Reduction of sulfur compounds in the sediments of a eutrophic lake basin

Using stable isotopes to discriminate anthropogenic impacts of the sedimentary organic matter pollution in the Rodrigo de Freitas Lagoon (RJ, Brazil)

Over the last decades, the Rodrigo de Freitas Lagoon (RFL), Rio de Janeiro, Brazil, has been impacted by the release of untreated domestic sewage, causing eutrophication processes with negative effects on its biota. Recently, the RFL underwent urban interventions to fulfill the demands of the 2016 Olympic Games, which included building the waist gallery and monitoring clandestine waste discharges into the underground drainage network. Organic-source tracing methods can be successfully used to characterize the organic matter transported from the urbanized areas to the RLF. The application of the elemental (C, N) and stable isotope (δ¹⁵N and δ¹³C) fingerprint methods in sediments from the RLF indicated a reduction in the domestic sewage inputs from 32 ± 16 to 12 ± 13% between 2015 and 2017. However, the sewage inputs continue being worrying. Our results also suggest that the main source of organic matter pollution in the lagoon comes from indiscriminate domestic sewage release from river channels. Secondary pollution sources are associated with the underground drainage network that still shows punctual and irregular releases of domestic sewage. Petroleum products, mainly from sewers, also show as possible organic pollution sources. Finally, the findings indicate that the interventions carried out in the RFL are promising. However, they were insufficient to cease the pollutant inputs and mitigate the negative impacts of eutrophication.

Increasing sulfate concentrations result in higher sulfide production and phosphorous mobilization in a shallow eutrophic freshwater lake

Increasing sulfate input has been seen as an issue in management of aquatic ecosystems, but its influences on eutrophic freshwater lakes is not clear. In this study, it was observed that increasing sulfate concentration without additional cyanobacterial bloom biomass (CBB) addition did not have an obvious effect on element cycling during 1-year continuous flow mesocosm experiments in which water and sediments were taken from a shallow eutrophic lake with sulfate levels near 1 mM. However, following addition of CBB to mesocosms, sulfate-reducing bacteria (SRB) were observed in the water column, and increasing numbers of SRB in the water column were associated with higher sulfate input. Sulfate amendment (0-70 mg L(-1)) also resulted in a larger amount of total dissolved sulfide (peak values of 5.90 ± 0.36 to 7.60 ± 0.12 mg L(-1)) in the water column and acid volatile sulfide (1081.71 ± 69.91 to 1557.98 ± 41.72 mg kg(-1)) in 0-1 cm surface sediments due to sulfate reduction. During the period of CBB decomposition, increasing sulfate levels in the water column were positively correlated with increasing diffusive phosphate fluxes of 1.23 ± 0.32 to 2.17 ± 0.01 mg m(-2) d(-1) at the water-sediment interface. As increases in sulfide and phosphate release rates deteriorated the water quality/ecosystem and even spurred the occurrence of a black water problem in lakes, the control of sulfate input level should be considered for shallow eutrophic lake management, especially during cyanobacterial bloom periods.

Kinetics of sulfate reduction and sulfide precipitation rates in sediments of a bar-built estuary (Pescadero, California)

The bar-built Pescadero Estuary in Northern California is a major fish rearing habitat, though recently threatened by near-annual fish kill events, which occur when the estuary transitions from closed to open state. The direct and indirect effects of hydrogen sulfide are suspected to play a role in these mortalities, but the spatial variability of hydrogen sulfide production and its link to fish kills remains poorly understood. Using flow-through reactors containing intact littoral sediment slices, we measured potential sulfate reduction rates, kinetic parameters of microbial sulfate reduction (Rmax, the maximum sulfate reduction rate, and Km, the half-saturation constant for sulfate), potential sulfide precipitation rates, and potential hydrogen sulfide export rates to water at four sites in the closed and open states. At all sites, the Michaelis-Menten kinetic rate equation adequately describes the utilization of sulfate by the complex resident microbial communities. We estimate that 94-96% of hydrogen sulfide produced through sulfate reduction precipitates in the sediment and that only 4-6% is exported to water, suggesting that elevated sulfide concentrations in water, which would affect fish through toxicity and oxygen consumption, cannot be responsible for fish deaths. However, the indirect effects of sulfide precipitates, which chemically deplete, contaminate, and acidify the water column during sediment re-suspension and re-oxidation in the transition from closed to open state, can be implicated in fish mortalities at Pescadero Estuary.

