling across the wide range of environmental conditions present in this ecosystem.
We present nearly 9 yrs (June 2005–December 2013) of measurements of upper‐ocean (0 m to 125 m) d... more We present nearly 9 yrs (June 2005–December 2013) of measurements of upper‐ocean (0 m to 125 m) dinitrogen (N2) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N2 fixation rates were measured based on adding the 15N2 tracer as a gas bubble. Beginning in August 2012, 15N2 was first dissolved into filtered seawater and the 15N2‐enriched water was subsequently added to N2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15N2‐enriched seawater resulting in twofold greater depth‐integrated rates than those derived from adding a 15N2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N2 fixation averaged 230 ± 136 μmol N m−2 d−1 for the full time series (n = 71). Analysis of the 15N isotopic composition of sinking PN, together w...
The partitioning of photosynthetically-derived organic carbon between particulate and dissolved p... more The partitioning of photosynthetically-derived organic carbon between particulate and dissolved phases has important implications for marine carbon cycling. In this study we utilized 14 C-bicarbonate assimilation to quantify rates of photosynthetic production of both particulate and dissolved organic carbon (DOC) at Station ALOHA (22 • 45 ′ N, 158 • W) in the North Pacific Subtropical Gyre (NPSG). At near-monthly time scales over ∼5 years, we examined retention of 14 C-labeled organic matter by both glass fiber filters and 0.2 µm pore size polycarbonate membrane filters that are commonly used for measurements of 14 C-based plankton productivity. Use of polycarbonate filters resulted in significantly lower (averaging 60%) estimates of 14 C-production compared to glass fiber filters. Coincident measurements of chlorophyll a concentrations from both 0.2 µm polycarbonate and glass fiber filters were not significantly different, suggesting the differences in 14 C-productivity between these filter types did not derive from differences in retention of photosynthetic biomass by these filters. Moreover, consistent with previous studies, results from experiments aimed at quantifying retention of organic matter by these filters suggested differences resulted from retention of DOC by glass fiber filters. We also quantified rates of 14 C-DOC production to evaluate the partitioning of photosynthetic production between dissolved and particulate phases over daily to monthly time scales in this ecosystem. Unlike the strong depth dependence observed in measurements of particulate organic carbon production, measured rates of 14 C-DOC demonstrated no clear depth dependence. On average, depth-integrated (0-75 m) rates of 14 C-DOC production rates were equivalent to 18 ± 10% of the total (particulate and dissolved) productivity. Our findings indicate that in this oligotrophic ecosystem, rates of dissolved and particulate production can be temporally decoupled over daily to monthly time scales.
One challenge in field-based marine microbial ecology is to achieve sufficient spatial resolution... more One challenge in field-based marine microbial ecology is to achieve sufficient spatial resolution to obtain representative information about microbial distributions and biogeochemical processes. The challenges are exacerbated when conducting rate measurements of biological processes due to potential perturbations during sampling and incubation. Here we present the first application of a robotic microlaboratory, the 4 L-submersible incubation device (SID), for conducting in situ measurements of the rates of biological nitrogen (N 2) fixation (BNF). The free-drifting autonomous instrument obtains samples from the water column that are incubated in situ after the addition of 15 N 2 tracer. After each of up to four consecutive incubation experiments, the 4-L sample is filtered and chemically preserved. Measured BNF rates from two deployments of the SID in the oligotrophic North Pacific ranged from 0.8 to 2.8 nmol N L 21 day 21 , values comparable with simultaneous rate measurements obtained using traditional conductivity-temperature-depth (CTD)-rosette sampling followed by on-deck or in situ incubation. Future deployments of the SID will help to better resolve spatial variability of oceanic BNF, particularly in areas where recovery of seawater samples by CTD compromises their integrity, e.g. anoxic habitats.
Methane is a potent greenhouse gas that has contributed approximately 20% to the Earth's warming ... more Methane is a potent greenhouse gas that has contributed approximately 20% to the Earth's warming since pre-industrial times. The world's oceans are an important source of methane, comprising 1-4% of annual global emissions. But despite its global significance, oceanic methane production is poorly understood. In particular, methane concentrations in the surface waters of most of the world's oceans are supersaturated with respect to atmospheric concentrations, but the origin of this methane, which has been thought to be produced exclusively in anaerobic environments, is not known. Here, we measure methane production in seawater samples amended with methylphosphonate, an organic, phosphorus-containing compound. We show that methane is produced aerobically as a by-product of methylphosphonate decomposition in phosphate-stressed waters. Methylphosphonate decomposition, and thus methane production, may be enhanced by the activity of nitrogen-fixing microorganisms. We suggest that aerobic marine methane production will be sensitive to the changes in water-column stratification and nutrient limitation that are likely to result from greenhouse-gas-induced ocean warming.
