Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading... more Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading within the watershed. The main sources of N are thought to be in the form of nitrate from surface water runoff, agricultural fields, dairy farms, and urban runoff. More poorly quantified sources of nitrate may also include significant groundwater discharge as well as tidal inputs from Monterey Bay. Traditional studies, involving natural abundance stable isotopes within the N cycle, have been limited to delta 15 N and thus to a'one-dimensional' ...
Abstract As nitrate is utilized by phytoplankton in the euphotic zone, the isotopic compositions ... more Abstract As nitrate is utilized by phytoplankton in the euphotic zone, the isotopic compositions of both oxygen and nitrogen in the residual nitrate become enriched in the heavier isotopes (15 N and 18 O). Recent evidence from phytoplankton culture experiments showed that the enrichments in 15 N and 18 O occur intracellularly during the reduction of nitrate to nitrite. Furthermore, the isotopic composition of the external nitrate pool reflects the balance between intracellular reduction and efflux of the intracellular nitrate. It has also ...
Nitrate isotopes in wet deposition are useful indicators of NOx source contributions to nitrate f... more Nitrate isotopes in wet deposition are useful indicators of NOx source contributions to nitrate formation and NOx oxidation pathways at local and regional scales. Here, we examine whether nitrogen and oxygen isotopes (d15N and d18O, respectively) provide similarly useful information in: 1) wet deposition at the continental scale; and 2) dry deposition at the regional scale. For wet deposition analyses,
Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading... more Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading within the watershed. The main sources of N are thought to be in the form of nitrate from surface water runoff, agricultural fields, dairy farms, and urban runoff. More poorly quantified sources of nitrate may also include significant groundwater discharge as well as tidal inputs from Monterey Bay. Traditional studies, involving natural abundance stable isotopes within the N cycle, have been limited to delta 15 N and thus to a'one-dimensional' ...
Chemoautotrophic symbionts of deep sea hydrothermal vent tubeworms are known to provide their hos... more Chemoautotrophic symbionts of deep sea hydrothermal vent tubeworms are known to provide their hosts with all their primary nutrition. While studies have examined how chemoautotrophic symbionts provide the association with nitrogen, fewer have examined if symbiont nitrogen metabolism varies as a function of environmental conditions. Ridgeia piscesae tubeworms flourish at Northeastern Pacific vents, occupy a range of microhabitats, and exhibit a high degree of morphological plasticity [e.g. long-skinny (LS) and short-fat (SF) phenotypes] that may relate to environmental conditions. This plasticity affords an opportunity to examine whether symbiont nitrogen metabolism varies among host phenotypes. LS and SF R. piscesae were recovered from the Axial and Main Endeavour Field hydrothermal vents. Nitrate and ammonium were quantified in Ridgeia blood, and the expression of key nitrogen metabolism genes, as well as stable nitrogen isotope ratios, was quantified in host branchial plume and symbiont-containing tissues. Nitrate and ammonium were abundant in the blood of both phenotypes though environmental ammonium concentrations were, paradoxically, lowest among individuals with the highest blood ammonium. Assimilatory nitrate reductase transcripts were always below detection, though in both LS and SF R. piscesae symbionts, we observed elevated expression of dissimilatory nitrate reductase genes, as well as symbiont and host ammonium assimilation genes. Site-specific differences in expression, along with tissue stable isotope analyses, suggest that LS and SF Ridgeia symbionts are engaged in both dissimilatory nitrate reduction and ammonia assimilation to varying degrees. As such, it appears that environmental conditions -not host phenotype-primarily dictates symbiont nitrogen metabolism.