Natural stressors in uncontaminated sediments of shallow freshwaters: the prevalence of sulfide, ammonia, and reduced iron

Potentially toxic levels of 3 naturally occurring chemical stressors (dissolved sulfide, ammonia, and iron) can appear in freshwater sediments, although their roles in shaping ecosystem structure (i.e., plant and animal communities) and function (e.g., biologically mediated elemental cycles) have received little study. The present critical review discusses the prevalence and ecological effects of potentially toxic concentrations of sulfide, ammonia, and iron in uncontaminated freshwater sediments, including a review of the literature as well as a case study presenting previously unpublished data on sediment porewaters from a diverse set of shallow (<2 m) freshwater ecosystems in southwest Michigan, USA. Measured concentrations are compared with surface water quality criteria established by the US Environmental Protection Agency (USEPA) and with acute and chronic toxic thresholds in the published literature, where available. Based on USEPA criteria for aquatic life for these 3 stressors, the benthic environment of almost every freshwater ecosystem sampled was theoretically stressful to some component of aquatic life in some area or at some time (i.e., in at least 1 sample), and 54% of samples exceeded more than 1 criterion simultaneously. Organismal tolerances to chemical stressors vary, so the observed concentrations are likely shaping benthic animal communities and influencing rates of ecosystem processes. Consideration of the role of natural chemical stressors is important in shaping freshwater benthic environments and in developing bioassessments, restoration goals, and remediation plans. Environ Toxicol Chem 2015;34:467-479. © 2014 SETAC.

Effect of hydrogen sulfide on phosphorus lability in lake sediments amended with drinking water treatment residuals

The use of drinking water treatment residuals (WTRs) to immobilize P in sediments is a novel approach for lake restoration. However, the lability of P in WTRs-amended sediments may vary with many factors, e.g., hydrogen sulfide content. Earlier works in our laboratory have demonstrated that WTRs are effective sorbents for hydrogen sulfide in water. Thus, we hypothesized that the lability of P in WTRs-amended sediments would not be increased by hydrogen sulfide. The results of this work suggested that this hypothesis was tenable. Compared to the raw sediments, the amended sediments had significantly lower P desorption potential in the presence of hydrogen sulfide at different times, pH and concentrations. Moreover, the amended sediments were also better able to adsorb hydrogen sulfide. In the amended sediments, the P, which was easily desorbed due to the effect of hydrogen sulfide, was transformed into the Fe/Al bound P.

Sulfate-reducing microorganisms in wetlands - fameless actors in carbon cycling and climate change

Freshwater wetlands are a major source of the greenhouse gas methane but at the same time can function as carbon sink. Their response to global warming and environmental pollution is one of the largest unknowns in the upcoming decades to centuries. In this review, we highlight the role of sulfate-reducing microorganisms (SRM) in the intertwined element cycles of wetlands. Although regarded primarily as methanogenic environments, biogeochemical studies have revealed a previously hidden sulfur cycle in wetlands that can sustain rapid renewal of the small standing pools of sulfate. Thus, dissimilatory sulfate reduction, which frequently occurs at rates comparable to marine surface sediments, can contribute up to 36–50% to anaerobic carbon mineralization in these ecosystems. Since sulfate reduction is thermodynamically favored relative to fermentative processes and methanogenesis, it effectively decreases gross methane production thereby mitigating the flux of methane to the atmosphere. However, very little is known about wetland SRM. Molecular analyses using dsrAB [encoding subunit A and B of the dissimilatory (bi)sulfite reductase] as marker genes demonstrated that members of novel phylogenetic lineages, which are unrelated to recognized SRM, dominate dsrAB richness and, if tested, are also abundant among the dsrAB-containing wetland microbiota. These discoveries point toward the existence of so far unknown SRM that are an important part of the autochthonous wetland microbiota. In addition to these numerically dominant microorganisms, a recent stable isotope probing study of SRM in a German peatland indicated that rare biosphere members might be highly active in situ and have a considerable stake in wetland sulfate reduction. The hidden sulfur cycle in wetlands and the fact that wetland SRM are not well represented by described SRM species explains their so far neglected role as important actors in carbon cycling and climate change.

Inhibition of hydrogen sulfide generation from disposed gypsum drywall using chemical inhibitors

Disposal of gypsum drywall in landfills has been demonstrated to elevate hydrogen sulfide (H(2)S) concentrations in landfill gas, a problem with respect to odor, worker safety, and deleterious effect on gas-to-energy systems. Since H(2)S production in landfills results from biological activity, the concept of inhibiting H(2)S production through the application of chemical agents to drywall during disposal was studied. Three possible inhibition agents - sodium molybdate (Na(2)MoO(4)), ferric chloride (FeCl(3)), and hydrated lime (Ca(OH)(2)) - were evaluated using flask and column experiments. All three agents inhibited H(2)S generation, with Na(2)MoO(4) reducing H(2)S generation by interrupting the biological sulfate reduction process and Ca(OH)(2) providing an unfavorable pH for biological growth. Although FeCl(3) was intended to provide an electron acceptor for a competing group of bacteria, the mechanism found responsible for inhibiting H(2)S production in the column experiment was a reduction in pH. Application of both Na(2)MoO(4) and FeCl(3) inhibited H(2)S generation over a long period (over 180 days), but the impact of Ca(OH)(2) decreased with time as the alkalinity it contributed was neutralized by the generated H(2)S. Practical application and potential environmental implications need additional exploration.