Microbial community response to nitrogen (N) amendments and induced phosphorus (P) stress was inv... more Microbial community response to nitrogen (N) amendments and induced phosphorus (P) stress was investigated in the North Pacific Subtropical Gyre (NPSG). Samples amended with reduced sources of N, in the form of nitrate plus ammonium, showed significant increases in microbial cell abundance and biomass and decreases in dissolved inorganic phosphate (Pi) and silicate concentrations during an incubation period of 6 d. Primary productivity, P uptake rates (as both Pi and adenosine-5'-triphosphate [ATP]) and alkaline phosphatase activity (APA) all increased following N amendment. Dissolved organic P (DOP) concentrations did not change, but the large increase in APA and ATP uptake rates suggests that DOP was a dynamic pool and an important source for microbial P nutrition in P-stressed samples. Significant changes were also observed in the structure of the microbial community, with Synecho coccus and picoalgae abundances increasing substantially in the N-amended treatments, while non-pigmented picoplankton abundances were unchanged. Data on P resource partitioning among groups of picoplankton separated by size using membrane filters of different porosities, or by scattering and fluorescence properties using flow cytometry sorting, indicate that Synechococcus could have a greater role in the NPSG P cycling following episodic N inputs. This experimental manipulation of nutrient loading combined with observations at the total population to the microbial group levels constitutes a unique approach to improve our understanding of microbial community structure and function in response to environmental forcing.
Studies on upper ocean plankton production and respiration in oligotrophic ecosystems are taken t... more Studies on upper ocean plankton production and respiration in oligotrophic ecosystems are taken to imply that these regions are net heterotrophic (i.e. gross primary production [GPP] < respiration [R]). In this study we examined the hypothesis that the regularly observed state of net heterotrophy in the oligotrophic North Pacific Ocean may be interrupted by periodic bursts of net autotrophy when GPP becomes decoupled from R. To examine this hypothesis, we conducted a series of nutrient addition experiments in the oligotrophic North Pacific Subtropical Gyre (NPSG) in which nutrient-replete deep seawater was mixed with nutrient-depleted mixed-layer water. We monitored time-dependent changes in rates of GPP and R as well as dynamics of chlorophyll and nutrient concentrations. Our results consistently demonstrated that following the addition of nutrients: (1) phytoplankton biomass (chl a) and primary production increase; (2) the photoautotrophic plankton assemblage shifts from small (< 2 µm diameter) to large (>10 µm diameter), chl c-containing and Siutilizing cells; and (3) community metabolism shifts from near balance, or slightly net heterotrophic, to demonstrably net autotrophic metabolism. These results suggest that nutrient availability partly regulates plankton community structure and dynamics, and could serve as a mechanism to temporally and spatially decouple GPP and R in oligotrophic open ocean ecosystems.
We examined the phytoplankton response to the addition of nutrient-enriched deep seawater (DSW) a... more We examined the phytoplankton response to the addition of nutrient-enriched deep seawater (DSW) and of nitrate only during shipboard experiments performed between July 2004 and May 2007 in the North Pacific Subtropical Gyre (NPSG). Chlorophyll a (chl a), community size structure and composition, carbon fixation (CF) rates, and nutrient concentrations were measured daily for 5 to 7 d under simulated in situ conditions. Despite the fact that the NPSG is a permanently stratified, oligotrophic biome, there was a seasonal response to the addition of DSW and nitrate. In summertime experiments, chl a and CF rates increased up to 18-and 22-fold, respectively, relative to unamended controls after an incubation period of 5 to 6 d. Nutrients were assimilated to below control concentrations. A shift from a Prochlorococcus-dominated to a Synechococcus-and diatom-dominated community, and an 8-to 12-fold increase in the chl a content of the > 2 µm size fraction were observed. Addition of nitrate only increased chl a and CF rates 10-and 7-fold, respectively. In wintertime experiments, chl a and CF rates increased up to 4-and 9-fold, respectively, after an incubation period of up to 7 d. Only 50% of nutrients were assimilated and nitrate alone stimulated a 2-fold increase in chl a and CF rates. We explore the role of nutrient limitation, community composition, grazing, and light in explaining our observations. Findings from this seasonal and multi-year field experiment demonstrate how little we know about bloom development in the NPSG and highlight the potential risk in extrapolating the response of phytoplankton to natural or artificial fertilization from short-term studies.
