Dissimilatory Arsenate and Sulfate Reduction in Sediments of Two Hypersaline, Arsenic-Rich Soda Lakes: Mono and Searles Lakes, California

A radioisotope method was devised to study bacterial respiratory reduction of arsenate in sediments. The following two arsenic-rich soda lakes in California were chosen for comparison on the basis of their different salinities: Mono Lake (~90 g/liter) and Searles Lake (~340 g/liter). Profiles of arsenate reduction and sulfate reduction were constructed for both lakes. Reduction of [⁷³As]arsenate occurred at all depth intervals in the cores from Mono Lake (rate constant [k] = 0.103 to 0.04 h⁻¹) and Searles Lake (k = 0.012 to 0.002 h⁻¹), and the highest activities occurred in the top sections of each core. In contrast, [³⁵S]sulfate reduction was measurable in Mono Lake (k = 7.6 ×10⁴ to 3.2 × 10⁻⁶ h⁻¹) but not in Searles Lake. Sediment DNA was extracted, PCR amplified, and separated by denaturing gradient gel electrophoresis (DGGE) to obtain phylogenetic markers (i.e., 16S rRNA genes) and a partial functional gene for dissimilatory arsenate reduction (arrA). The amplified arrA gene product showed a similar trend in both lakes; the signal was strongest in surface sediments and decreased to undetectable levels deeper in the sediments. More arrA gene signal was observed in Mono Lake and was detectable at a greater depth, despite the higher arsenate reduction activity observed in Searles Lake. A partial sequence (about 900 bp) was obtained for a clone (SLAS-3) that matched the dominant DGGE band found in deeper parts of the Searles Lake sample (below 3 cm), and this clone was found to be closely related to SLAS-1, a novel extremophilic arsenate respirer previously cultivated from Searles Lake.     

Microbial Manganese and Sulfate Reduction in Black Sea Shelf Sediments

The microbial ecology of anaerobic carbon oxidation processes was investigated in Black Sea shelf sediments from mid-shelf with well-oxygenated bottom water to the oxic-anoxic chemocline at the shelf-break. At all stations, organic carbon (C_org) oxidation rates were rapidly attenuated with depth in anoxically incubated sediment. Dissimilatory Mn reduction was the most important terminal electron-accepting process in the active surface layer to a depth of ~1 cm, while SO₄²⁻ reduction accounted for the entire C_org oxidation below. Manganese reduction was supported by moderately high Mn oxide concentrations. A contribution from microbial Fe reduction could not be discerned, and the process was not stimulated by addition of ferrihydrite. Manganese reduction resulted in carbonate precipitation, which complicated the quantification of C_org oxidation rates. The relative contribution of Mn reduction to C_org oxidation in the anaerobic incubations was 25 to 73% at the stations with oxic bottom water. In situ, where Mn reduction must compete with oxygen respiration, the contribution of the process will vary in response to fluctuations in bottom water oxygen concentrations. Total bacterial numbers as well as the detection frequency of bacteria with fluorescent in situ hybridization scaled to the mineralization rates. Most-probable-number enumerations yielded up to 10⁵ cells of acetate-oxidizing Mn-reducing bacteria (MnRB) cm⁻³, while counts of Fe reducers were <10² cm⁻³. At two stations, organisms affiliated with Arcobacter were the only types identified from 16S rRNA clone libraries from the highest positive MPN dilutions for MnRB. At the third station, a clone type affiliated with Pelobacter was also observed. Our results delineate a niche for dissimilatory Mn-reducing bacteria in sediments with Mn oxide concentrations greater than ~10 μmol cm⁻³ and indicate that bacteria that are specialized in Mn reduction, rather than known Mn and Fe reducers, are important in this niche.     