Microbial physiology-based model of ethanol metabolism in subsurface sediments

A biogeochemical reaction model was developed based on microbial physiology to simulate ethanol metabolism and its influence on the chemistry of anoxic subsurface environments. The model accounts for potential microbial metabolisms that degrade ethanol, including those that oxidize ethanol directly or syntrophically by reducing different electron acceptors. Out of the potential metabolisms, those that are active in the environment can be inferred by fitting the model to experimental observations. This approach was applied to a batch sediment slurry experiment that examined ethanol metabolism in uranium-contaminated aquifer sediments from Area 2 at the U.S. Department of Energy Field Research Center in Oak Ridge, TN. According to the simulation results, complete ethanol oxidation by denitrification, incomplete ethanol oxidation by ferric iron reduction, ethanol fermentation to acetate and H(2), hydrogenotrophic sulfate reduction, and acetoclastic methanogenesis: all contributed significantly to the degradation of ethanol in the aquifer sediments. The assemblage of the active metabolisms provides a frame work to explore how ethanol amendment impacts the chemistry of the environment, including the occurrence and levels of uranium. The results can also be applied to explore how diverse microbial metabolisms impact the progress and efficacy of bioremediation strategies.

Comparative aspects of sulfur mineralization in sediments of a eutrophic lake basin

The net mineralization of organic sulfur compounds in surface sediments of Wintergreen Lake was estimated from a mass-balance budget of sulfur inputs and sediment sulfur concentrations. The net mineralization of organic sulfur inputs is <50% complete, which is consistent with the dominance of organic sulfur (>80% of total sulfur) in sediment. Although sediment sulfur is predominantly organic, sulfate reduction is the most significant process in terms of the quantities of sulfur transformed in surface sediments. Rates of sulfate reduction in these sediments average 7 mmol/m per day. On an annual basis, this rate is 19-fold greater than net rates of organic sulfur mineralization and 65-fold greater than sulfate ester hydrolysis.

Kinetic analysis of competition between sulfate reducers and methanogens for hydrogen in sediments

The competition between sulfate-reducing and methanogenic bacteria for hydrogen was investigated in eutrophic lake sediments that contained low in situ sulfate concentrations and in sulfate-amended sediments. Sulfate reduction and methane production coexisted in situ in lake surface sediments (0 to 2 cm), but methane production was the dominant terminal process. Addition of 10 to 20 mM sulfate to sediments resulted in a decrease in the hydrogen partial pressure and a concomitant inhibition of methane production over time. Molybdate inhibition of sulfate reduction in sulfate-amended sediments was followed by an increase in the hydrogen partial pressure and the methane production rate to values comparable to those in sediments not amended with sulfate. The sulfate reducer population had a half-saturation constant for hydrogen uptake of 141 pascals versus 597 pascals for the methanogen population. Thus, when sulfate was not limiting, the lower half-saturation constant of sulfate reducers enabled them to inhibit methane production by lowering the hydrogen partial pressure below levels that methanogens could effectively utilize. However, methanogens coexisted with sulfate reducers in the presence of sulfate, and the outcome of competition at any time was a function of the rate of hydrogen production, the relative population sizes, and sulfate availability.

Intermediary metabolism of organic matter in the sediments of a eutrophic lake

The rates, products, and controls of the metabolism of fermentation intermediates in the sediments of a eutrophic lake were examined. C-fatty acids were directly injected into sediment subcores for turnover rate measurements. The highest rates of acetate turnover were in surface sediments (0- to 2-cm depth). Methane was the dominant product of acetate metabolism at all depths. Simultaneous measurements of acetate, propionate, and lactate turnover in surface sediments gave turnover rates of 159, 20, and 3 muM/h, respectively. [2-C]propionate and [U-C]lactate were metabolized to [C]acetate, CO(2), and CH(4). [C]formate was completely converted to CO(2) in less than 1 min. Inhibition of methanogenesis with chloroform resulted in an immediate accumulation of volatile fatty acids and hydrogen. Hydrogen inhibited the metabolism of C(3)-C(5) volatile fatty acids. The rates of fatty acid production were estimated from the rates of fatty acid accumulation in the presence of chloroform or hydrogen. The mean molar rates of production were acetate, 82%; propionate, 13%; butyrates, 2%; and valerates, 3%. A working model for carbon and electron flow is presented which illustrates that fermentation and methanogenesis are the predominate steps in carbon flow and that there is a close interaction between fermentative bacteria, acetogenic hydrogen-producing bacteria, and methanogens.

Anaerobic oxalate degradation: widespread natural occurrence in aquatic sediments

Significant concentrations of oxalate (dissolved plus particulate) were present in sediments taken from a diversity of aquatic environments, ranging from 0.1 to 0.7 mmol/liter of sediment. These included pelagic and littoral sediments from two freshwater lakes (Searsville Lake, Calif., and Lake Tahoe, Calif.), a hypersaline, meromictic, alkaline lake (Big Soda Lake, Nev.), and a South San Francisco Bay mud flat and salt marsh. The oxalate concentration of several plant species which are potential detrital inputs to these aquatic sediments ranged from 0.1 to 5.0% (wt/wt). In experiments with litter bags, the oxalate content of Myriophyllum sp. samples buried in freshwater littoral sediments decreased to 7% of the original value in 175 days. This suggests that plant detritus is a potential source of the oxalate within these sediments. [C]oxalic acid was anaerobically degraded to CO(2) in all sediment types tested, with higher rates evident in littoral sediments than in the pelagic sediments of the lakes studied. The turnover time of the added [C]oxalate was less than 1 day in Searsville Lake littoral sediments. The total sediment oxalate concentration did not vary significantly between littoral and pelagic sediments and therefore did not appear to be controlling the rate of oxalate degradation. However, depth profiles of [C]oxalate mineralization and dissolved oxalate concentration were closely correlated in freshwater littoral sediments; both were greatest in the surface sediments (0 to 5 cm) and decreased with depth. The dissolved oxalate concentration (9.1 mumol/liter of sediment) was only 3% of the total extractable oxalate (277 mumol/liter of sediment) at the sediment surface. These results suggest that anaerobic oxalate degradation is a widespread phenomenon in aquatic sediments and may be limited by the dissolved oxalate concentration within these sediments.