Oceanic habitats may select for different organisms, thereby tuning genomic capabilities to local... more Oceanic habitats may select for different organisms, thereby tuning genomic capabilities to local environmental conditions. To understand the relationship between microbial assemblage composition, functional capability, and habitat, a random genome shotgun sequencing (metagenomic) survey was conducted with surface‐water microbial assemblages (0.2–5‐µm size fraction) collected at seven locations along a meridional transect from the northern edge of the South Pacific subtropical gyre to the southern edge of the North Pacific subtropical gyre (16°S–13.5°N). A total of 1.1 million unique sequence reads were obtained, of which ~45% could be annotated to metabolic category. Microbial assemblages in equatorial divergence and countercurrent habitats were distinct phylogenetically from those in gyre waters. Ecotypes of dominant Cyanobacteria (Prochlorococcus and Synechococcus) had distinct distributions congruent with their physiological characteristics in cultivation. The metagenomic distri...
Previously published data on vertical fluxes of particulate carbon (PC), nitrogen (PN), organisms... more Previously published data on vertical fluxes of particulate carbon (PC), nitrogen (PN), organisms (MICRO), and extracted adenosine triphosphate (ATP) into screened sediment traps (335/xm) from the VERTEX 5 and ADIOS I programs are reexamined as they relate to biogeochemical cycling and oceanic productivity. The four stations discussed represent an oligotrophic to mesotrophic gradient in total primary production (PT), ranging from 245 to 1141 mg C m-2 d-1 and a gradient in PC flux from the euphotic zone, ranging from 12 to 164 mg C m-2 d-• for particles <335/xm in diameter. Vertical fluxes of PC, PN, MICRO, and ATP decreased as negative power functions of depth with significantly higher depth-dependent losses for ATP fluxes. The flux of intact biota (free, particleassociated, and some active "swimmers," measured microscopically and by extracted ATP) decreased rapidly in the upper 200 m, contributing as much as 52.4% at the most productive station and as little as 1.6% to the flux of PC at oligotrophic stations, remaining relatively constant or increasing slightly (to 3.4-9.6% PC flux) between 200 and 2000 m. Multiple regression analyses, expressing fluxes as • Now at Marine Sciences Research Center, State University of New York at Stony Brook.
The inventories and dynamics of dissolved organic matter (DOM) in the surface water at Station AL... more The inventories and dynamics of dissolved organic matter (DOM) in the surface water at Station ALOHA were analyzed from the Hawaii Ocean Time-series (HOT) data set for the period 1989-1999. Euphotic zone, depthintegrated (0-175 m) concentrations of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) were temporally variable. In particular, during the period 1993-1999, concentrations of DOC and DON increased while inventories of DOP remained unchanged. DOC inventories increased by 303 mmol C m Ϫ2 yr Ϫ1 , a value equivalent to approximately 2% of measured primary production (14 C method) at this site. DON increased at 11 mmol N m Ϫ2 yr Ϫ1 , resulting in a mean molar DOC : DON ratio of 27.5 for the accumulated DOM. Accumulation of DOC and DON without corresponding accumulation of DOP resulted in changes to the bulk organic C : N : P stoichiometry; bulk DOC : DOP ratios increased 16% and DON : DOP ratios increased by 17%. These results indicate that a small fraction of the annually produced organic matter escaped biological utilization on time scales of months to years. More importantly, the accumulated DOM inventories grew progressively enriched in C and N relative to P. Fundamental changes in the North Pacific Subtropical Gyre (NPSG) habitat appear to have altered microbial processes that regulate organic matter fluxes. Considered together, the long-term increases in DOC and DON inventories are consistent with previous observations, indicating that a recent reorganization of plankton community dynamics may have altered organic matter cycling in this ecosystem. * Range of water column temperatures (0-175 m). † Top numbers are mean integrated stocks; numbers in parentheses are standard deviations of the means. ‡ SRP, soluble reactive phosphorus. § n, annual number of observations from which mean integrated stocks were calculated. Typically 8-10 depths were sampled between the surface and 175 m for each observation.-, no data available.