Characterization of Microbial Arsenate Reduction in the Anoxic Bottom Waters of Mono Lake,California

Dissimilatory reduction of arsenate (DAsR) occurs in the arsenic-rich, anoxic water column of Mono Lake, California, yet the microorganisms responsible for this observed in situ activity have not been identified. To gain insight as to which microorganisms mediate this phenomenon, as well as to some of the biogeochemical constraints on this activity, we conducted incubations of arsenate-enriched bottom water coupled with inhibition/amendment studies and Denaturing Gradient Gel Electrophoresis (DGGE) characterization techniques. DAsR was totally inhibited by filter-sterilization and by nitrate, partially inhibited (~50%) by selenate, but only slightly (~25%) inhibited by oxyanions that block sulfate-reduction (molybdate and tungstate). The apparent inhibition by nitrate, however, was not due to action as a preferred electron acceptor to arsenate. Rather, nitrate addition caused a rapid, microbial re-oxidation of arsenite to arsenate, which gave the overall appearance of no arsenate loss. A similar microbial oxidation of As(III) was also found with Fe(III), a fact that has implications for the recycling of As(V) in Mono Lake's anoxic bottom waters. DAsR could be slightly (10%) stimulated by substrate amendments of lactate, succinate, malate, or glucose, but not by acetate, suggesting that the DAsR microflora is not electron donor limited. DGGE analysis of amplified 16S rDNA gene fragments from incubated arsenate-enriched bottom waters revealed the presence of two bands that were not present in controls without added arsenate. The resolved sequences of these excised bands indicated the presence of members of the epsilon ( Sulfurospirillum ) and delta ( Desulfovibrio ) subgroups of the Proteobacteria , both of which have representative species that are capable of anaerobic growth using arsenate as their electron acceptor.     

Effects of Metals on Methanogenesis, Sulfate Reduction, Carbon Dioxide Evolution, and Microbial Biomass in Anoxic Salt Marsh Sediments

The effects of several metals on microbial methane, carbon dioxide, and sulfide production and microbial ATP were examined in sediments from Spartina alterniflora communities. Anaerobically homogenized sediments were amended with 1,000 ppm (ratio of weight of metal to dry weight of sediment) of various metals. Time courses in controls were similar for CH₄, H₂S, and CO₂, with short initial lags (0 to 4 h) followed by periods of constant gas production (1 to 2 days) and declining rates thereafter. Comparisons were made between control and experimental assays with respect to initial rates of production (after lag) and overall production. Methane evolution was inhibited both initially and overall by CH₃HgCl, HgS, and NaAsO₂. A period of initial inhibition was followed by a period of overall stimulation with Hg, Pb, Ni, Cd, and Cu, all as chlorides, and with ZnSO₄, K₂CrO₄, and K₂Cr₂O₇. Production of CO₂ was generally less affected by the addition of metals. Inhibition was noted with NaAsO₂, CH₃HgCl, and Na₂MoO₄. Minor stimulation of CO₂ production occurred over the long term with chlorides of Hg, Pb, and Fe. Sulfate reduction was inhibited in the short term by all metals tested and over the long term by all but FeCl₂ and NiCl₂. Microbial biomass was decreased by FeCl₂, K₂Cr₂O₇, ZnSO₄, CdCl₂, and CuCl₂ but remained generally unaffected by PbCl₂, HgCl₂, and NiCl₂. Although the majority of metals produced an immediate inhibition of methanogenesis, for several metals this was only a transient phenomenon followed by an overall stimulation. The initial suppression of methanogenesis may be relieved by precipitation, complexation, or transformation of the metal (possibly by methylation), with the subsequent stimulation resulting from a sustained inhibition of competing organisms (e.g., sulfate-reducing bacteria). For several environmentally significant metals, severe metal pollution may substantially alter the flow of carbon in sediments.     

Effects of Copper,Nickel, and Sulfate from the Smelters at Sudbury,Ontario (Canada) on Microbial Communities in Lakes