Deep-sea biogeochemical cycles are, in general, poorly understood owing to the difficulties of ma... more Deep-sea biogeochemical cycles are, in general, poorly understood owing to the difficulties of making measurements in situ, recovering samples with minimal perturbation, and, in many cases, coping with high spatial and temporal heterogeneity. In particular, biogeochemical fluxes of volatiles such as methane remain largely unconstrained because of the difficulties with accurate quantification in situ and the patchiness of point sources such as seeps and brine pools. To better constrain biogeochemical fluxes and cycling, we have developed a deep-sea in situ mass spectrometer (ISMS) to enable high-resolution quantification of volatiles in situ. Here we report direct measurements of methane concentrations made in a Gulf of Mexico brine pool located at a depth of over 2300 m. Concentrations of up to 33 mM methane were observed within the brine pool, whereas concentrations in the water directly above were three orders of magnitude lower. These direct measurements enabled us to make the first accurate estimates of the diffusive flux from a brine pool, calculated to be 1.1 7 0.2 mol m À 2 yr À 1. Integrated rate measurements of aerobic methane oxidation in the water column overlying the brine pool were $ 320 mmol m À 2 yr À 1 , accounting at most for just 0.03% of the diffusive methane flux from the brine pool. Calculated rates of anaerobic methane oxidation were 600–1200 mM yr À 1 , one to two orders of magnitude higher than previously published values of AOM in anoxic fluids. These findings suggest that brine pools are enormous point sources of methane in the deep sea, and may, in aggregate, have a pronounced impact on the global marine methane cycle.
Fish migration through the deep-water channel in the San Joaquin River at Stockton, California is... more Fish migration through the deep-water channel in the San Joaquin River at Stockton, California is inhibited by low oxygen concentrations during the summer months. The cause for this condition appears to be stagnation and decomposition of algae with attendant oxygen consumption. Algae growth in the San Joaquin River is promoted by nutrients entering the river mainly in the form of nitrate. Possible significant sources of nitrate include soil, fertilizer from agriculture, manure from dairy operations, and N derived from municipal sewage. A 2000 CALFED pilot study investigated the sources and cycling of nitrate at four sites along the San Joaquin River upstream of Stockton using the carbon and nitrogen isotopes of total dissolved and particulate organic matter, together with hydrological measurements and various concentration data, including chlorophyll-a. The nitrate source, its relationship to phytoplankton, and the effect of the nitrate source and cycling on the N isotopic composition of dissolved and particulate organic matter were the primary concerns of the study. The d15N values of dissolved organic nitrogen (DON) were used as a proxy for nitrate d15N because nitrate comprised about 90% of DON. Chlorophyll-a and C:N ratios indicated that the particulate organic matter (POM) consisted largely of plankton and therefore the d15N of POM was used as a proxy for the d15N of plankton. A tentative interpretation of the pilot study was that nitrate was a major nutrient for the plankton and the nitrate was of anthropogenic origin, possibly sewage or animal waste. To test these assumptions and interpretations, we are currently analyzing a set of samples collected in 2001. In addition to the previous sample types, a subset of samples will be measured directly for nitrate d15N to assess the validity of using d15N of DON as a proxy for nitrate.
Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contribu... more Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contributor to eutrophication of surface waters worldwide. In order to reduce N loads to surface waters, it is important to understand the relative contributions of major NOx sources to wet and dry deposition to watersheds. In the United States, the two largest NOx sources are vehicular emissions (54 percent) and stationary fuel combustion (40 percent). Reducing emissions from these sources is critical to improving air and surface water quality. However, using nitrate concentration data alone, it is difficult to establish relationships between individual NOx sources and wet deposition of nitrate. Previous research has shown that different NOx sources can have different isotopic compositions and can be used to identify NOx sources to wet deposition. To address this research need, we have completed the first national survey of nitrate isotopes in wet deposition using samples collected by the Natio...
Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contribu... more Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contributor to eutrophication of surface waters worldwide. In order to reduce N loads to surface waters, it is important to understand the relative contributions of major NOx sources to wet and dry deposition to watersheds. In the United States, the two largest NOx sources are vehicular emissions (54 percent) and stationary fuel combustion (40 percent). Reducing emissions from these sources is critical to improving air and surface water quality. However, using nitrate concentration data alone, it is difficult to establish relationships between individual NOx sources and wet deposition of nitrate. Previous research has shown that different NOx sources can have different isotopic compositions and can be used to identify NOx sources to wet deposition. To address this research need, we have completed the first national survey of nitrate isotopes in wet deposition using samples collected by the Natio...