Sulfate reducers can outcompete methanogens at freshwater sulfate concentrations

Acetate and hydrogen metabolism by sulfate reducers and methanogens in the profundal sediments of an oligotrophic lake were examined. Inhibition of sulfate reduction with molybdate stimulated methane production from both hydrogen and acetate. Molybdate did not stimulate methane production in sediments that were preincubated to deplete the sulfate pool. Sulfate reduction accounted for 30 to 81% of the total of terminal metabolism proceeding through sulfate reduction and methane production in Eckman grab samples of surface sediments. The ability of sulfate reducers to effectively compete with methanogens for acetate was related to the sulfate reducers' lower half-saturation constant for acetate metabolism at in situ sulfate concentrations. Processes other than sulfate reduction and methanogenesis consumed hydrogen at elevated hydrogen partial pressures and prevented a kinetic analysis of hydrogen uptake by sulfate reducers and methanogens. The demonstration that sulfate reducers can successfully compete with methanogens for hydrogen and acetate in sediments at in situ sulfate concentrations of 60 to 105 muM extends the known range of sediment habitats in which sulfate reduction can be a dominant terminal process.

Glucose metabolism in sediments of a eutrophic lake: tracer analysis of uptake and product formation

The uptake of glucose and the formation of end products from glucose catabolism have been measured for sediments of eutrophic Wintergreen Lake with a combination of tritiated and C-labeled tracers. Time course analyses of the loss of [H]glucose from sediments were used to establish rate constants for glucose uptake at natural substrate concentrations. Turnover times from these analyses were about 1 min for littoral and profundal sediments. No seasonal or site differences were noted in turnover times. Time course analyses of [U-C]glucose uptake and C-labeled end product formation indicated that glucose mass flow could not be calculated from end product formation since the specific activity of added [C]glucose was significantly diluted by pools of intracellular glucose and glucose metabolites. Mass flow could only be accurately estimated by use of rates of uptake from tracer studies. Intermediate fermentation end products included acetate (71%), propionate (15%), lactate (9%), and only minor amounts of butyrates or valerates. Addition of H(2) to sediments resulted in greater production of lactate (28%) and decreased formation of acetate (50%), but did not affect glucose turnover. Depth profiles of glucose uptake indicated that rates of uptake decreased with depth over the 0- to 18-cm interval and that glucose uptake accounted for 30 to 40% of methanogenesis in profundal sediments.

Bacterial disproportionation of elemental sulfur coupled to chemical reduction of iron or manganese

A new chemolithotrophic bacterial metabolism was discovered in anaerobic marine enrichment cultures. Cultures in defined medium with elemental sulfur (S) and amorphous ferric hydroxide (FeOOH) as sole substrates showed intense formation of sulfate. Furthermore, precipitation of ferrous sulfide and pyrite was observed. The transformations were accompanied by growth of slightly curved, rod-shaped bacteria. The quantification of the products revealed that S was microbially disproportionated to sulfate and sulfide, as follows: 4S + 4H(2)O --> SO(4) + 3H(2)S + 2H. Subsequent chemical reactions between the formed sulfide and the added FeOOH led to the observed precipitation of iron sulfides. Sulfate and iron sulfides were also produced when FeOOH was replaced by FeCO(3). Further enrichment with manganese oxide, MnO(2), instead of FeOOH yielded stable cultures which formed sulfate during concomitant reduction of MnO(2) to Mn. Growth of small rod-shaped bacteria was observed. When incubated without MnO(2), the culture did not grow but produced small amounts of SO(4) and H(2)S at a ratio of 1:3, indicating again a disproportionation of S. The observed microbial disproportionation of S only proceeds significantly in the presence of sulfide-scavenging agents such as iron and manganese compounds. The population density of bacteria capable of S disproportionation in the presence of FeOOH or MnO(2) was high, > 10 cm in coastal sediments. The metabolism offers an explanation for recent observations of anaerobic sulfide oxidation to sulfate in anoxic sediments.

Effect of salinity on mercury-methylating activity of sulfate-reducing bacteria in estuarine sediments

The biomethylation of mercury was measured in anoxic estuarine sediments that ranged in salinity from 0.03 to 2.4% with or without added molybdate, an inhibitor of sulfate reducers. Mercury methylation was inhibited by molybdate by more than 95%, regardless of sediment salinity. In the absence of inhibitor, high-salinity sediments methylated mercury at only 40% of the level observed in low-salinity sediments. In response to molybdate inhibition of sulfate reducers, methanogenesis increased up to 258% in high-salinity sediments but only up to 25% in low-salinity sediments. In contrast to an earlier low-salinity isolate, a Desulfovibrio desulfuricans strain from high-salinity sediment required 0.5 M sodium for optimal growth and mercury methylation activity. The formation of negatively charged mercuric chloride complexes at high salinity did not noticeably interfere with the methylation process. Results of these studies demonstrate that sulfate reducers are responsible for mercury methylation in anoxic estuarine sediments, regardless of the prevailing salinity.