Analysis of water and sediments from deep and shallow environments in lakes located 6–154 km east or southeast of the smelters at Sudbury, Ontario (Canada) revealed variable, interactive effects of copper, nickel, and sulfate from smelter fallout on lacustrine microfloras. Metal species in sediments were differentiated by sequential extractions, and the nature, abundances, and activities of microbial populations were represented by chlorophyll-a in water and by CO₂ production, fatty acids, phospholipids, dehydrogenase, alkaline phosphatase, and spectral properties of humic matter in sediments. Smelter fallout declined logarithmically with distance from the smelters, and its effects on microfloras depended on the type of microorganism or microbial process and on environmental factors and the abundances of metal species and detoxifying agents. Extractable copper and nickel had toxic effects, which were not attributable solely to the exchangeable fractions, but in certain cases nickel counteracted copper toxicity. Sedimentary sulfide as a whole or sulfide produced by bacterial sulfate reduction, or low oxidation–reduction potential regardless of sulfide concentration, ameliorated metal toxicity by making the metals less bioavailable; and toxicity showed a “quantum jump” when detoxifying agents fell below certain critical concentrations, implying the existence of threshold levels of bioavailable metals above which toxicity increased abruptly. In some cases metal toxicity was lowest in the lakes closest to the smelters (because sulfate concentrations were highest) as well as in the lakes furthest away, and was highest at intermediate distances. The results also suggest that nickel pollution led to ecological succession whereby nickel-tolerant microbial populations replaced nickel-sensitive ones.     

Dissimilatory Arsenate Reduction with Sulfide as Electron Donor: Experiments with Mono Lake Water and Isolation of Strain MLMS-1, a Chemoautotrophic Arsenate Respirer

Anoxic bottom water from Mono Lake, California, can biologically reduce added arsenate without any addition of electron donors. Of the possible in situ inorganic electron donors present, only sulfide was sufficiently abundant to drive this reaction. We tested the ability of sulfide to serve as an electron donor for arsenate reduction in experiments with lake water. Reduction of arsenate to arsenite occurred simultaneously with the removal of sulfide. No loss of sulfide occurred in controls without arsenate or in sterilized samples containing both arsenate and sulfide. The rate of arsenate reduction in lake water was dependent on the amount of available arsenate. We enriched for a bacterium that could achieve growth with sulfide and arsenate in a defined, mineral medium and purified it by serial dilution. The isolate, strain MLMS-1, is a gram-negative, motile curved rod that grows by oxidizing sulfide to sulfate while reducing arsenate to arsenite. Chemoautotrophy was confirmed by the incorporation of H¹⁴CO₃⁻ into dark-incubated cells, but preliminary gene probing tests with primers for ribulose-1,5-biphosphate carboxylase/oxygenase did not yield PCR-amplified products. Alignment of 16S rRNA sequences indicated that strain MLMS-1 was in the δ-Proteobacteria, located near sulfate reducers like Desulfobulbus sp. (88 to 90% similarity) but more closely related (97%) to unidentified sequences amplified previously from Mono Lake. However, strain MLMS-1 does not grow with sulfate as its electron acceptor.     

Effects of Iron and Nitrogen Limitation on Sulfur Isotope Fractionation during Microbial Sulfate Reduction

Sulfate-reducing microbes utilize sulfate as an electron acceptor and produce sulfide that is depleted in heavy isotopes of sulfur relative to sulfate. Thus, the distribution of sulfur isotopes in sediments can trace microbial sulfate reduction (MSR), and it also has the potential to reflect the physiology of sulfate-reducing microbes. This study investigates the relationship between the availability of iron and reduced nitrogen and the magnitude of S-isotope fractionation during MSR by a marine sulfate-reducing bacterium, DMSS-1, a Desulfovibrio species, isolated from salt marsh in Cape Cod, MA. Submicromolar levels of iron increase sulfur isotope fractionation by about 50% relative to iron-replete cultures of DMSS-1. Iron-limited cultures also exhibit decreased cytochrome c-to-total protein ratios and cell-specific sulfate reduction rates (csSRR), implying changes in the electron transport chain that couples carbon and sulfur metabolisms. When DMSS-1 fixes nitrogen in ammonium-deficient medium, it also produces larger fractionation, but it occurs at faster csSRRs than in the ammonium-replete control cultures. The energy and reducing power required for nitrogen fixation may be responsible for the reverse trend between S-isotope fractionation and csSRR in this case. Iron deficiency and nitrogen fixation by sulfate-reducing microbes may lead to the large observed S-isotope effects in some euxinic basins and various anoxic sediments.     