This is the author manuscript accepted for publication and has undergone full peer review but has... more This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as
Scientists working on the Hawaii Ocean Time-series (HOT) program have been making repeated observ... more Scientists working on the Hawaii Ocean Time-series (HOT) program have been making repeated observations of the hydrography, chemistry and biology of the water column at a station north of Oahu, Hawaii since October 1988. The objective of this research is to provide a comprehensive description of the ocean at a site representative of the North Pacific subtropical gyre. Cruises are made approximately once per month to the deep-water Station ALOHA (A Long-term Oligotrophic Habitat Assessment; 22 45' N, 158 00' W) located 100 km north of Oahu, Hawaii. Measurements of the thermohaline structure, water column chemistry, currents, optical properties, primary production, plankton community structure, and rates of particle export are made on each cruise. This document reports the data collected in 2005. However, we have included some data from 1988-2004 to place the 2005 measurements in the context of ongoing time-series observations. The data reported here are a subset of the complete data set. Summary plots are given for CTD, biogeochemical, optical, meteorological, navigational, thermosalinograph and ADCP observations. The complete data set resides on a pair of Workstations at the University of Hawaii. These data are in ASCII format, and can easily be accessed using either anonymous file transfer protocol (FTP) ,the World Wide Web (WWW) or the Hawaii Ocean Time-series Data Organization and Graphical System (HOT-DOGS).
The marine cyanobacterium Prochlorococcus can utilize glucose as a source of carbon. However, the... more The marine cyanobacterium Prochlorococcus can utilize glucose as a source of carbon. However, the relative importance of inorganic and organic carbon assimilation and the timing of glucose assimilation are still poorly understood in these numerically dominant cyanobacteria. Here we investigated whole microbial community and group-specific primary production and glucose assimilation, using incubations with radioisotopes combined with flow cytometry cell sorting. We also studied changes in the microbial community structure in response to glucose enrichments and analyzed the transcription of Prochlorocccus genes involved in carbon metabolism and photosynthesis.Our results showed a circadian rhythm for glucose assimilation in Prochlorococcus, with maximum assimilation during the midday and minimum at midnight, which was different compared with that of the total microbial community. This suggests that rhythms in glucose assimilation have been adapted in Prochlorococcus to couple the acti...
Global analyses of satellite and modeled data suggest decreased phytoplankton abundance and prima... more Global analyses of satellite and modeled data suggest decreased phytoplankton abundance and primary productivity in oligotrophic gyres as they expand in response to increased surface temperatures, shoaling of surface mixed layers, and decreased supply of subsurface macronutrients. However, analogous changes in the phytoplankton have not been evident in situ at Hawaii Ocean Time-series (HOT) Station ALOHA (22 • 45 ′ N, 158 • 00 ′ W), suggesting that physiological or structural reorganization not observed from space, uncorrected sensor drift, or uncharacterized geographic variability may be responsible for the apparent discrepancy. To address the latter, we compared interannual patterns of in situ phytoplankton dynamics and mixed layer properties to gyre extent and boundary location based on multiple definitions including dynamic topography, a threshold of satellite surface chlorophyll (chl a) ≤0.07 mg m −3 , and multivariate biophysical seascapes using modeled or satellite data. Secular increases in gyre extent were apparent, although the rate of expansion was much slower than previously reported, whereas strong interannual oscillations were evident for all definitions of the gyre. Modeled and satellite-based multivariate seascapes agreed well in terms of expansion (surface area of seascapes) and isolation of Station ALOHA (distance to seascape boundary) resulting in a combined data record of nearly three decades. Isolation was associated positively with the North Pacific Gyre Oscillation (NPGO), and negatively with Multivariate ENSO Index (MEI), and Pacific Decadal Oscillation (PDO). The converse was true for the gyre's expansion. Expansion followed a shoaling and freshening of the surface mixed layer and declines of in situ net primary production (PP) suggesting that Station ALOHA may serve as an early indicator of gyre biogeographic patterns. Lags between geographic indicators and in situ conditions appear to partially explain past observed discrepancies between patterns from satellite remote sensing and those from in situ conditions at Station ALOHA.
Proceedings of the National Academy of Sciences, 2021
Significance The ocean’s “biological pump” exports sinking particles containing carbon, nutrients... more Significance The ocean’s “biological pump” exports sinking particles containing carbon, nutrients, and energy to the deep sea, contributing centrally to the global carbon cycle. Here, we identify key organisms and biological processes associated with elevated carbon flux to the abyss. Our analyses reveal that, during summer export, specific populations of photosynthetic algae, heterotrophic protists, and bacteria reach the abyss on sinking particles. Deep-sea bacteria respond rapidly to this elevated nutrient delivery to the abyss in summer. During other seasons, different organisms and processes appear responsible for particle export to the deep sea. Our analyses reveal key biota and biological processes that interconnect surface productivity, particle export, and the deep-sea ecosystem, thereby influencing the function and efficiency of the ocean’s biological pump.