Electron donors utilized by sulfate-reducing bacteria in eutrophic lake sediments

Mineralization rates of C-labeled substrates were determined in the presence and absence of Na(2)MoO(4), an inhibitor of sulfate reduction, in the profundal sediments of a shallow eutrophic lake. Sulfate reduction was inhibited by Na(2)MoO(4) at all concentrations tested (0.2 to 200 mM), whereas methane production was inhibited at Na(2)MoO(4) concentrations greater than 20 mM. Initial mineralization rates of glucose were unaffected by Na(2)MoO(4); however, Na(2)MoO(4) decreased the mineralization rates of lactate (58%), propionate (52%), an amino acid mixture (85%), and acetate (14%). These decreases in the rates of mineralization were attributed to inhibition of sulfate reduction. Hydrogen stimulated the reduction of SO(4) 2.5- to 2.8-fold, demonstrating potential hydrogen oxidation by sulfate-reducing bacteria. These results indicate that sulfate reducers utilize an array of substrates as electron donors and are of potential significance to the in situ mineralization of lactate, propionate, and free amino acids in these sediments.

Ecology of sulfate-reducing bacteria in an iron-dominated, mining-impacted freshwater sediment

A legacy of lead and silver mining in its headwaters left Lake Coeur d'Alene, Idaho with a sediment body that is highly reduced and contains up to 100 g kg(-1) iron and a smaller fraction of chemically active sulfide phases. The dynamic character of these sulfides and their importance for the sequestering of contaminating trace elements prompted this study of the sulfate-reducing bacteria (SRB) involved in their production. We estimated parameters indicative of the distribution and activity of SRB in relation to season, site, and depth. Most probable number estimates and quantitative PCR assays of an SRB-specific functional gene, alpha-adenosine 5'-phosphosulfate reductase, indicated 10(3) to 10(6) cultivable cells and 10(5) to 10(7) gene copy numbers g(-1) dry wt sediment, respectively. Although culture-based estimates of SRB abundance correlated poorly with site, season, depth, total S, or pore water SO(4), non-culture-based estimates of SRB abundance were markedly higher at contaminated sites and positively correlated with pore water SO(4). Ex situ estimates of (35)SO(4) respiration and acid volatile sulfides abundance also showed strong among-site effects, indicating elevated sulfidogenesis at contaminated sites. These observations support the view that biogenic sulfides may act in concert with reduced iron to retain soluble metal(loid)s in the solid phase.

Natural alkalinity generation in neutral lakes affected by acid mine drainage

Lakes in surface mining areas are often subject to continuous loads of acid mine drainage. The knowledge of internal alkalinity generation in a lake is necessary to predict if the lake will stay circumneutral or may acidify. The most important processes of alkalinity production in lakes are sulfate reduction, denitrification, and the burial of N in the sediment. By summarizing data from the literature, we present probable rates of these different processes in circumneutral mining lakes. The critical acidity load that can probably be compensated for by internal processes, is 5.09 mmol(-) m(-2) d(-1) in productive lakes and 0.50 mmol(-) m(-2) d(-1) in less productive lakes. Under the assumption that methanogenesis is inhibited by high sulfate concentrations, the highest probable acidity loads in such lakes are 6.85 mmol(-) m(-2) d(-1) and 1.06 mmol(-) m(-2) d(-1), respectively. Denitrification, sulfate reduction, and N burial contributed significantly to total alkalinity production. Sulfate reduction had the largest potential. However, existing models cannot predict alkalinity generation from sulfate concentrations alone because the long-term stability of reduced S compounds in the sediment is crucial for a sustainable biological alkalinity generation. The larger acid-neutralizing potential of higher trophic lakes is caused both by higher rates of microbial activity and by a greater stability of reduced reaction products in the sediment. The largest uncertainties in our knowledge with respect to the total alkalinity budget are related to microbial processes in sulfate-rich freshwater lakes and the long-term stability of reduced reaction products in the sediment.

A review of acidity generation and consumption in acidic coal mine lakes and their watersheds

Lakes developing in former coal mine pits are often characterized by high concentrations of sulfate and iron and low pH. The review focuses on the causes for and fate of acidity in these lakes and their watersheds. Acidification is primarily caused by the generation of ferrous iron bearing and mineralized groundwater, transport through the groundwater-surface water interface, and subsequent iron oxidation and precipitation. Rates of acidity generation in mine tailings and dumps, and surface water are often similar (1 to >10 mol m(-2) yr(-1)). Weathering processes, however, often suffice to buffer groundwaters to only moderately acidic or neutral pH, depending on the suite of minerals present. In mine lakes, the acidity balance is further influenced by proton release from transformation of metastable iron hydroxysulfate minerals to goethite, and proton and ferrous iron sequestration by burial of iron sulfides and carbonates in sediments. These processes mostly cannot compensate acidity loading from the watershed, though. A master variable for almost all processes is the pH: rates of pyrite oxidation, ferrous iron oxidation, mineral dissolution, iron precipitation, iron hydroxide transformation, and iron and sulfate reduction are strongly pH dependent. While the principle mechanism of acidity generation and consumption and several controls are mostly understood, this cannot be said about the fate of acidity on larger spatial and temporal scales. Little is also known about critical loads and the internal regulation of biogeochemical iron, sulfur, and carbon cycling in acidic mine lakes.