Dissimilatory arsenate and sulfate reduction in sediments of two hypersaline, arsenic-rich soda lakes: Mono and Searles Lakes, California

A radioisotope method was devised to study bacterial respiratory reduction of arsenate in sediments. The following two arsenic-rich soda lakes in California were chosen for comparison on the basis of their different salinities: Mono Lake (approximately 90 g/liter) and Searles Lake (approximately 340 g/liter). Profiles of arsenate reduction and sulfate reduction were constructed for both lakes. Reduction of [73As]arsenate occurred at all depth intervals in the cores from Mono Lake (rate constant [k] = 0.103 to 0.04 h(-1)) and Searles Lake (k = 0.012 to 0.002 h(-1)), and the highest activities occurred in the top sections of each core. In contrast, [35S]sulfate reduction was measurable in Mono Lake (k = 7.6 x10(4) to 3.2 x 10(-6) h(-1)) but not in Searles Lake. Sediment DNA was extracted, PCR amplified, and separated by denaturing gradient gel electrophoresis (DGGE) to obtain phylogenetic markers (i.e., 16S rRNA genes) and a partial functional gene for dissimilatory arsenate reduction (arrA). The amplified arrA gene product showed a similar trend in both lakes; the signal was strongest in surface sediments and decreased to undetectable levels deeper in the sediments. More arrA gene signal was observed in Mono Lake and was detectable at a greater depth, despite the higher arsenate reduction activity observed in Searles Lake. A partial sequence (about 900 bp) was obtained for a clone (SLAS-3) that matched the dominant DGGE band found in deeper parts of the Searles Lake sample (below 3 cm), and this clone was found to be closely related to SLAS-1, a novel extremophilic arsenate respirer previously cultivated from Searles Lake.     

Depth Distribution of Microbial Diversity in Mono Lake, a Meromictic Soda Lake in California

We analyzed the variation with depth in the composition of members of the domain Bacteria in samples from alkaline, hypersaline, and currently meromictic Mono Lake in California. DNA samples were collected from the mixolimnion (2 m), the base of the oxycline (17.5 m), the upper chemocline (23 m), and the monimolimnion (35 m). Composition was assessed by sequencing randomly selected cloned fragments of 16S rRNA genes retrieved from the DNA samples. Most of the 212 sequences retrieved from the samples fell into five major lineages of the domain Bacteria: α- and γ-Proteobacteria (6 and 10%, respectively), Cytophaga-Flexibacter-Bacteroides (19%), high-G+C-content gram-positive organisms (Actinobacteria; 25%), and low-G+C-content gram-positive organisms (Bacillus and Clostridium; 19%). Twelve percent were identified as chloroplasts. The remaining 9% represented β- and δ-Proteobacteria, Verrucomicrobiales, and candidate divisions. Mixolimnion and oxycline samples had low microbial diversity, with only 9 and 12 distinct phylotypes, respectively, whereas chemocline and monimolimnion samples were more diverse, containing 27 and 25 phylotypes, respectively. The compositions of microbial assemblages from the mixolimnion and oxycline were not significantly different from each other (P = 0.314 and 0.877), but they were significantly different from those of chemocline and monimolimnion assemblages (P < 0.001), and the compositions of chemocline and monimolimnion assemblages were not significantly different from each other (P = 0.006 and 0.124). The populations of sequences retrieved from the mixolimnion and oxycline samples were dominated by sequences related to high-G+C-content gram-positive bacteria (49 and 63%, respectively) distributed in only three distinct phylotypes, while the population of sequences retrieved from the monimolimnion sample was dominated (52%) by sequences related to low-G+C-content gram-positive bacteria distributed in 12 distinct phylotypes. Twelve and 28% of the sequences retrieved from the chemocline sample were also found in the mixolimnion and monimolimnion samples, respectively. None of the sequences retrieved from the monimolimnion sample were found in the mixolimnion or oxycline samples. Elevated diversity in anoxic bottom water samples relative to oxic surface water samples suggests a greater opportunity for niche differentiation in bottom versus surface waters of this lake.     