Do different sources of atmospheric nitrate (power plants, vehicles, agricultural emissions) have... more Do different sources of atmospheric nitrate (power plants, vehicles, agricultural emissions) have distinctive isotopic signatures? To answer this question, we conducted a national survey of nitrate isotopes in wet deposition samples collected throughout the USA from 156 NADP sites. Archived samples from the year 2000 were pooled into bimonthly composites and analyzed for d15N and d18O, with a subset analyzed for D17O. In this presentation, we present our current thoughts about why the different sources have distinctive isotopic signatures, focusing mainly on oxygen isotopes of nitrate. Our original conceptual model was based largely on (1) Heaton's 1990 paper that showed that NOx emissions from power plants in South Africa had d15N values significantly higher than exhaust from vehicles, and (2) anecdotal data from several studies showing higher d15N and/or d18O values of nitrate in precipitation downwind of areas dominated by power plants. Our model proposed that atmospheric nit...
As part of a multi-agency study of alligator health, 28 American alligators (Alligator mississipp... more As part of a multi-agency study of alligator health, 28 American alligators (Alligator mississippiensis) were captured along a transect through the Florida Everglades in 1999. Liver and tail muscle tissues were sampled and analyzed on a wet weight basis for total mercury (THg) using cold-vapor atomic absorption spectrophotometry. All tissues had detectable concentrations of THg that ranged from 0.6 to 17 mg/kg in liver and from 0.1 to 1.8 mg/kg in tail muscle. THg was more concentrated in liver tissue than tail muscle, but levels were highly correlated between tissues. THg concentrations in tissue differed significantly among locations, with animals from Everglades National Park (ENP) having mean concentrations of THg in liver (10.4 mg/kg) and tail muscle (1.2 mg/kg) that were two-fold higher than basin-wide averages (4.9 and 0.64 mg/kg, respectively). The reasons for higher contamination of ENP alligators were unclear and could not be explained by differences in sex, length, weight...
A clear understanding of the aquatic food web is essential for determining the entry points and s... more A clear understanding of the aquatic food web is essential for determining the entry points and subsequent biomagnification pathways of contaminants such as methyl-mercury (MeHg) in the Everglades. Anthropogenic changes in nutrients can significantly affect the entry points of MeHg by changing food web structure from one dominated by algal productivity to one dominated by macrophytes and associated microbial activity. These changes in the base of the food web can also influence the distribution of animals within the ecosystem, and subsequently the bioaccumulation of MeHg up the food chain. As part of several collaborations with local and other federal agencies, more than 7000 Everglades samples were collected in 1995-99, and analysed for d13C and d15N. Many organisms were also analysed for d34S, gut contents, total Hg, and MeHg. Carbon isotopes effectively distinguish between two main types of food webs: ones where algae is the dominant base of the food web, which are characteristic...
Stable isotopes can be used to determine the relative trophic positions of biota within a food we... more Stable isotopes can be used to determine the relative trophic positions of biota within a food web, and to improve our understanding of the biomagnification of contaminants. Plants at the base of the food web uptake dissolved organic carbon (DIC) and nitrogen (DIN) for growth, and their tissue reflects the isotopic composition of these sources. Animals then mirror the isotopic composition of the primary producers, as modified by consumer-diet fractionations at successive trophic steps. During 1995-99, we collected algae, macrophyte, invertebrate, and fish samples from 15 USGS sites in the Everglades and analyzed them for d13C and d15N with the goal of characterizing seasonal and spatial differences in food web relations. Carbon isotopes effectively distinguish between two main types of food webs: ones where algae is the dominant base of the food web, which are characteristic of relatively pristine marsh sites with long hydroperiods, and ones where macrophyte debris appears to be a s...