Trichodesmium is an important dinitrogen (N2)-fixing cyanobacterium in marine ecosystems. Recent ... more Trichodesmium is an important dinitrogen (N2)-fixing cyanobacterium in marine ecosystems. Recent nucleic acid analyses indicate that Trichodesmium colonies with their diverse epibionts support various nitrogen (N) transformations beyond N2 fixation. However, rates of these transformations and concentration gradients of N compounds in Trichodesmium colonies remain largely unresolved. We combined isotope-tracer incubations, micro-profiling and numeric modelling to explore carbon fixation, N cycling processes as well as oxygen, ammonium and nitrate concentration gradients in individual field-sampled Trichodesmium colonies. Colonies were net-autotrophic, with carbon and N2 fixation occurring mostly during the day. Ten percent of the fixed N was released as ammonium after 12-h incubations. Nitrification was not detectable but nitrate consumption was high when nitrate was added. The consumed nitrate was partly reduced to ammonium, while denitrification was insignificant. Thus, the potenti...
The fluorescence and scattering properties of Prochlorococcus and Synechococcus at Station ALOHA ... more The fluorescence and scattering properties of Prochlorococcus and Synechococcus at Station ALOHA as measured by flow cytometry (termed the FCM phenotype) vary with depth and over a variety of time scales. The variation in FCM phenotypes may reflect population selection or physiological acclimation to local conditions. Observations before, during, and after a storm with deep water mixing show a short-term homogenization of the FCM phenotypes with depth, followed by a return to the stable pattern over the time span of a few days. These dynamics indicate that, within the upper mixed-layer, the FCM phenotype distribution represents acclimation to ambient light. The populations in the pycnocline (around 100 m and below), remain stable and are invariant with light conditions. In samples where both cyanobacteria coexist, fluorescence properties of Prochlorococcus and Synechococcus are tightly correlated providing further evidence that FCM phenotype variability is caused by a common environmental factor or factors. Measurements of the dynamics of FCM phenotypes provide insights into phytoplankton physiology and adaptation. Alternatively, FCM phenotype census of a water mass may provide information about its origin and illumination history.
The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligo... more The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligotrophic oceans, the foundation of marine food webs, and an important component of global biogeochemical cycles. The prominence of Prochlorococcus across the environmental gradients of the open ocean is attributed to its extensive genetic diversity and flexible chlorophyll physiology, enabling light capture over a wide range of intensities. What remains unknown is the balance between temporal dynamics of genetic diversity and chlorophyll physiology in the ability of Prochlorococcus to respond to a variety of short (approximately 1 day) and longer (months to year) changes in the environment. Previous field research established depth-dependent Prochlorococcus single cell chlorophyll distributions in the North Pacific Subtropical Gyre. Here, we examined whether the shifts in chlorophyll distributions correspond to changes in Prochlorococcus genetic diversity (i.e., ecotype-level community structure) or photoacclimation of stable communities over short time intervals. We report that community structure was relatively stable despite abrupt shifts in Prochlorococcus chlorophyll physiology, due to unexpected physiological plasticity of highlight adapted Prochlorococcus ecotypes. Through comparison with seasonalscale changes, our data suggest that variability on daily scales triggers shifts in Prochlorococcus photoacclimation, while seasonal-scale dynamics trigger shifts in community structure. Together, these data highlight the importance of incorporating the process of photoacclimation and chlorophyll dynamics into interpretations of phytoplankton population dynamics from chlorophyll data as well as the importance of daily-scale variability to Prochlorococcus ecology.