Enhanced hydrolysis of carbohydrates in primary sludge under biosulfidogenic conditions

The potential for using readily available and cost-effective complex carbon sources, such as primary sludge (PS), for the bioremediation of sulfate-rich effluent streams, including acid mine drainage, has been constrained by the slow rate of solubilization and low yield of soluble products. Disposal of PS also remains a global problem. Recent studies of a patented recycling sludge bed reactor have shown that the solubilization of PS is enhanced under biosulfidogenic conditions. The current study investigated the enhanced solubilization of the carbohydrate fraction of PS under these conditions, using selective metabolic inhibitors. The mean maximum rate of reducing sugar production was significantly higher under sulfidogenic (167 mg L(-1)h(-1)) than methanogenic (51 mg L(-1)h(-1)) conditions and the utilization of volatile fatty acids under sulfidogenic conditions was rapid. Analysis of VFA profiles indicated preferential utilization of longer chain acids under sulfidogenic conditions and of acetate in the methanogenic systems and that the acetogenic step was unlikely to be rate-limiting in the solubilization of complex carbon.

Variations temporelles des paramètres physicochimiques et microbiologiques de trois écosystèmes aquatiques (Sud-Est de la France) envahis par des Ludwigia

In France, two amphibious hydrophytes of alien Ludwigia (Onagraceae) have for about the past twenty years been causing serious ecological and economic problems: L. peploides (Kunth) Raven et L. grandiflora (Michaux) Greuter & Burdet. This bacteriological and physicochemical study, focused on three different Mediterranean aquatic ecosystems, reveals, for the first time, a direct negative impact of these American invaders. During summer, while plant growth is intensive, and the appearance in the water column of anoxic conditions and production of toxic compounds may be observed, notably in L. grandiflora stands. The toxicity is linked to a proliferation of sulphate-reducing bacteria producing sulphides that are very harmful for aquatic organisms.

Sulfate reduction processes in sediments at different sites in Lake Kinneret, Israel

Lake Kinneret, Israel, is a warm (13-30°C) monomictic lake that stratifies in April and turns over in December. Between January and June each year, a heavy bloom (up to 250 g wet weight n(-2) 2) of the dinoflagellate Peridinium gatunense dominates the phytoplankton biomass. In early summer, the bloom collapses, and the sinking Peridinium biomass serves as a trigger for intense sulfate-reduction activity throughout the hypolimnion and within the sediments. The availability of organic matter and sulfate was high shortly after the bloom crash and the beginning of stratification and was lowest in December before overturn. Sulfate-reduction rates at three different sites in the lake were studied. In the sediments, the rates varied seasonally and among stations from 5 to 1600 nmol SO4 (-2) reduced cm(-3) day(-1), with respect to the distance from the Jordan River, depth, organic content, and stratification period. During years of low lake water levels, intense sulfate reduction occurred in the hypolimnion, resulting in anoxia and high concentrations of H2S (>400 μM). In years with high water levels, early bloom, and delayed stratification, higher rates of sulfate reduction were recorded in the sediments, probably as a result of a greater fraction of the primary production (organic matter) reaching the bottom.

Thermophilic Sulfate Reduction in Hydrothermal Sediment of Lake Tanganyika, East Africa

In environments with temperatures above 60°C, thermophilic prokaryotes are the only metabolically active life-forms. By using the 35 SO 4 2- tracer technique, we studied the activity of sulfate-reducing microorganisms (SRM) in hot sediment from a hydrothermal vent site in the northern part of freshwater Lake Tanganyika (East Africa). Incubation of slurry samples at 8 to 90°C demonstrated meso- and thermophilic sulfate reduction with optimum temperatures of 34 to 45°C and 56 to 65°C, respectively, and with an upper temperature limit of 80°C. Sulfate reduction was stimulated at all temperatures by the addition of short-chain fatty acids and benzoate or complex substrates (yeast extract and peptone). A time course experiment showed that linear thermophilic sulfate consumption occurred after a lag phase (12 h) and indicated the presence of a large population of SRM in the hydrothermal sediment. Thermophilic sulfate reduction had a pH optimum of about 7 and was completely inhibited at pH 8.8 to 9.2. SRM could be enriched from hydrothermal chimney and sediment samples at 60 and 75°C. In lactate-grown enrichments, sulfide production occurred at up to 70 and 75°C, with optima at 63 and 71°C, respectively. Several sporulating thermophilic enrichments were morphologically similar to Desulfotomaculum spp. Dissimilatory sulfate reduction in the studied hydrothermal area of Lake Tanganyika apparently has an upper temperature limit of 80°C.

