The isotope pairing technique (IPT) is a well-established 15 N method for estimation of denitrifi... more The isotope pairing technique (IPT) is a well-established 15 N method for estimation of denitrification. Presence of anammox, the anaerobic oxidation of NH4 + to N2 with NO2 - results in violation of central assump- tions on which the IPT is built. It is shown that anammox activity causes overestimation of the N2 production calculated by the IPT. However, experiments
The discovery of electric currents in marine sediments arose from a simple observation that conve... more The discovery of electric currents in marine sediments arose from a simple observation that conventional biogeochemistry could not explain: Sulfide oxidation in one place is closely coupled to oxygen reduction in another place, centimeters away. After experiments demonstrated that this resulted from electric coupling, the conductors were found to be long, multicellular, filamentous bacteria, now known as cable bacteria. The spatial separation of oxidation and reduction processes by these bacteria represents a shortcut in the conventional cascade of redox processes and may drive most of the oxygen consumption. In addition, it implies a separation of strong proton generators and consumers and the formation of measurable electric fields, which have several effects on mineral development and ion migration. This article reviews the work on electric currents and cable bacteria published through April 2014, with an emphasis on general trends, thought-provoking consequences, and new questio...
... by Hahn et al. (1982). The applied microsensor reacted to concentration changes with 90% resp... more ... by Hahn et al. (1982). The applied microsensor reacted to concentration changes with 90% response times of 1.5 s for 02 and 12 s for N20. The detection limits for both gases were about 1 #M. The sensitivity to N20 was almost unaffected by flow in the 867 868 NIELS PETER ...
Filamentous bacteria of the Desulfobulbaceae family can conduct electrons over centimeter-long di... more Filamentous bacteria of the Desulfobulbaceae family can conduct electrons over centimeter-long distances thereby coupling oxygen reduction at the surface of marine sediment to sulfide oxidation in deeper anoxic layers. The ability of these cable bacteria to use alternative electron acceptors is currently unknown. Here we show that these organisms can use also nitrate or nitrite as an electron acceptor thereby coupling the reduction of nitrate to distant oxidation of sulfide. Sulfidic marine sediment was incubated with overlying nitrate-amended anoxic seawater. Within 2 months, electric coupling of spatially segregated nitrate reduction and sulfide oxidation was evident from: (1) the formation of a 4-6-mm-deep zone separating sulfide oxidation from the associated nitrate reduction, and (2) the presence of pH signatures consistent with proton consumption by cathodic nitrate reduction, and proton production by anodic sulfide oxidation. Filamentous Desulfobulbaceae with the longitudinal structures characteristic of cable bacteria were detected in anoxic, nitrate-amended incubations but not in anoxic, nitrate-free controls. Nitrate reduction by cable bacteria using longdistance electron transport to get privileged access to distant electron donors is a hitherto unknown mechanism in nitrogen and sulfur transformations, and the quantitative importance for elements cycling remains to be addressed.
Filamentous Desulfobulbaceae have been reported to conduct electrons over centimetre-long distanc... more Filamentous Desulfobulbaceae have been reported to conduct electrons over centimetre-long distances, thereby coupling oxygen reduction at the surface of marine sediment to sulphide oxidation in sub-surface layers. To understand how these 'cable bacteria' establish and sustain electric conductivity, we followed a population for 53 days after exposing sulphidic sediment with initially no detectable filaments to oxygen. After 10 days, cable bacteria and electric currents were established throughout the top 15 mm of the sediment, and after 21 days the filament density peaked with a total length of 2 km cm À 2 . Cells elongated and divided at all depths with doubling times over the first 10 days of o20 h. Active, oriented movement must have occurred to explain the separation of O 2 and H 2 S by 15 mm. Filament diameters varied from 0.4-1.7 lm, with a general increase over time and depth, and yet they shared 16S rRNA sequence identity of 498%. Comparison of the increase in biovolume and electric current density suggested high cellular growth efficiency. While the vertical expansion of filaments continued over time and reached 30 mm, the electric current density and biomass declined after 13 and 21 days, respectively. This might reflect a breakdown of short filaments as their solid sulphide sources became depleted in the top layers of the anoxic zone. In conclusion, cable bacteria combine rapid and efficient growth with oriented movement to establish and exploit the spatially separated half-reactions of sulphide oxidation and oxygen consumption.