Temporal variability in dissolved inorganic, organic phosphate (Pi, DOP) and particulate phosphor... more Temporal variability in dissolved inorganic, organic phosphate (Pi, DOP) and particulate phosphorus (PPO 4) concentrations, and microbial utilization of Pi and dissolved adenosine-5 ′-triphosphate (DATP) was studied at Station ALOHA (22.75 • N, 158 • W) in the North Pacific Subtropical Gyre (NPSG) over a multi-year period. Spatial variability of the same properties was investigated along two transects, to and from Hawaii, that traversed the NPSG boundaries to the east (2014) and north (2016). Radiotracer techniques were employed to measure the turnover time of Pi and DATP pools to calculate Pi uptake rates and the Pi hydrolysis rates of DATP. Pi concentrations were more variable, both in time and space, than DOP, ranging two orders of magnitude compared to a factor of two for DOP. The DATP pool, while constituting on average <0.15% of the total DOP-P, was as dynamic as Pi (∼1-200 pmol l −1), with lowest concentrations coinciding with Pi depletion. The Pi turnover times ranged from a few hours to several weeks, and were correlated with measured Pi concentrations (r = 0.9; Station ALOHA, n = 28; 2014, n = 14; 2016, n = 12). Pi uptake rates averaged 3.6 ± 1.3 nmol-P l −1 d −1 (n = 28: Station ALOHA), 9.2 ± 4.7 nmol-P l −1 d −1 , (n = 15; 2014) and 5.1 ± 2.5 nmol-P l −1 d −1 , (n = 12; 2016). The turnover time of the DATP pool was typically substantially shorter (0.4-5 days) than for the Pi-pool, and uptake rates ranged from 1 to 115 pmol l −1 d −1. However, at very low Pi and ATP concentrations, ATP turnover was longer than Pi turnover and ATP uptake rates lower. Total ATP hydrolysis was high along both transects, exceeding the ATP taken up by the microbial community, resulting in a net release of Pi into the ambient seawater. This net release was positively correlated to Pi concentration. The relative contribution by microbial size classes to total P-uptake depended on whether P was derived from ambient Pi or from DATP, with the <0.6>0.2 µm size class dominating the DATP uptake. Our results indicate that during Pi limiting conditions, regenerated P is rapidly consumed, and that Pi limitation occurs locally and transiently but does not appear to be the predominant condition in the upper water column of the NPSG.
The southeast subtropical Pacific Ocean was sampled along a zonal transect between the coasts of ... more The southeast subtropical Pacific Ocean was sampled along a zonal transect between the coasts of Chile and Easter Island. This remote area of the world's ocean presents strong gradients in physical (e.g., temperature, density and light), chemical (e.g., salinity and nutrient concentrations) and microbiological (e.g., cell abundances, biomass and specific growth rates) properties. The goal of this study was to describe the phosphorus (P) dynamics in three main ecosystems along this transect: the upwelling regime off the northern Chilean coast, the oligotrophic area associated with the southeast subtropical Pacific gyre and the transitional area in between these two biomes. We found that inorganic phosphate (Pi) concentrations were high and turnover times were long (>210 nmol l À1 and >31 d, respectively) in the upper water column, along the entire transect. Pi uptake rates in the gyre were low (euphotic layer integrated rates were 0.26 mmol m À2 d À1 in the gyre and 1.28 mmol m À2 d À1 in the upwelling region), yet not only driven by decreases in particle mass or cell abundance (particulate P-and cell-normalized Pi uptake rates in the euphotic layer were $1-4 times and $3-15 times lower in the gyre than in the upwelling, respectively). However these Pi uptake rates were at or near the maximum Pi uptake velocity (i.e., uptake rates in Pi amended samples were not significantly different from those at ambient concentration: 1.5 and 23.7 nmol l À1 d À1 at 50% PAR in the gyre and upwelling, respectively). Despite the apparent Pi replete conditions, selected dissolved organic P (DOP) compounds were readily hydrolyzed. Nucleotides were the most bioavailable of the DOP substrates tested. Microbes actively assimilated adenosine-5 0triphosphate (ATP) leading to Pi and adenosine incorporation as well as Pi release to the environment. The southeast subtropical Pacific Ocean is a Pi-sufficient environment, yet DOP hydrolytic processes are maintained and contribute to P-cycling across the wide range of environmental conditions present in this ecosystem.
We present nearly 9 yrs (June 2005–December 2013) of measurements of upper‐ocean (0 m to 125 m) d... more We present nearly 9 yrs (June 2005–December 2013) of measurements of upper‐ocean (0 m to 125 m) dinitrogen (N2) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N2 fixation rates were measured based on adding the 15N2 tracer as a gas bubble. Beginning in August 2012, 15N2 was first dissolved into filtered seawater and the 15N2‐enriched water was subsequently added to N2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15N2‐enriched seawater resulting in twofold greater depth‐integrated rates than those derived from adding a 15N2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N2 fixation averaged 230 ± 136 μmol N m−2 d−1 for the full time series (n = 71). Analysis of the 15N isotopic composition of sinking PN, together w...