Diversity, Composition, and Geographical Distribution of Microbial Communities in California Salt Marsh Sediments

The Pacific Estuarine Ecosystem Indicators Research Consortium seeks to develop bioindicators of toxicant-induced stress and bioavailability for wetland biota. Within this framework, the effects of environmental and pollutant variables on microbial communities were studied at different spatial scales over a 2-year period. Six salt marshes along the California coastline were characterized using phospholipid fatty acid (PLFA) analysis and terminal restriction fragment length polymorphism (TRFLP) analysis. Additionally, 27 metals, six currently used pesticides, total polychlorinated biphenyls and polycyclic aromatic hydrocarbons, chlordanes, nonachlors, dichlorodiphenyldichloroethane, and dichlorodiphenyldichloroethylene were analyzed. Sampling was performed over large (between salt marshes), medium (stations within a marsh), and small (different channel depths) spatial scales. Regression and ordination analysis suggested that the spatial variation in microbial communities exceeded the variation attributable to pollutants. PLFA analysis and TRFLP canonical correspondence analysis (CCA) explained 74 and 43% of the variation, respectively, and both methods attributed 34% of the variation to tidal cycles, marsh, year, and latitude. After accounting for spatial variation using partial CCA, we found that metals had a greater effect on microbial community composition than organic pollutants had. Organic carbon and nitrogen contents were positively correlated with PLFA biomass, whereas total metal concentrations were positively correlated with biomass and diversity. Higher concentrations of heavy metals were negatively correlated with branched PLFAs and positively correlated with methyl- and cyclo-substituted PLFAs. The strong relationships observed between pollutant concentrations and some of the microbial indicators indicated the potential for using microbial community analyses in assessments of the ecosystem health of salt marshes.     

Effects of Temperature and Pressure on Sulfate Reduction and Anaerobic Oxidation of Methane in Hydrothermal Sediments of Guaymas Basin

Rates of sulfate reduction (SR) and anaerobic oxidation of methane (AOM) in hydrothermal deep-sea sediments from Guaymas Basin were measured at temperatures of 5 to 200°C and pressures of 1 × 10⁵, 2.2 × 10⁷, and 4.5 × 10⁷ Pa. A maximum SR of several micromoles per cubic centimeter per day was found at between 60 and 95°C and 2.2 × 10⁷ and 4.5 × 10⁷ Pa. Maximal AOM was observed at 35 to 90°C but generally accounted for less than 5% of SR.     

The Process of Microbial Sulfate Reduction in Sediments of the Coastal Zone and Littoral of the Kandalaksha Bay of the White Sea

Microbiological and biogeochemical investigations of the coastal zone and the littoral of the Kandalaksha Bay of the White Sea were carried out. The material for investigations was obtained in the series of expeditions of the Institute of Microbiology, Russian Academy of Sciences, in August 1999, 2000, 2001, and in March 2003. The studies were conducted on the littoral and in the water area of the Kandalaksha Preserve, the Moscow University Belomorsk Biological Station, and the Zoological Institute Biological Station, Russian Academy of Sciences. Sediment sampling on the littoral was carried out in the typical microlandscapes differing in the sediment properties and macrobenthos distribution. The maximal sulfate reduction rate (SRR) was shown for the shallow part of the Chernorechenskaya Bay (up to 2550 g S/(dm³ day)) and in the Bab'ye More Bay (up to 3191 g S/(dm³ day)). During the winter season, at a temperature of –0.5 × 0.5°C, the SRR in the sediments of the Kartesh Bay was 7.9 × 13 g S/(dm³ day). In the widest limits, the SRR values varied in the sediment cores sampled on the littoral. The minimal values (11 g S/(dm³ day)) were obtained in the core samples on the silt–sandy littoral. The littoral finely dispersed sediments rich in organic matter were characterized by high SRR values (524–1413 g S/(dm³ day)). The maximal SRR values were shown for the sediments present within the stretch of decomposing macrophytes, in local pits at the lower littoral waterline, and in the mouth of a freshwater stream (51–159 mg S/(dm³ day)). A sharp difference in the level of H₂S production in the type microlandscapes was shown. The average hydrogen sulfide production in finely dispersed sediments constituted 125 mg S/(m² day); in stormy discharge deposits, 1950 mg S/(m² day); in depressions under stones and in silted pits, 4300 mg S/(m² day). A calculation made with regard to the area of microlandscapes with increased productivity shows that the daily H₂S production per 1 km² of the littoral (August) is 60.8 to 202 kg S/(km² day), while the organic carbon consumption for sulfate reduction per 1 km² of the littoral is 46 to 152 kg C_org/(km² day).     