Reviews in Environmental Science and Bio/Technology, 2002
In order to meet increasingly stringent European discharge standards, new applications and contro... more In order to meet increasingly stringent European discharge standards, new applications and control strategies for the sustainable removal of ammonia from wastewater have to be implemented. In this paper we discuss a nitrogen removal system based on the processes of partial nitrification and anoxic ammonia oxidation (anammox). The anammox process offers great opportunities to remove ammonia in fully autotrophic systems with biomass retention. No organic carbon is needed in such nitrogen removal system, since ammonia is used as electron donor for nitrite reduction. The nitrite can be produced from ammonia in oxygen-limited biofilm systems or in continuous processes without biomass retention. For successful implementation of the combined processes, accurate biosensors for measuring ammonia and nitrite concentrations, insight in the complex microbial communities involved, and new control strategies have to be developed and evaluated.
Proceedings of the National Academy of Sciences, 2009
A large variety of aquatic animals was found to emit the potent greenhouse gas nitrous oxide when... more A large variety of aquatic animals was found to emit the potent greenhouse gas nitrous oxide when nitrate was present in the environment. The emission was ascribed to denitrification by ingested bacteria in the anoxic animal gut, and the exceptionally high N2O-to-N2 production ratio suggested delayed induction of the last step of denitrification. Filter-and deposit-feeding animal species showed the highest rates of nitrous oxide emission and predators the lowest, probably reflecting the different amounts of denitrifying bacteria in the diet. We estimate that nitrous oxide emission by aquatic animals is quantitatively important in nitraterich aquatic environments like freshwater, coastal marine, and deep-sea ecosystems. The contribution of this source to overall nitrous oxide emission from aquatic environments might further increase because of the projected increase of nitrate availability in tropical regions and the numeric dominance of filter-and depositfeeders in eutrophic ecosystems.
Figure 1 | Foraminifera, oxygen and nitrate in sediment. a, Depth distribution of Rose-Bengal sta... more Figure 1 | Foraminifera, oxygen and nitrate in sediment. a, Depth distribution of Rose-Bengal stained G. pseudospinescens (hatched area) and oxygen (open circles). b, Porewater nitrate/nitrite (squares) and cell-bound nitrate (grey area) in sediment cores from the Gullmar Fjord, Sweden, August 2005. The error bars indicate the s.e.m (n ¼ 3).
Some bacteria are capable of extracellular electron transfer, thereby enabling them to use electr... more Some bacteria are capable of extracellular electron transfer, thereby enabling them to use electron acceptors and donors without direct cell contact . Beyond the micrometre scale, however, no firm evidence has previously existed that spatially segregated biogeochemical processes can be coupled by electric currents in nature.
The nitrogen balance of a shallow water eelgrass Zostera marina L. bed was assessed in April and ... more The nitrogen balance of a shallow water eelgrass Zostera marina L. bed was assessed in April and August 1995 by quantifying pools of nitrogen in plants and sediment and by measunng rates of nitrogen inputs and losses in vegetated and bare sediments. The total pool of nitrogen in the vegetated sediment doubled from Apnl to August. The exchange of inorganic nitrogen between water column and sediment was measured in benthic flux chambers, and showed that vegetated sediment was a sink for water column nitrogen in both April and August. Net nitrogen fluxes of NO3-and NH,' were controlled mainly by light dependent nitrogen uptake in eelgrass leaves, which accounted for 60% of the estimated nitrogen requirements for plant growth. Nitrogen fixation was stimulated by eelgrass photosynthesis, but contributed less than 4 % to total nitrogen input. The nitrogen mass balance suggested a large import of particulate nitrogen in addition to net uptake of inorganic nitrogen. Nitrogen was lost from the vegetated sediment mainly via export of leaves. Rates of denitrification measured with In s~t u techniques were low compared to the activity in bare sedlrnents of similar areas. Increased denitrification was found in the eelgrass rhizosphere in Apr~l, but the overall denitrification activity within the bed barely balanced nitrogen fixation. The study shows that eelgrass vegetated sediments may influence the nitrogen cycling of shallow waters substantially by incorporating large pools of nitrogen into slowly degradable material, making nitrogen at least temporally unavailable to phytoplankton and ephemeral macroalgae. The results also suggest that neither nitrogen fixation nor denitrification plays a major role in the nitrogen dynamics of eelgrass beds.