A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable ... more A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable sequence of terminal electron acceptors based on the energetic yield for the reaction. It is thereby oftentimes assumed that microbial respiration of ferric iron outcompetes sulfate in all but high-sulfate systems, and thus sulfide has little influence on freshwater or terrestrial iron cycling. Observations of sulfate reduction in low-sulfate environments have been attributed to the presumed presence of highly crystalline iron oxides allowing sulfate reduction to be more energetically favored. Here we identified the iron-reducing processes under low-sulfate conditions within columns containing freshwater sediments amended with structurally diverse iron oxides and fermentation products that fuel anaerobic respiration. We show that despite low sulfate concentrations and regardless of iron oxide substrate (ferrihydrite, Al-ferrihydrite, goethite, hematite), sulfidization was a dominant path...
Iron (Fe) oxides exist in a spectrum of structures in the environment, with ferrihydrite widely c... more Iron (Fe) oxides exist in a spectrum of structures in the environment, with ferrihydrite widely considered the most bioavailable phase. Yet, ferrihydrite is unstable and rapidly transforms to more crystalline Fe(III) oxides (e.g., goethite, hematite), which are poorly reduced by model dissimilatory Fe(III)-reducing microorganisms. This begs the question, what processes and microbial groups are responsible for reduction of crystalline Fe(III) oxides within sedimentary environments? Further, how do changes in Fe mineralogy shape oxide-hosted microbial populations? To address these questions, we conducted a large-scale cultivation effort using various Fe(III) oxides (ferrihydrite, goethite, hematite) and carbon substrates (glucose, lactate, acetate) along a dilution gradient to enrich for microbial populations capable of reducing Fe oxides spanning a wide range of crystallinities and reduction potentials. While carbon source was the most important variable shaping community composition...
Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading... more Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading within the watershed. The main sources of N are thought to be in the form of nitrate from surface water runoff, agricultural fields, dairy farms, and urban runoff. More poorly quantified sources of nitrate may also include significant groundwater discharge as well as tidal inputs from Monterey Bay. Traditional studies, involving natural abundance stable isotopes within the N cycle, have been limited to delta 15 N and thus to a'one-dimensional' ...
Abstract As nitrate is utilized by phytoplankton in the euphotic zone, the isotopic compositions ... more Abstract As nitrate is utilized by phytoplankton in the euphotic zone, the isotopic compositions of both oxygen and nitrogen in the residual nitrate become enriched in the heavier isotopes (15 N and 18 O). Recent evidence from phytoplankton culture experiments showed that the enrichments in 15 N and 18 O occur intracellularly during the reduction of nitrate to nitrite. Furthermore, the isotopic composition of the external nitrate pool reflects the balance between intracellular reduction and efflux of the intracellular nitrate. It has also ...
Nitrate isotopes in wet deposition are useful indicators of NOx source contributions to nitrate f... more Nitrate isotopes in wet deposition are useful indicators of NOx source contributions to nitrate formation and NOx oxidation pathways at local and regional scales. Here, we examine whether nitrogen and oxygen isotopes (d15N and d18O, respectively) provide similarly useful information in: 1) wet deposition at the continental scale; and 2) dry deposition at the regional scale. For wet deposition analyses,
Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading... more Abstract Elkhorn Slough is a tidally flushed estuary with exceptionally high amounts of N loading within the watershed. The main sources of N are thought to be in the form of nitrate from surface water runoff, agricultural fields, dairy farms, and urban runoff. More poorly quantified sources of nitrate may also include significant groundwater discharge as well as tidal inputs from Monterey Bay. Traditional studies, involving natural abundance stable isotopes within the N cycle, have been limited to delta 15 N and thus to a'one-dimensional' ...