Filamentous Anoxygenic Phototrophic Bacteria in Microbial Communities of the Kulunda Steppe Soda Lakes (Altai Krai,Russia)

Microbiology - Soda lakes are relic ecosystems inhabited by unique microorganisms, which are doubly extremophilic: both haloalkaliphilic and natronophilic. Two morphologically and physiologically...     

Effects of Urbanization on the Dynamics of Organic Sediments in Temperate Lakes

Residential development of lakeshores affects the structure and function of riparian and littoral habitats. Organic detritus in sediments is a critical component of littoral food webs, but the effects of urbanization on sediment characteristics are unexplored. We characterized the quantity of organic sediments in Pacific Northwest lakes along a development gradient and found a 10-fold decline in the proportion of detritus in littoral sediments associated with density of lakeshore dwellings. In a comparison between two fully developed lakes and two undeveloped reference lakes, we examined several possible controls on sedimentary organic content, including terrestrial inputs, decomposition rates and associated macroinvertebrate communities, and physical retention by coarse wood. The littoral sediments of undeveloped lakes ranged from 34 to 77% organic by mass, whereas the range on urban lakes was an order of magnitude less, ranging from 1 to 3% organic. We found no significant differences in terrestrial litter inputs between the two sets of lakes. Leaf litter decomposition rates did not vary significantly between the two sets of lakes, and we found higher densities of shredder macroinvertebrate taxa in the littoral zones of undeveloped lakes. Sedimentary organic matter on undeveloped lakes accumulated in shallow waters and declined with distance from shore, whereas the opposite pattern existed on urban lakes. Our results suggest that coarse wood physically retains organic matter in littoral zones where it can enter the detrital energy pathway, and the loss of this feature on urban lakes alters littoral sediment characteristics, with potentially far-reaching consequences for lake food webs.     

Archaeal and Bacterial Communities Respond Differently to Environmental Gradients in Anoxic Sediments of a California Hypersaline Lake,the Salton Sea