Benthic nitrogen processes have received substantial attention because the release of nutrients f... more Benthic nitrogen processes have received substantial attention because the release of nutrients from sediments can contribute to the requirements of pelagic primary production; their study can also give an estimation of the importance of the sediment as a source or a sink of nutrients. Concepción Bay is located in central Chile and is the largest (167.4 km 2 ) and most enclosed embayment on the Chilean coastline. The bay is characterized by a strong hydrographic variability produced by the spring/summer seasonal upwelling of Equatorial subsurface waters (ESSW), rich in nutrients (ϳ25 M NO 3 Ϫ ) and poor in oxygen (Ͻ44.6 M). The area was studied in order to understand the consequences of phytodetrital deposition and oxygen deficiency on the environment and benthic communities. The study was carried out by sampling at a single station (28-m depth) in the inner part of the bay during winter (June 1998) and spring/summertime (November 1998 and January and March 1999). It was focused on measurements of benthic nitrogen fluxes, sulfate reduction, and denitrification rates before and after a phytoplankton bloom. Additionally, samples from the flocculent layer and from a semipurified bacterial mat were incubated under controlled oxygen conditions to determine NH 4 ϩ production. NH 4 ϩ exchange showed a clear seasonal pattern, with influxes during the winter (Ϫ7.6 Ϯ 4.9 mmol m Ϫ2 d Ϫ1 ) and high effluxes during the summer (36.6 and 20.8 mmol m Ϫ2 d Ϫ1 ) when the accumulation of fresh organic matter (evidenced as chlorophyll a) produced a flocculent layer over the sediments. Besides natural hypoxia of the bottom water associated with ESSW, the large input of organic matter resulted in anoxia within the sediment, as a consequence of respiration processes, and an enhancement in sulfate reduction rates (up to 200 mmol m Ϫ2 d Ϫ1 ). The flocculent layer then provided a favorable environment for the extensive development of Beggiatoa spp. mats. Overall, during the sampling period, NO 3
The effect of light on benthic photosynthesis, denitrification, and assimilation of NH,+ and NO,i... more The effect of light on benthic photosynthesis, denitrification, and assimilation of NH,+ and NO,in stream sediments was studied with whole-core techniques and with 0, and N,O microsensors. Photosynthetic oxygen production increased the thickness of the aerobic surface layer from 1.5 mm in the dark to -3.5 mm at a light intensity saturating photosynthesis. The 0, flux changed concurrently from net uptake to net release and the overall rate of denitrification was reduced by 70%. Denitrification was always restricted to a narrow zone immediately below the aerobio anaerobic interface. Calculated NO,-microprofiles showed that overall denitrification was primarily dependent on the thickness of the aerobic layer which acted as a barrier for diffusion of NO,-from the overlying water. In the light, algal NO,-assimilation could exceed NO,-consumption by denitrification when availability of NH,+ was low. Assimilation of N03-, however, had no influence on the flux of NO,-to the denitrification zone, since assimilation occurred relatively close to the sediment surface.
Seasonal variation of chlorophyll content, photosynthesis, 0, respiration, and denitrification wa... more Seasonal variation of chlorophyll content, photosynthesis, 0, respiration, and denitrification was measured under light and dark conditions in the sediment of a nutrient-rich Danish lowland stream. Exponential growth of benthic microalgae was observed in early spring (April-May) and photosynthetic capacity persisted until fall. The benthic algae were a major C source for heterotrophic activity as indicated by a close correlation between 0, respiration and Chl content in the sediment. Denitrilication activity was related to Chl content, NO,-availability, and 0, conditions. Diffusion from the overlying water was always the major NO,-source for denitrification. Under lighted conditions, photosynthetic 0, production increased the oxic zone and reduced denitrification activity by up to 85% in spring. A simple diffusion-reaction model allowed denitrification rates to be estimated from 0, respiration rates and concentrations of O2 and N03-in the stream water. Throughout the season, estimated denitrification rates correlated well with those actually measured. The model demonstrated that denitrification activity was controlled primarily by the thickness of the oxic surface layer which served as a diffusion barrier for NO,-to the denitrification zone.