Chemoautotrophic symbionts of deep sea hydrothermal vent tubeworms are known to provide their hos... more Chemoautotrophic symbionts of deep sea hydrothermal vent tubeworms are known to provide their hosts with all their primary nutrition. While studies have examined how chemoautotrophic symbionts provide the association with nitrogen, fewer have examined if symbiont nitrogen metabolism varies as a function of environmental conditions. Ridgeia piscesae tubeworms flourish at Northeastern Pacific vents, occupy a range of microhabitats, and exhibit a high degree of morphological plasticity [e.g. long-skinny (LS) and short-fat (SF) phenotypes] that may relate to environmental conditions. This plasticity affords an opportunity to examine whether symbiont nitrogen metabolism varies among host phenotypes. LS and SF R. piscesae were recovered from the Axial and Main Endeavour Field hydrothermal vents. Nitrate and ammonium were quantified in Ridgeia blood, and the expression of key nitrogen metabolism genes, as well as stable nitrogen isotope ratios, was quantified in host branchial plume and symbiont-containing tissues. Nitrate and ammonium were abundant in the blood of both phenotypes though environmental ammonium concentrations were, paradoxically, lowest among individuals with the highest blood ammonium. Assimilatory nitrate reductase transcripts were always below detection, though in both LS and SF R. piscesae symbionts, we observed elevated expression of dissimilatory nitrate reductase genes, as well as symbiont and host ammonium assimilation genes. Site-specific differences in expression, along with tissue stable isotope analyses, suggest that LS and SF Ridgeia symbionts are engaged in both dissimilatory nitrate reduction and ammonia assimilation to varying degrees. As such, it appears that environmental conditions -not host phenotype-primarily dictates symbiont nitrogen metabolism.
Deep-sea biogeochemical cycles are, in general, poorly understood owing to the difficulties of ma... more Deep-sea biogeochemical cycles are, in general, poorly understood owing to the difficulties of making measurements in situ, recovering samples with minimal perturbation, and, in many cases, coping with high spatial and temporal heterogeneity. In particular, biogeochemical fluxes of volatiles such as methane remain largely unconstrained because of the difficulties with accurate quantification in situ and the patchiness of point sources such as seeps and brine pools. To better constrain biogeochemical fluxes and cycling, we have developed a deep-sea in situ mass spectrometer (ISMS) to enable high-resolution quantification of volatiles in situ. Here we report direct measurements of methane concentrations made in a Gulf of Mexico brine pool located at a depth of over 2300 m. Concentrations of up to 33 mM methane were observed within the brine pool, whereas concentrations in the water directly above were three orders of magnitude lower. These direct measurements enabled us to make the first accurate estimates of the diffusive flux from a brine pool, calculated to be 1.1 7 0.2 mol m À 2 yr À 1. Integrated rate measurements of aerobic methane oxidation in the water column overlying the brine pool were $ 320 mmol m À 2 yr À 1 , accounting at most for just 0.03% of the diffusive methane flux from the brine pool. Calculated rates of anaerobic methane oxidation were 600–1200 mM yr À 1 , one to two orders of magnitude higher than previously published values of AOM in anoxic fluids. These findings suggest that brine pools are enormous point sources of methane in the deep sea, and may, in aggregate, have a pronounced impact on the global marine methane cycle.