Mercury methylation in aquatic systems affected by acid deposition

Recently, it has been noted that fish in acidified lakes may contain elevated levels of mercury. While there is correlation among lakes between depressed pH and high mercury concentrations in fish, the cause of this problem is unknown. A number of hypotheses have been advanced in explanation, including increased mercury deposition, changes in mercury mobility due to acidification, pH dependent changes in mercury uptake by biota, and alterations in population size and/or structure which result in increased bioaccumulation in fish. Because fish accumulate mercury mainly in an organic form, methylmercury, changes in the biogeochemical cycling of this compound might account for elevated bioaccumulation. Mercury methylation is predominantly a microbial process which occurs in situ in lakes. This review focuses on microbiological and biogeochemical changes that may lead to increased levels of methylmercury in fresh waters impacted by acid-deposition. In particular, we focus on the hypothesis that sulfate-reducing bacteria are important mediators of metal methylation in aquatic systems and, moreover, that sulfate-deposition may stimulate methylmercury production by enhancing the activity of sulfate-reducing bacteria in sediments.

Dynamics of Methane Production, Sulfate Reduction, and Denitrification in a Permanently Waterlogged Alder Swamp

The dynamics of sulfate reduction, methane production, and denitrification were investigated in a permanently waterlogged alder swamp. Molybdate, an inhibitor of sulfate reduction, stimulated methane production in soil slurries, thus suggesting competition for common substrates between sulfate-reducing and methane-producing bacteria. Acetate, hydrogen, and methanol were found to stimulate both sulfate reduction and methane production, while trimethylamine mainly stimulated methane production. Nitrate addition reduced both methane production and sulfate reduction, either as a consequence of competition or poisoning of the bacteria. Sulfate-reducing bacteria were only slightly limited by the availability of electron acceptors, while denitrifying bacteria were seriously limited by low nitrate concentrations. Arrhenius plots of the three processes revealed different responses to temperature changes in the slurries. Methane production was most sensitive to temperature changes, followed by denitrification and sulfate reduction. No significant differences between slope patterns were observed when comparing summer and winter measurements, indicating similar populations regarding temperature responses.

Reduction of Selenate to Selenide by Sulfate-Respiring Bacteria: Experiments with Cell Suspensions and Estuarine Sediments

Washed cell suspensions of Desulfovibrio desulfuricans subsp. aestuarii were capable of reducing nanomolar levels of selenate to selenide as well as sulfate to sulfide. Reduction of these species was inhibited by 1 mM selenate or tungstate. The addition of 1 mM sulfate decreased the reduction of selenate and enhanced the reduction of sulfate. Increasing concentrations of sulfate inhibited rates of selenate reduction but enhanced sulfate reduction rates. Cell suspensions kept in 1 mM selenate were incapable of reducing either selenate or sulfate when the selenate/sulfate ratio was ≥0.02, indicating that irreversible inhibition occurs at high selenate concentrations. Anoxic estuarine sediments having an active flora of sulfate-respiring bacteria were capable of a small amount of selenate reduction when ambient sulfate concentrations were low (<4 mM). These results indicate that sulfate is an inhibitor of the reduction of trace quantities of selenate. Therefore, direct reduction of traces of selenate to selenide by sulfate-respiring bacteria in natural environments is constrained by the ambient concentration of sulfate ions. The significance of this observation with regard to the role sediments play in sequestering selenium is discussed.

Flowthrough Reactor Flasks for Study of Microbial Metabolism in Sediments

Flowthrough reactor flasks are described that allow continuous low-level nutrient input to mixed anoxic sediments without dilution of the sediment. The flasks were tested by simulating sulfate inputs into sediments collected from a freshwater eutrophic lake. After an initial 2-day adaptation within the reactor system, rates of methane production and sulfate consumption were constant for the duration of a 12-day incubation. A sulfate input rate of 0.15 mmol liter of sediment −1 day −1 resulted in an equivalent rate of sulfate removal, which was unaffected by inputs of acetate (1.0 mmol liter of sediment −1 day −1 ). The rate of methane production in control reactors, 0.18 mmol liter of sediment −1 day −1 , was doubled by the addition of acetate, whereas sulfate consumption was only stimulated by additions of high concentrations of sulfate plus acetate (1.5 and 1.0 mmol liter of sediment −1 day −1 , respectively). The reactor system appears to be effective in maintaining the balance between sulfate reduction and methane production in freshwater sediments and is potentially useful for study of the response of sediment populations to varying inputs of naturally occurring substrates, selected inhibitors, or xenobiotic compounds.

Sulfate-Reducing Bacteria: Principal Methylators of Mercury in Anoxic Estuarine Sediment

Substrate-electron acceptor combinations and specific metabolic inhibitors were applied to anoxic saltmarsh sediment spiked with mercuric ions (Hg 2+ ) in an effort to identify, by a direct approach, the microorganisms responsible for the synthesis of hazardous monomethylmercury. 2-Bromoethane sulfonate (30 mM), a specific inhibitor of methanogens, increased monomethylmercury synthesis, whereas sodium molybdate (20 mM), a specific inhibitor of sulfate reducers, decreased Hg 2+ methylation by more than 95%. Anaerobic enrichment and isolation procedures yielded a Desulfovibrio desulfuricans culture that vigorously methylated Hg 2+ in culture solution and also in samples of presterilized sediment. The Hg 2+ methylation activity of sulfate reducers is fully expressed only when sulfate is limiting and fermentable organic substrates are available. To date, sulfate reducers have not been suspected of Hg 2+ methylation. Identification of these bacteria as the principal methylators of Hg 2+ in anoxic sediments raises questions about the environmental relevance of previous pure culture-based methylation work.