Diurnal variation of denitrification in sediments with benthic microphytes was investigated by a ... more Diurnal variation of denitrification in sediments with benthic microphytes was investigated by a 15N isotope-pairing technique in order to distinguish between coupled nitrification-denitrification and deni- trification of nitrate supplied from the water column. Sediments were incubated in a continuous flow- through system and exposed to diurnal light and dark cycles. Illumination of both limnetic and estuarine sediments doubled the rate
... 14N03-from autochthonous sources takes place in the denitrifying microenvironment. Local vari... more ... 14N03-from autochthonous sources takes place in the denitrifying microenvironment. Local variation of the nitrification activity caused by heterogeneous topography, biotu-bation, etc. can lead to the existence of sub-pools with ...
The groundwater concentrations of dinitrogen (N2) and argon (Ar) were measured in a transect of a... more The groundwater concentrations of dinitrogen (N2) and argon (Ar) were measured in a transect of a Danish riparian wetland in order to estimate N2 production from denitrification, the concentration of Ar being used as an internal tracer for degassing. Dissolved N 2 increased by 135 #M N 2-N. Simple calculations incorporating the decrease in groundwater dissolved Ar and the partial pressure of N2 revealed that 376/xM N2-N was degassed along the transect. Total denitrification thus amounted to 511 #M N2-N, correlating well with the measured NO3 reduction of 503 #M NO3-N. The total partial pressures of dissolved gases (N2, O2, Ar, and CO2) exceeded the hydrostatic pressure, thus confirming that degassing was a plausible mechanism for loss of dissolved gases in this riparian wetland. This study provides strong evidence that losses of dissolved Ar along a transect parallel to groundwater flow can be used to assess the amount of dissolved N2 degassed and thereby permit estimates of N2 production under conditions without conservation of gases within the groundwater system.
In the Completely Autotrophic Nitrogen removal Over Nitrite (CANON) process, aerobic and anaerobi... more In the Completely Autotrophic Nitrogen removal Over Nitrite (CANON) process, aerobic and anaerobic ammonia oxidizing bacteria cooperate to remove ammonia in one oxygen-limited reactor. Kinetic studies, microsensor analysis, and fluorescence in situ hybridization on CANON biomass showed a partial differentiation of processes and organisms within and among aggregates. Under normal oxygen-limited conditions ($5 lM O 2 ), aerobic ammonia oxidation (nitrification) was restricted to an outer shell (<100 lm) while anaerobic ammonia oxidation (anammox) was found in the central anoxic parts. Larger type aggregates (>500 lm) accounted for 68% of the anammox potential whereas 65% of the nitrification potential was found in the smaller aggregates (<500 lm). Analysis with O 2 and NO À 2 microsensors showed that the thickness of the activity zones varied as a function of bulk O 2 and NO À 2 concentrations and flow rate.
The effects of the new antifouling biocides, zinc pyrithione (ZPT) and copper pyrithione (CPT), o... more The effects of the new antifouling biocides, zinc pyrithione (ZPT) and copper pyrithione (CPT), on microbial communities in estuarine sediments were studied in microcosms. As functional endpoints, fluxes of nutrients (NO3-, NH4+, HPO4(2-), Si(OH)4) and protein synthesis ([14C]leucine incorporation) were used, whereas molecular fingerprinting methods (polymerase chain reaction/denaturing gradient gel electrophoresis) were used to describe the bacterial community structure. The lowest-observed-effect concentration (LOEC) for ZPT was 0.001 nmol/g dry sediment for the phosphate flux and total DNA content, whereas the LOEC for CPT was 0.1 nmol/g dry sediment for the nitrate flux and total DNA content. Nitrate fluxes increased significantly following additions of both ZPT and CPT, whereas ammonium fluxes decreased significantly after ZPT addition, suggesting changes in the nitrification and denitrification processes. The total DNA content decreased significantly following addition of both ZPT and CPT, but at the highest addition of ZPT (10 nmol ZPT/g dry sediment), an increase in total DNA content was found. Increased protein synthesis and bacterial diversity were also observed at this concentration of ZPT, suggesting growth of tolerant opportunistic species.