Fish migration through the deep-water channel in the San Joaquin River at Stockton, California is... more Fish migration through the deep-water channel in the San Joaquin River at Stockton, California is inhibited by low oxygen concentrations during the summer months. The cause for this condition appears to be stagnation and decomposition of algae with attendant oxygen consumption. Algae growth in the San Joaquin River is promoted by nutrients entering the river mainly in the form of nitrate. Possible significant sources of nitrate include soil, fertilizer from agriculture, manure from dairy operations, and N derived from municipal sewage. A 2000 CALFED pilot study investigated the sources and cycling of nitrate at four sites along the San Joaquin River upstream of Stockton using the carbon and nitrogen isotopes of total dissolved and particulate organic matter, together with hydrological measurements and various concentration data, including chlorophyll-a. The nitrate source, its relationship to phytoplankton, and the effect of the nitrate source and cycling on the N isotopic composition of dissolved and particulate organic matter were the primary concerns of the study. The d15N values of dissolved organic nitrogen (DON) were used as a proxy for nitrate d15N because nitrate comprised about 90% of DON. Chlorophyll-a and C:N ratios indicated that the particulate organic matter (POM) consisted largely of plankton and therefore the d15N of POM was used as a proxy for the d15N of plankton. A tentative interpretation of the pilot study was that nitrate was a major nutrient for the plankton and the nitrate was of anthropogenic origin, possibly sewage or animal waste. To test these assumptions and interpretations, we are currently analyzing a set of samples collected in 2001. In addition to the previous sample types, a subset of samples will be measured directly for nitrate d15N to assess the validity of using d15N of DON as a proxy for nitrate.
Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contribu... more Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contributor to eutrophication of surface waters worldwide. In order to reduce N loads to surface waters, it is important to understand the relative contributions of major NOx sources to wet and dry deposition to watersheds. In the United States, the two largest NOx sources are vehicular emissions (54 percent) and stationary fuel combustion (40 percent). Reducing emissions from these sources is critical to improving air and surface water quality. However, using nitrate concentration data alone, it is difficult to establish relationships between individual NOx sources and wet deposition of nitrate. Previous research has shown that different NOx sources can have different isotopic compositions and can be used to identify NOx sources to wet deposition. To address this research need, we have completed the first national survey of nitrate isotopes in wet deposition using samples collected by the Natio...
Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contribu... more Atmospheric deposition is a major source of nitrate exported to coastal waters and a key contributor to eutrophication of surface waters worldwide. In order to reduce N loads to surface waters, it is important to understand the relative contributions of major NOx sources to wet and dry deposition to watersheds. In the United States, the two largest NOx sources are vehicular emissions (54 percent) and stationary fuel combustion (40 percent). Reducing emissions from these sources is critical to improving air and surface water quality. However, using nitrate concentration data alone, it is difficult to establish relationships between individual NOx sources and wet deposition of nitrate. Previous research has shown that different NOx sources can have different isotopic compositions and can be used to identify NOx sources to wet deposition. To address this research need, we have completed the first national survey of nitrate isotopes in wet deposition using samples collected by the Natio...
Do different sources of atmospheric nitrate (power plants, vehicles, agricultural emissions) have... more Do different sources of atmospheric nitrate (power plants, vehicles, agricultural emissions) have distinctive isotopic signatures? To answer this question, we conducted a national survey of nitrate isotopes in wet deposition samples collected throughout the USA from 156 NADP sites. Archived samples from the year 2000 were pooled into bimonthly composites and analyzed for d15N and d18O, with a subset analyzed for D17O. In this presentation, we present our current thoughts about why the different sources have distinctive isotopic signatures, focusing mainly on oxygen isotopes of nitrate. Our original conceptual model was based largely on (1) Heaton's 1990 paper that showed that NOx emissions from power plants in South Africa had d15N values significantly higher than exhaust from vehicles, and (2) anecdotal data from several studies showing higher d15N and/or d18O values of nitrate in precipitation downwind of areas dominated by power plants. Our model proposed that atmospheric nit...
As part of a multi-agency study of alligator health, 28 American alligators (Alligator mississipp... more As part of a multi-agency study of alligator health, 28 American alligators (Alligator mississippiensis) were captured along a transect through the Florida Everglades in 1999. Liver and tail muscle tissues were sampled and analyzed on a wet weight basis for total mercury (THg) using cold-vapor atomic absorption spectrophotometry. All tissues had detectable concentrations of THg that ranged from 0.6 to 17 mg/kg in liver and from 0.1 to 1.8 mg/kg in tail muscle. THg was more concentrated in liver tissue than tail muscle, but levels were highly correlated between tissues. THg concentrations in tissue differed significantly among locations, with animals from Everglades National Park (ENP) having mean concentrations of THg in liver (10.4 mg/kg) and tail muscle (1.2 mg/kg) that were two-fold higher than basin-wide averages (4.9 and 0.64 mg/kg, respectively). The reasons for higher contamination of ENP alligators were unclear and could not be explained by differences in sex, length, weight...