Kinetic Studies of Bacterial Sulfate Reduction in Freshwater Sediments by High-Pressure Liquid Chromatography and Microdistillation

Indirect photometric chromatography and microdistillation enabled a simultaneous measurement of sulfate depletion and sulfide production in the top 3 cm of freshwater sediments to be made. The simultaneous measurement of sulfate depletion and sulfide production rates provided added insight into microbial sulfur metabolism. The lower sulfate reduction rates, as derived from the production of acid-volatile 35 S 2− only, were explained by a conversion of this pool to an undistillable fraction under acidic conditions during incubation. A mathematical model was applied to calculate sulfate reduction from sulfate gradients at the sediment-water interface. To avoid disturbance of these gradients, the sample volume was reduced to 0.2 g (wet weight) of sediment. Sulfate diffusion coefficients in the model were determined ( D s = 0.3 × 10 −5 cm 2 s −1 at 6°C). The results of the model were compared with those of radioactive sulfate turnover experiments by assessing the actual turnover rate constants (2 to 5 day −1 ) and pool sizes of sulfate at different sediment depths.

Sulfate Reduction in Freshwater Sediments Receiving Acid Mine Drainage

One arm of Lake Anna, Va., receives acid mine drainage (AMD) from Contrary Creek (SO 4 2− concentration = 2 to 20 mM, pH = 2.5 to 3.5). Acid-volatile sulfide concentrations, SO 4 2− reduction rates, and interstitial SO 4 2− concentrations were measured at various depths in the sediment at four stations in four seasons to assess the effects of the AMD-added SO 4 2− on bacterial SO 4 2− reduction. Acid-volatile sulfide concentrations were always an order of magnitude higher at the stations receiving AMD than at a control station in another arm of the lake that received no AMD. Summer SO 4 2− reduction rates were also an order of magnitude higher at stations that received AMD than at the control station (226 versus 13.5 mmol m −2 day −1 ), but winter values were inconclusive, probably due to low sediment temperature (6°C). Profiles of interstitial SO 4 2− concentrations at the AMD stations showed a rapid decrease with depth (from 1,270 to 6 μM in the top 6 cm) due to rapid SO 4 2− reduction. Bottom-water SO 4 2− concentrations in the AMD-receiving arm were highest in winter and lowest in summer. These data support the conclusion that there is a significant enhancement of SO 4 2− reduction in sediments receiving high SO 4 2− inputs from AMD.

Kinetics of Sulfate and Acetate Uptake by Desulfobacter postgatei

The kinetics of sulfate and acetate uptake was studied in the sulfate-reducing bacterium Desulfobacter postgatei (DSM 2034). Kinetic parameters ( K m and V max ) were estimated from substrate consumption curves by resting cell suspensions with [ 35 S]sulfate and [ 14 C]acetate. Both sulfate and acetate consumption followed Michaelis-Menten saturation kinetics. The half-saturation constant ( K m ) for acetate uptake was 70 μM with cells from either long-term sulfate- or long-term acetate-limited chemostat cultures. The average K m value for sulfate uptake by D. postgatei was about 200 μM. K m values for sulfate uptake did not differ significantly when determined with cells derived either from batch cultures or sulfate- or acetate-limited chemostat cultures. Acetate consumption was observed at acetate concentrations of ≤1 μM, whereas sulfate uptake usually ceased at 5 to 20 μM. The results show that D. postgatei is not freely permeable to sulfate ions and further indicate that sulfate uptake is an energy-requiring process.

Sulfate reducers can outcompete methanogens at freshwater sulfate concentrations

Acetate and hydrogen metabolism by sulfate reducers and methanogens in the profundal sediments of an oligotrophic lake were examined. Inhibition of sulfate reduction with molybdate stimulated methane production from both hydrogen and acetate. Molybdate did not stimulate methane production in sediments that were preincubated to deplete the sulfate pool. Sulfate reduction accounted for 30 to 81% of the total of terminal metabolism proceeding through sulfate reduction and methane production in Eckman grab samples of surface sediments. The ability of sulfate reducers to effectively compete with methanogens for acetate was related to the sulfate reducers' lower half-saturation constant for acetate metabolism at in situ sulfate concentrations. Processes other than sulfate reduction and methanogenesis consumed hydrogen at elevated hydrogen partial pressures and prevented a kinetic analysis of hydrogen uptake by sulfate reducers and methanogens. The demonstration that sulfate reducers can successfully compete with methanogens for hydrogen and acetate in sediments at in situ sulfate concentrations of 60 to 105 muM extends the known range of sediment habitats in which sulfate reduction can be a dominant terminal process.