The isotope pairing technique (IPT) is a well-established 15 N method for estimation of denitrifi... more The isotope pairing technique (IPT) is a well-established 15 N method for estimation of denitrification. Presence of anammox, the anaerobic oxidation of NH4 + to N2 with NO2 - results in violation of central assump- tions on which the IPT is built. It is shown that anammox activity causes overestimation of the N2 production calculated by the IPT. However, experiments
The discovery of electric currents in marine sediments arose from a simple observation that conve... more The discovery of electric currents in marine sediments arose from a simple observation that conventional biogeochemistry could not explain: Sulfide oxidation in one place is closely coupled to oxygen reduction in another place, centimeters away. After experiments demonstrated that this resulted from electric coupling, the conductors were found to be long, multicellular, filamentous bacteria, now known as cable bacteria. The spatial separation of oxidation and reduction processes by these bacteria represents a shortcut in the conventional cascade of redox processes and may drive most of the oxygen consumption. In addition, it implies a separation of strong proton generators and consumers and the formation of measurable electric fields, which have several effects on mineral development and ion migration. This article reviews the work on electric currents and cable bacteria published through April 2014, with an emphasis on general trends, thought-provoking consequences, and new questio...
... by Hahn et al. (1982). The applied microsensor reacted to concentration changes with 90% resp... more ... by Hahn et al. (1982). The applied microsensor reacted to concentration changes with 90% response times of 1.5 s for 02 and 12 s for N20. The detection limits for both gases were about 1 #M. The sensitivity to N20 was almost unaffected by flow in the 867 868 NIELS PETER ...
Filamentous bacteria of the Desulfobulbaceae family can conduct electrons over centimeter-long di... more Filamentous bacteria of the Desulfobulbaceae family can conduct electrons over centimeter-long distances thereby coupling oxygen reduction at the surface of marine sediment to sulfide oxidation in deeper anoxic layers. The ability of these cable bacteria to use alternative electron acceptors is currently unknown. Here we show that these organisms can use also nitrate or nitrite as an electron acceptor thereby coupling the reduction of nitrate to distant oxidation of sulfide. Sulfidic marine sediment was incubated with overlying nitrate-amended anoxic seawater. Within 2 months, electric coupling of spatially segregated nitrate reduction and sulfide oxidation was evident from: (1) the formation of a 4-6-mm-deep zone separating sulfide oxidation from the associated nitrate reduction, and (2) the presence of pH signatures consistent with proton consumption by cathodic nitrate reduction, and proton production by anodic sulfide oxidation. Filamentous Desulfobulbaceae with the longitudinal structures characteristic of cable bacteria were detected in anoxic, nitrate-amended incubations but not in anoxic, nitrate-free controls. Nitrate reduction by cable bacteria using longdistance electron transport to get privileged access to distant electron donors is a hitherto unknown mechanism in nitrogen and sulfur transformations, and the quantitative importance for elements cycling remains to be addressed.
Filamentous Desulfobulbaceae have been reported to conduct electrons over centimetre-long distanc... more Filamentous Desulfobulbaceae have been reported to conduct electrons over centimetre-long distances, thereby coupling oxygen reduction at the surface of marine sediment to sulphide oxidation in sub-surface layers. To understand how these 'cable bacteria' establish and sustain electric conductivity, we followed a population for 53 days after exposing sulphidic sediment with initially no detectable filaments to oxygen. After 10 days, cable bacteria and electric currents were established throughout the top 15 mm of the sediment, and after 21 days the filament density peaked with a total length of 2 km cm À 2 . Cells elongated and divided at all depths with doubling times over the first 10 days of o20 h. Active, oriented movement must have occurred to explain the separation of O 2 and H 2 S by 15 mm. Filament diameters varied from 0.4-1.7 lm, with a general increase over time and depth, and yet they shared 16S rRNA sequence identity of 498%. Comparison of the increase in biovolume and electric current density suggested high cellular growth efficiency. While the vertical expansion of filaments continued over time and reached 30 mm, the electric current density and biomass declined after 13 and 21 days, respectively. This might reflect a breakdown of short filaments as their solid sulphide sources became depleted in the top layers of the anoxic zone. In conclusion, cable bacteria combine rapid and efficient growth with oriented movement to establish and exploit the spatially separated half-reactions of sulphide oxidation and oxygen consumption.