Sulfidic, anoxic sediments of the moderately hypersaline Salton Sea contain gradients in salinity and carbon that potentially structure the sedimentary microbial community. We investigated the abundance, community structure, and diversity of Bacteria and Archaea along these gradients to further distinguish the ecologies of these domains outside their established physiological range. Quantitative PCR was used to enumerate 16S rRNA gene abundances of Bacteria, Archaea, and Crenarchaeota. Community structure and diversity were evaluated by terminal restriction fragment length polymorphism (T-RFLP), quantitative analysis of gene (16S rRNA) frequencies of dominant microorganisms, and cloning and sequencing of 16S rRNA. Archaea were numerically dominant at all depths and exhibited a lesser response to environmental gradients than that of Bacteria. The relative abundance of Crenarchaeota was low (0.4 to 22%) at all depths but increased with decreased carbon content and increased salinity. Salinity structured the bacterial community but exerted no significant control on archaeal community structure, which was weakly correlated with total carbon. Partial sequencing of archaeal 16S rRNA genes retrieved from three sediment depths revealed diverse communities of Euryarchaeota and Crenarchaeota, many of which were affiliated with groups previously described from marine sediments. The abundance of these groups across all depths suggests that many putative marine archaeal groups can tolerate elevated salinity (5.0 to 11.8% [wt/vol]) and persist under the anaerobic conditions present in Salton Sea sediments. The differential response of archaeal and bacterial communities to salinity and carbon patterns is consistent with the hypothesis that adaptations to energy stress and availability distinguish the ecologies of these domains.The vast majority of cultured Archaea isolates are characterized as extremophiles, which thrive under environmental extremes of temperature, pH, salinity, and oxygen availability. Unlike Bacteria, these organisms are well defined by select physiologies or catabolic activities. Cultivated halophilic archaea are obligate aerobes, and with a few exceptions (58), most 16S rRNA gene sequences affiliated with this physiological group have been recovered primarily from environments with oxygen present. Thermophilic archaea, many of which utilize hydrogen-based metabolisms, have temperature requirements that preclude their survival and growth in more moderate environments. Other archaeal physiological groups include acidophiles, which thrive in acidic and mostly high-temperature environments, the obligate anaerobic methanogens, which are capable of competing with Bacteria when more energetically favorable electron acceptors are not available (i.e., sulfate), and methane-oxidizing archaea, which require methane for energy production. Recent work on several Crenarchaeota isolates points to nitrification as their primary energy metabolism, but these organisms have been detected in cold, predominantly aerobic environments, such as open ocean waters and soil (47), and in hyperthermophilic environments (24).Several archaeal groups identified using only 16S rRNA genes, for which no current isolates exist, have been detected in anaerobic sediments of the marine subsurface (6), estuaries (42), freshwater (46), and salt lakes (29). While their physiology and catabolism remain a source of speculation, the environmental distribution patterns of these mesophilic, presumably anaerobic, groups seemingly exclude the physiological and catabolic types outlined above. That is, the persistence of diverse archaeal populations in anoxic sediments at moderate temperature and salinity and at circumneutral pH with only trace levels of methane strongly suggests that alternative metabolic or physiological activities must characterize these populations.Saline lakes are ubiquitous and can be found on all continents. Although many saline lakes are labeled “extreme” environments, microbial diversity within their sediments is often equivalent to that reported for studies of freshwater and marine systems (28). Most studies of the microbial ecology within saline lakes have focused on gradients within the water column, with very few studies on patterns within the sediments. Specifically, these studies have examined how changes in water column salinity lead to shifts in microbial productivity and diversity (8). However, particle-associated microbial communities are known to differ fundamentally from water column or free-living populations (1, 18). These observed differences could be explained by the type and strength of environmental gradients that microbial communities in sediments experience, as opposed to those encountered by pelagic communities.Sediments contain strong environmental gradients, such as time (e.g., sediment age at depth), nutrient and carbon availability, and the dominant terminal electron-accepting process (TEAP) resulting from the sequential use of available oxidants by the microbial community (41). These gradients can lead to changes in the dominant microbial groups (i.e., a shift from sulfate reducers to methanogens with depth and age). Many saline lakes are highly productive and shallow and experience large fluctuations in water level due to climatic changes or to changes in inflows due to urban and agricultural activities. Changes in lake level can lead to dramatic shifts in mixing regimens, nutrient cycling, and water chemistry. Historic fluctuations in water column salinity are often recorded within the sediments in the form of evaporite deposits, which may act as additional sources of ionic loading of the water column (62). These sedimentary salinity gradients may modulate the metabolic activity of some microbial groups. For example, Oren (44) proposed bioenergetic constraints as a possible explanation for the reduced activity or absence of some microbial groups within high-salinity environments. Thus, saline lake sediments are excellent natural laboratories in which to study changes and adaptations of microbial communities due to large-scale changes in environmental gradients.The Salton Sea is a large (980 km²), eutrophic, moderately hypersaline (48 to 50 g liter⁻¹), terminal lake located 69 m below sea level in the Salton Basin, CA. Several large lakes have formed in the Salton Basin over geologic history, the most recent of which was Lake Cahuilla ca. 300 years ago (7). The current lake was unintentionally created in 1905-1907, when the Colorado River flooded the Salton Basin for a period of 16 months. Profundal sediments are highly sulfidic, and sulfate reduction is suspected to be the dominant TEAP within these sediments (54). Based on elemental analysis (51) and ¹³⁷Cs activity (37) of sediment layers, a depth of ∼22 cm marks the point when flooding of the Salton Basin occurred. Sediment above this depth represents the ca. 102 years of historical change within the Salton Sea, including a shift from a water column salinity of 35 g liter⁻¹ to the hypersaline conditions that currently exist. Sediments below this depth consist of low-carbon, gypsum-rich evaporite deposits that were present on the older dry lake bed prior to the formation of the current lake. A previous study reported several strong geochemical gradients within pore water across this relatively small depth range (62).In this work, a suite of cultivation-independent techniques and geochemical analyses was utilized to correlate shifts in abundance, community structure, and diversity of Archaea and Bacteria in Salton Sea sediments with changes in environmental gradients. Large differences in abundance and community structure patterns of Archaea and Bacteria were found along the gradients. In addition, the majority of archaeal sequences retrieved were affiliated with previously described but as yet uncultivated groups identified from various marine sedimentary environments. This indicates that these groups are able to tolerate the higher salinity and anaerobic conditions characteristic of Salton Sea sediments. Fundamental differences between the metabolic capacities and ecologies of Archaea and Bacteria are discussed to explain these patterns.     