The partitioning of photosynthetically-derived organic carbon between particulate and dissolved p... more The partitioning of photosynthetically-derived organic carbon between particulate and dissolved phases has important implications for marine carbon cycling. In this study we utilized 14 C-bicarbonate assimilation to quantify rates of photosynthetic production of both particulate and dissolved organic carbon (DOC) at Station ALOHA (22 • 45 ′ N, 158 • W) in the North Pacific Subtropical Gyre (NPSG). At near-monthly time scales over ∼5 years, we examined retention of 14 C-labeled organic matter by both glass fiber filters and 0.2 µm pore size polycarbonate membrane filters that are commonly used for measurements of 14 C-based plankton productivity. Use of polycarbonate filters resulted in significantly lower (averaging 60%) estimates of 14 C-production compared to glass fiber filters. Coincident measurements of chlorophyll a concentrations from both 0.2 µm polycarbonate and glass fiber filters were not significantly different, suggesting the differences in 14 C-productivity between these filter types did not derive from differences in retention of photosynthetic biomass by these filters. Moreover, consistent with previous studies, results from experiments aimed at quantifying retention of organic matter by these filters suggested differences resulted from retention of DOC by glass fiber filters. We also quantified rates of 14 C-DOC production to evaluate the partitioning of photosynthetic production between dissolved and particulate phases over daily to monthly time scales in this ecosystem. Unlike the strong depth dependence observed in measurements of particulate organic carbon production, measured rates of 14 C-DOC demonstrated no clear depth dependence. On average, depth-integrated (0-75 m) rates of 14 C-DOC production rates were equivalent to 18 ± 10% of the total (particulate and dissolved) productivity. Our findings indicate that in this oligotrophic ecosystem, rates of dissolved and particulate production can be temporally decoupled over daily to monthly time scales.
One challenge in field-based marine microbial ecology is to achieve sufficient spatial resolution... more One challenge in field-based marine microbial ecology is to achieve sufficient spatial resolution to obtain representative information about microbial distributions and biogeochemical processes. The challenges are exacerbated when conducting rate measurements of biological processes due to potential perturbations during sampling and incubation. Here we present the first application of a robotic microlaboratory, the 4 L-submersible incubation device (SID), for conducting in situ measurements of the rates of biological nitrogen (N 2) fixation (BNF). The free-drifting autonomous instrument obtains samples from the water column that are incubated in situ after the addition of 15 N 2 tracer. After each of up to four consecutive incubation experiments, the 4-L sample is filtered and chemically preserved. Measured BNF rates from two deployments of the SID in the oligotrophic North Pacific ranged from 0.8 to 2.8 nmol N L 21 day 21 , values comparable with simultaneous rate measurements obtained using traditional conductivity-temperature-depth (CTD)-rosette sampling followed by on-deck or in situ incubation. Future deployments of the SID will help to better resolve spatial variability of oceanic BNF, particularly in areas where recovery of seawater samples by CTD compromises their integrity, e.g. anoxic habitats.
Methane is a potent greenhouse gas that has contributed approximately 20% to the Earth's warming ... more Methane is a potent greenhouse gas that has contributed approximately 20% to the Earth's warming since pre-industrial times. The world's oceans are an important source of methane, comprising 1-4% of annual global emissions. But despite its global significance, oceanic methane production is poorly understood. In particular, methane concentrations in the surface waters of most of the world's oceans are supersaturated with respect to atmospheric concentrations, but the origin of this methane, which has been thought to be produced exclusively in anaerobic environments, is not known. Here, we measure methane production in seawater samples amended with methylphosphonate, an organic, phosphorus-containing compound. We show that methane is produced aerobically as a by-product of methylphosphonate decomposition in phosphate-stressed waters. Methylphosphonate decomposition, and thus methane production, may be enhanced by the activity of nitrogen-fixing microorganisms. We suggest that aerobic marine methane production will be sensitive to the changes in water-column stratification and nutrient limitation that are likely to result from greenhouse-gas-induced ocean warming.
Microbial community response to nitrogen (N) amendments and induced phosphorus (P) stress was inv... more Microbial community response to nitrogen (N) amendments and induced phosphorus (P) stress was investigated in the North Pacific Subtropical Gyre (NPSG). Samples amended with reduced sources of N, in the form of nitrate plus ammonium, showed significant increases in microbial cell abundance and biomass and decreases in dissolved inorganic phosphate (Pi) and silicate concentrations during an incubation period of 6 d. Primary productivity, P uptake rates (as both Pi and adenosine-5'-triphosphate [ATP]) and alkaline phosphatase activity (APA) all increased following N amendment. Dissolved organic P (DOP) concentrations did not change, but the large increase in APA and ATP uptake rates suggests that DOP was a dynamic pool and an important source for microbial P nutrition in P-stressed samples. Significant changes were also observed in the structure of the microbial community, with Synecho coccus and picoalgae abundances increasing substantially in the N-amended treatments, while non-pigmented picoplankton abundances were unchanged. Data on P resource partitioning among groups of picoplankton separated by size using membrane filters of different porosities, or by scattering and fluorescence properties using flow cytometry sorting, indicate that Synechococcus could have a greater role in the NPSG P cycling following episodic N inputs. This experimental manipulation of nutrient loading combined with observations at the total population to the microbial group levels constitutes a unique approach to improve our understanding of microbial community structure and function in response to environmental forcing.