Reviews in Environmental Science and Bio/Technology, 2002
In order to meet increasingly stringent European discharge standards, new applications and contro... more In order to meet increasingly stringent European discharge standards, new applications and control strategies for the sustainable removal of ammonia from wastewater have to be implemented. In this paper we discuss a nitrogen removal system based on the processes of partial nitrification and anoxic ammonia oxidation (anammox). The anammox process offers great opportunities to remove ammonia in fully autotrophic systems with biomass retention. No organic carbon is needed in such nitrogen removal system, since ammonia is used as electron donor for nitrite reduction. The nitrite can be produced from ammonia in oxygen-limited biofilm systems or in continuous processes without biomass retention. For successful implementation of the combined processes, accurate biosensors for measuring ammonia and nitrite concentrations, insight in the complex microbial communities involved, and new control strategies have to be developed and evaluated.
Proceedings of the National Academy of Sciences, 2009
A large variety of aquatic animals was found to emit the potent greenhouse gas nitrous oxide when... more A large variety of aquatic animals was found to emit the potent greenhouse gas nitrous oxide when nitrate was present in the environment. The emission was ascribed to denitrification by ingested bacteria in the anoxic animal gut, and the exceptionally high N2O-to-N2 production ratio suggested delayed induction of the last step of denitrification. Filter-and deposit-feeding animal species showed the highest rates of nitrous oxide emission and predators the lowest, probably reflecting the different amounts of denitrifying bacteria in the diet. We estimate that nitrous oxide emission by aquatic animals is quantitatively important in nitraterich aquatic environments like freshwater, coastal marine, and deep-sea ecosystems. The contribution of this source to overall nitrous oxide emission from aquatic environments might further increase because of the projected increase of nitrate availability in tropical regions and the numeric dominance of filter-and depositfeeders in eutrophic ecosystems.
Figure 1 | Foraminifera, oxygen and nitrate in sediment. a, Depth distribution of Rose-Bengal sta... more Figure 1 | Foraminifera, oxygen and nitrate in sediment. a, Depth distribution of Rose-Bengal stained G. pseudospinescens (hatched area) and oxygen (open circles). b, Porewater nitrate/nitrite (squares) and cell-bound nitrate (grey area) in sediment cores from the Gullmar Fjord, Sweden, August 2005. The error bars indicate the s.e.m (n ¼ 3).
Some bacteria are capable of extracellular electron transfer, thereby enabling them to use electr... more Some bacteria are capable of extracellular electron transfer, thereby enabling them to use electron acceptors and donors without direct cell contact . Beyond the micrometre scale, however, no firm evidence has previously existed that spatially segregated biogeochemical processes can be coupled by electric currents in nature.
The nitrogen balance of a shallow water eelgrass Zostera marina L. bed was assessed in April and ... more The nitrogen balance of a shallow water eelgrass Zostera marina L. bed was assessed in April and August 1995 by quantifying pools of nitrogen in plants and sediment and by measunng rates of nitrogen inputs and losses in vegetated and bare sediments. The total pool of nitrogen in the vegetated sediment doubled from Apnl to August. The exchange of inorganic nitrogen between water column and sediment was measured in benthic flux chambers, and showed that vegetated sediment was a sink for water column nitrogen in both April and August. Net nitrogen fluxes of NO3-and NH,' were controlled mainly by light dependent nitrogen uptake in eelgrass leaves, which accounted for 60% of the estimated nitrogen requirements for plant growth. Nitrogen fixation was stimulated by eelgrass photosynthesis, but contributed less than 4 % to total nitrogen input. The nitrogen mass balance suggested a large import of particulate nitrogen in addition to net uptake of inorganic nitrogen. Nitrogen was lost from the vegetated sediment mainly via export of leaves. Rates of denitrification measured with In s~t u techniques were low compared to the activity in bare sedlrnents of similar areas. Increased denitrification was found in the eelgrass rhizosphere in Apr~l, but the overall denitrification activity within the bed barely balanced nitrogen fixation. The study shows that eelgrass vegetated sediments may influence the nitrogen cycling of shallow waters substantially by incorporating large pools of nitrogen into slowly degradable material, making nitrogen at least temporally unavailable to phytoplankton and ephemeral macroalgae. The results also suggest that neither nitrogen fixation nor denitrification plays a major role in the nitrogen dynamics of eelgrass beds.