Competitive Mechanisms for Inhibition of Sulfate Reduction and Methane Production in the Zone of Ferric Iron Reduction in Sediments

Mechanisms for inhibition of sulfate reduction and methane production in the zone of Fe(III) reduction in sediments were investigated. Addition of amorphic iron(III) oxyhydroxide to sediments in which sulfate reduction was the predominant terminal electron-accepting process inhibited sulfate reduction 86 to 100%. The decrease in electron flow to sulfate reduction was accompanied by a corresponding increase in electron flow to Fe(III) reduction. In a similar manner, Fe(III) additions also inhibited methane production in sulfate-depleted sediments. The inhibition of sulfate reduction and methane production was the result of substrate limitation, because the sediments retained the potential for sulfate reduction and methane production in the presence of excess hydrogen and acetate. Sediments in which Fe(III) reduction was the predominant terminal electron-accepting process had much lower concentrations of hydrogen and acetate than sediments in which sulfate reduction or methane production was the predominant terminal process. The low concentrations of hydrogen and acetate in the Fe(III)-reducing sediments were the result of metabolism by Fe(III)-reducing organisms of hydrogen and acetate at concentrations lower than sulfate reducers or methanogens could metabolize them. The results indicate that when Fe(III) is in a form that Fe(III)-reducing organisms can readily reduce, Fe(III)-reducing organisms can inhibit sulfate reduction and methane production by outcompeting sulfate reducers and methanogens for electron donors.     

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 ³⁵S²⁻ 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 cm² 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.     

Activity,Distribution, and Diversity of Sulfate Reducers and Other Bacteria in Sediments above Gas Hydrate (Cascadia Margin,Oregon)

Cold seep environments such as sediments above outcropping hydrate at Hydrate Ridge (Cascadia margin off Oregon) are characterized by methane venting, high sulfide fluxes caused by the anaerobic oxidation of methane, and the presence of chemosynthetic communities. Recent investigations showed that another characteristic feature of cold seeps is the occurrence of methanotrophic archaea, which can be identified by specific biomarker lipids and 16S rDNA analysis. This investigation deals with the diversity and distribution of sulfate-reducing bacteria, some of which are directly involved in the anaerobic oxidation of methane as syntrophic partners of the methanotrophic archaea. The composition and activity of the microbial communities at methane vented and nonvented sediments are compared by quantitative methods including total cell counts, fluorescence in situ hybridization (FISH), bacterial production, enzyme activity, and sulfate reduction rates. Bacteria involved in the degradation of particulate organic carbon (POC) are as active and diverse as at other productive margin sites of similar water depths. The availability of methane supports a two orders of magnitude higher microbial biomass (up to 9.6 2 10 10 cells cm m 3 ) and sulfate reduction rates (up to 8 w mol cm m 3 d m 1 ) in hydrate-bearing sediments, as well as a high bacterial diversity, especially in the group of i -proteobacteria including members of the branches Desulfosarcina/Desulfococcus , Desulforhopalus , Desulfobulbus , and Desulfocapsa . Most of the diversity of sulfate-reducing bacteria in hydrate-bearing sediments comprises seep-endemic clades, which share only low similarities with previously cultured bacteria.