Studies on upper ocean plankton production and respiration in oligotrophic ecosystems are taken t... more Studies on upper ocean plankton production and respiration in oligotrophic ecosystems are taken to imply that these regions are net heterotrophic (i.e. gross primary production [GPP] < respiration [R]). In this study we examined the hypothesis that the regularly observed state of net heterotrophy in the oligotrophic North Pacific Ocean may be interrupted by periodic bursts of net autotrophy when GPP becomes decoupled from R. To examine this hypothesis, we conducted a series of nutrient addition experiments in the oligotrophic North Pacific Subtropical Gyre (NPSG) in which nutrient-replete deep seawater was mixed with nutrient-depleted mixed-layer water. We monitored time-dependent changes in rates of GPP and R as well as dynamics of chlorophyll and nutrient concentrations. Our results consistently demonstrated that following the addition of nutrients: (1) phytoplankton biomass (chl a) and primary production increase; (2) the photoautotrophic plankton assemblage shifts from small (< 2 µm diameter) to large (>10 µm diameter), chl c-containing and Siutilizing cells; and (3) community metabolism shifts from near balance, or slightly net heterotrophic, to demonstrably net autotrophic metabolism. These results suggest that nutrient availability partly regulates plankton community structure and dynamics, and could serve as a mechanism to temporally and spatially decouple GPP and R in oligotrophic open ocean ecosystems.
We examined the phytoplankton response to the addition of nutrient-enriched deep seawater (DSW) a... more We examined the phytoplankton response to the addition of nutrient-enriched deep seawater (DSW) and of nitrate only during shipboard experiments performed between July 2004 and May 2007 in the North Pacific Subtropical Gyre (NPSG). Chlorophyll a (chl a), community size structure and composition, carbon fixation (CF) rates, and nutrient concentrations were measured daily for 5 to 7 d under simulated in situ conditions. Despite the fact that the NPSG is a permanently stratified, oligotrophic biome, there was a seasonal response to the addition of DSW and nitrate. In summertime experiments, chl a and CF rates increased up to 18-and 22-fold, respectively, relative to unamended controls after an incubation period of 5 to 6 d. Nutrients were assimilated to below control concentrations. A shift from a Prochlorococcus-dominated to a Synechococcus-and diatom-dominated community, and an 8-to 12-fold increase in the chl a content of the > 2 µm size fraction were observed. Addition of nitrate only increased chl a and CF rates 10-and 7-fold, respectively. In wintertime experiments, chl a and CF rates increased up to 4-and 9-fold, respectively, after an incubation period of up to 7 d. Only 50% of nutrients were assimilated and nitrate alone stimulated a 2-fold increase in chl a and CF rates. We explore the role of nutrient limitation, community composition, grazing, and light in explaining our observations. Findings from this seasonal and multi-year field experiment demonstrate how little we know about bloom development in the NPSG and highlight the potential risk in extrapolating the response of phytoplankton to natural or artificial fertilization from short-term studies.
Oceanic habitats may select for different organisms, thereby tuning genomic capabilities to local... more Oceanic habitats may select for different organisms, thereby tuning genomic capabilities to local environmental conditions. To understand the relationship between microbial assemblage composition, functional capability, and habitat, a random genome shotgun sequencing (metagenomic) survey was conducted with surface‐water microbial assemblages (0.2–5‐µm size fraction) collected at seven locations along a meridional transect from the northern edge of the South Pacific subtropical gyre to the southern edge of the North Pacific subtropical gyre (16°S–13.5°N). A total of 1.1 million unique sequence reads were obtained, of which ~45% could be annotated to metabolic category. Microbial assemblages in equatorial divergence and countercurrent habitats were distinct phylogenetically from those in gyre waters. Ecotypes of dominant Cyanobacteria (Prochlorococcus and Synechococcus) had distinct distributions congruent with their physiological characteristics in cultivation. The metagenomic distri...
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