Benthic nitrogen processes have received substantial attention because the release of nutrients f... more Benthic nitrogen processes have received substantial attention because the release of nutrients from sediments can contribute to the requirements of pelagic primary production; their study can also give an estimation of the importance of the sediment as a source or a sink of nutrients. Concepción Bay is located in central Chile and is the largest (167.4 km 2 ) and most enclosed embayment on the Chilean coastline. The bay is characterized by a strong hydrographic variability produced by the spring/summer seasonal upwelling of Equatorial subsurface waters (ESSW), rich in nutrients (ϳ25 M NO 3 Ϫ ) and poor in oxygen (Ͻ44.6 M). The area was studied in order to understand the consequences of phytodetrital deposition and oxygen deficiency on the environment and benthic communities. The study was carried out by sampling at a single station (28-m depth) in the inner part of the bay during winter (June 1998) and spring/summertime (November 1998 and January and March 1999). It was focused on measurements of benthic nitrogen fluxes, sulfate reduction, and denitrification rates before and after a phytoplankton bloom. Additionally, samples from the flocculent layer and from a semipurified bacterial mat were incubated under controlled oxygen conditions to determine NH 4 ϩ production. NH 4 ϩ exchange showed a clear seasonal pattern, with influxes during the winter (Ϫ7.6 Ϯ 4.9 mmol m Ϫ2 d Ϫ1 ) and high effluxes during the summer (36.6 and 20.8 mmol m Ϫ2 d Ϫ1 ) when the accumulation of fresh organic matter (evidenced as chlorophyll a) produced a flocculent layer over the sediments. Besides natural hypoxia of the bottom water associated with ESSW, the large input of organic matter resulted in anoxia within the sediment, as a consequence of respiration processes, and an enhancement in sulfate reduction rates (up to 200 mmol m Ϫ2 d Ϫ1 ). The flocculent layer then provided a favorable environment for the extensive development of Beggiatoa spp. mats. Overall, during the sampling period, NO 3
The effect of light on benthic photosynthesis, denitrification, and assimilation of NH,+ and NO,i... more The effect of light on benthic photosynthesis, denitrification, and assimilation of NH,+ and NO,in stream sediments was studied with whole-core techniques and with 0, and N,O microsensors. Photosynthetic oxygen production increased the thickness of the aerobic surface layer from 1.5 mm in the dark to -3.5 mm at a light intensity saturating photosynthesis. The 0, flux changed concurrently from net uptake to net release and the overall rate of denitrification was reduced by 70%. Denitrification was always restricted to a narrow zone immediately below the aerobio anaerobic interface. Calculated NO,-microprofiles showed that overall denitrification was primarily dependent on the thickness of the aerobic layer which acted as a barrier for diffusion of NO,-from the overlying water. In the light, algal NO,-assimilation could exceed NO,-consumption by denitrification when availability of NH,+ was low. Assimilation of N03-, however, had no influence on the flux of NO,-to the denitrification zone, since assimilation occurred relatively close to the sediment surface.
Seasonal variation of chlorophyll content, photosynthesis, 0, respiration, and denitrification wa... more Seasonal variation of chlorophyll content, photosynthesis, 0, respiration, and denitrification was measured under light and dark conditions in the sediment of a nutrient-rich Danish lowland stream. Exponential growth of benthic microalgae was observed in early spring (April-May) and photosynthetic capacity persisted until fall. The benthic algae were a major C source for heterotrophic activity as indicated by a close correlation between 0, respiration and Chl content in the sediment. Denitrilication activity was related to Chl content, NO,-availability, and 0, conditions. Diffusion from the overlying water was always the major NO,-source for denitrification. Under lighted conditions, photosynthetic 0, production increased the oxic zone and reduced denitrification activity by up to 85% in spring. A simple diffusion-reaction model allowed denitrification rates to be estimated from 0, respiration rates and concentrations of O2 and N03-in the stream water. Throughout the season, estimated denitrification rates correlated well with those actually measured. The model demonstrated that denitrification activity was controlled primarily by the thickness of the oxic surface layer which served as a diffusion barrier for NO,-to the denitrification zone.