A clear understanding of the aquatic food web is essential for determining the entry points and s... more A clear understanding of the aquatic food web is essential for determining the entry points and subsequent biomagnification pathways of contaminants such as methyl-mercury (MeHg) in the Everglades. Anthropogenic changes in nutrients can significantly affect the entry points of MeHg by changing food web structure from one dominated by algal productivity to one dominated by macrophytes and associated microbial activity. These changes in the base of the food web can also influence the distribution of animals within the ecosystem, and subsequently the bioaccumulation of MeHg up the food chain. As part of several collaborations with local and other federal agencies, more than 7000 Everglades samples were collected in 1995-99, and analysed for d13C and d15N. Many organisms were also analysed for d34S, gut contents, total Hg, and MeHg. Carbon isotopes effectively distinguish between two main types of food webs: ones where algae is the dominant base of the food web, which are characteristic...
Stable isotopes can be used to determine the relative trophic positions of biota within a food we... more Stable isotopes can be used to determine the relative trophic positions of biota within a food web, and to improve our understanding of the biomagnification of contaminants. Plants at the base of the food web uptake dissolved organic carbon (DIC) and nitrogen (DIN) for growth, and their tissue reflects the isotopic composition of these sources. Animals then mirror the isotopic composition of the primary producers, as modified by consumer-diet fractionations at successive trophic steps. During 1995-99, we collected algae, macrophyte, invertebrate, and fish samples from 15 USGS sites in the Everglades and analyzed them for d13C and d15N with the goal of characterizing seasonal and spatial differences in food web relations. Carbon isotopes effectively distinguish between two main types of food webs: ones where algae is the dominant base of the food web, which are characteristic of relatively pristine marsh sites with long hydroperiods, and ones where macrophyte debris appears to be a s...
A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable ... more A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable sequence of terminal electron acceptors based on the energetic yield for the reaction. It is thereby oftentimes assumed that microbial respiration of ferric iron outcompetes sulfate in all but high-sulfate systems, and thus sulfide has little influence on freshwater or terrestrial iron cycling. Observations of sulfate reduction in low-sulfate environments have been attributed to the presumed presence of highly crystalline iron oxides allowing sulfate reduction to be more energetically favored. Here we identified the iron-reducing processes under low-sulfate conditions within columns containing freshwater sediments amended with structurally diverse iron oxides and fermentation products that fuel anaerobic respiration. We show that despite low sulfate concentrations and regardless of iron oxide substrate (ferrihydrite, Al-ferrihydrite, goethite, hematite), sulfidization was a dominant path...
Iron (Fe) oxides exist in a spectrum of structures in the environment, with ferrihydrite widely c... more Iron (Fe) oxides exist in a spectrum of structures in the environment, with ferrihydrite widely considered the most bioavailable phase. Yet, ferrihydrite is unstable and rapidly transforms to more crystalline Fe(III) oxides (e.g., goethite, hematite), which are poorly reduced by model dissimilatory Fe(III)-reducing microorganisms. This begs the question, what processes and microbial groups are responsible for reduction of crystalline Fe(III) oxides within sedimentary environments? Further, how do changes in Fe mineralogy shape oxide-hosted microbial populations? To address these questions, we conducted a large-scale cultivation effort using various Fe(III) oxides (ferrihydrite, goethite, hematite) and carbon substrates (glucose, lactate, acetate) along a dilution gradient to enrich for microbial populations capable of reducing Fe oxides spanning a wide range of crystallinities and reduction potentials. While carbon source was the most important variable shaping community composition...
Uploads