Diurnal variation of denitrification in sediments with benthic microphytes was investigated by a ... more Diurnal variation of denitrification in sediments with benthic microphytes was investigated by a 15N isotope-pairing technique in order to distinguish between coupled nitrification-denitrification and deni- trification of nitrate supplied from the water column. Sediments were incubated in a continuous flow- through system and exposed to diurnal light and dark cycles. Illumination of both limnetic and estuarine sediments doubled the rate
... 14N03-from autochthonous sources takes place in the denitrifying microenvironment. Local vari... more ... 14N03-from autochthonous sources takes place in the denitrifying microenvironment. Local variation of the nitrification activity caused by heterogeneous topography, biotu-bation, etc. can lead to the existence of sub-pools with ...
The groundwater concentrations of dinitrogen (N2) and argon (Ar) were measured in a transect of a... more The groundwater concentrations of dinitrogen (N2) and argon (Ar) were measured in a transect of a Danish riparian wetland in order to estimate N2 production from denitrification, the concentration of Ar being used as an internal tracer for degassing. Dissolved N 2 increased by 135 #M N 2-N. Simple calculations incorporating the decrease in groundwater dissolved Ar and the partial pressure of N2 revealed that 376/xM N2-N was degassed along the transect. Total denitrification thus amounted to 511 #M N2-N, correlating well with the measured NO3 reduction of 503 #M NO3-N. The total partial pressures of dissolved gases (N2, O2, Ar, and CO2) exceeded the hydrostatic pressure, thus confirming that degassing was a plausible mechanism for loss of dissolved gases in this riparian wetland. This study provides strong evidence that losses of dissolved Ar along a transect parallel to groundwater flow can be used to assess the amount of dissolved N2 degassed and thereby permit estimates of N2 production under conditions without conservation of gases within the groundwater system.
In the Completely Autotrophic Nitrogen removal Over Nitrite (CANON) process, aerobic and anaerobi... more In the Completely Autotrophic Nitrogen removal Over Nitrite (CANON) process, aerobic and anaerobic ammonia oxidizing bacteria cooperate to remove ammonia in one oxygen-limited reactor. Kinetic studies, microsensor analysis, and fluorescence in situ hybridization on CANON biomass showed a partial differentiation of processes and organisms within and among aggregates. Under normal oxygen-limited conditions ($5 lM O 2 ), aerobic ammonia oxidation (nitrification) was restricted to an outer shell (<100 lm) while anaerobic ammonia oxidation (anammox) was found in the central anoxic parts. Larger type aggregates (>500 lm) accounted for 68% of the anammox potential whereas 65% of the nitrification potential was found in the smaller aggregates (<500 lm). Analysis with O 2 and NO À 2 microsensors showed that the thickness of the activity zones varied as a function of bulk O 2 and NO À 2 concentrations and flow rate.
The effects of the new antifouling biocides, zinc pyrithione (ZPT) and copper pyrithione (CPT), o... more The effects of the new antifouling biocides, zinc pyrithione (ZPT) and copper pyrithione (CPT), on microbial communities in estuarine sediments were studied in microcosms. As functional endpoints, fluxes of nutrients (NO3-, NH4+, HPO4(2-), Si(OH)4) and protein synthesis ([14C]leucine incorporation) were used, whereas molecular fingerprinting methods (polymerase chain reaction/denaturing gradient gel electrophoresis) were used to describe the bacterial community structure. The lowest-observed-effect concentration (LOEC) for ZPT was 0.001 nmol/g dry sediment for the phosphate flux and total DNA content, whereas the LOEC for CPT was 0.1 nmol/g dry sediment for the nitrate flux and total DNA content. Nitrate fluxes increased significantly following additions of both ZPT and CPT, whereas ammonium fluxes decreased significantly after ZPT addition, suggesting changes in the nitrification and denitrification processes. The total DNA content decreased significantly following addition of both ZPT and CPT, but at the highest addition of ZPT (10 nmol ZPT/g dry sediment), an increase in total DNA content was found. Increased protein synthesis and bacterial diversity were also observed at this concentration of ZPT, suggesting growth of tolerant opportunistic species.
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