Substrate uptake by uncultured bacteria from the genus Achromatium determined by microautoradiography.

Microautoradiography was used to investigate substrate uptake by natural communities of uncultured bacteria from the genus Achromatium. Studies of the uptake of (14)C-labelled substrates demonstrated that Achromatium cells from freshwater sediments were able to assimilate (14)C from bicarbonate, acetate, and protein hydrolysate; however, (14)C-labelled glucose was not assimilated. The pattern of substrate uptake by Achromatium spp. was therefore similar to those of a number of other freshwater and marine sulfur-oxidizing bacteria. Different patterns of radiolabelled bicarbonate uptake were noted for Achromatium communities from different geographical locations and indicated that one community (Rydal Water) possessed autotrophic potential, while the other (Hell Kettles) did not. Furthermore, the patterns of organic substrate uptake within a single population suggested that physiological diversity existed in natural communities of Achromatium. These observations are consistent with and may relate to the phylogenetic diversity observed in Achromatium communities. Incubation of Achromatium-bearing sediment cores from Rydal Water with (35)S-labelled sulfate in the presence and absence of sodium molybdate demonstrated that this bacterial population was capable of oxidizing sulfide to intracellular elemental sulfur. This finding supported the role of Achromatium in the oxidative component of a tightly coupled sulfur cycle in Rydal Water sediment. The oxidation of sulfide to sulfur and ultimately to sulfate by Achromatium cells from Rydal Water sediment is consistent with an ability to conserve energy from sulfide oxidation.     

Use of Combined Microautoradiography and Fluorescence In Situ Hybridization To Determine Carbon Metabolism in Mixed Natural Communities of Uncultured Bacteria from the Genus Achromatium

Combined microautoradiography and fluorescence in situ hybridization (FISH) was used to investigate carbon metabolism in uncultured bacteria from the genus Achromatium. All of the Achromatium species identified in a freshwater sediment from Rydal Water, Cumbria, United Kingdom, which were distinguishable only by FISH, assimilated both [¹⁴C]bicarbonate and [¹⁴C]acetate. This extends previous findings that Achromatium spp. present at another location could only utilize organic carbon sources. Achromatium spp., therefore, probably exhibit a range of physiologies, i.e., facultative chemolithoautotrophy, mixotrophy, and chemoorganoheterotrophy, similar to other large sulfur bacteria (e.g., Beggiatoa spp.).     

Natural Communities of Achromatium oxaliferum Comprise Genetically, Morphologically, and Ecologically Distinct Subpopulations

The diversity and ecology of natural communities of the uncultivated bacterium Achromatium oxaliferum were studied by use of culture-independent approaches. 16S rRNA gene sequences were PCR amplified from DNA extracted from highly purified preparations of cells that were morphologically identified as A. oxaliferum present in freshwater sediments from three locations in northern England (Rydal Water, Jenny Dam, Hell Kettles). Cloning and sequence analysis of the PCR-amplified 16S rRNA genes revealed that multiple related but divergent sequences were routinely obtained from the A. oxaliferum communities present in all the sediments examined. Whole-cell in situ hybridization with combinations of fluorescence-labelled oligonucleotide probes revealed that the divergent sequences recovered from purified A. oxaliferum cells corresponded to genetically distinct Achromatium subpopulations. Analysis of the cell size distribution of the genetically distinct subpopulations demonstrated that each was also morphologically distinct. Furthermore, there was a high degree of endemism in the Achromatium sequences recovered from different sediments; identical sequences were never recovered from different sampling locations. In addition to ecological differences that were apparent between Achromatium communities from different freshwater sediments, the distribution of different subpopulations of Achromatium in relation to sediment redox profiles indicated that the genetically and morphologically distinct organisms that coexisted in a single sediment were also ecologically distinct and were adapted to different redox conditions. This result suggests that Achromatium populations have undergone adaptive radiation and that the divergent Achromatium species occupy different niches in the sediments which they inhabit.     

Calcite-accumulating large sulfur bacteria of the genus Achromatium in Sippewissett Salt Marsh

Large sulfur bacteria of the genus Achromatium are exceptional among Bacteria and Archaea as they can accumulate high amounts of internal calcite. Although known for more than 100 years, they remain uncultured, and only freshwater populations have been studied so far. Here we investigate a marine population of calcite-accumulating bacteria that is primarily found at the sediment surface of tide pools in a salt marsh, where high sulfide concentrations meet oversaturated oxygen concentrations during the day. Dynamic sulfur cycling by phototrophic sulfide-oxidizing and heterotrophic sulfate-reducing bacteria co-occurring in these sediments creates a highly sulfidic environment that we propose induces behavioral differences in the Achromatium population compared with reported migration patterns in a low-sulfide environment. Fluctuating intracellular calcium/sulfur ratios at different depths and times of day indicate a biochemical reaction of the salt marsh Achromatium to diurnal changes in sedimentary redox conditions. We correlate this calcite dynamic with new evidence regarding its formation/mobilization and suggest general implications as well as a possible biological function of calcite accumulation in large bacteria in the sediment environment that is governed by gradients. Finally, we propose a new taxonomic classification of the salt marsh Achromatium based on their adaptation to a significantly different habitat than their freshwater relatives, as indicated by their differential behavior as well as phylogenetic distance on 16S ribosomal RNA gene level. In future studies, whole-genome characterization and additional ecophysiological factors could further support the distinctive position of salt marsh Achromatium.     

Use of combined microautoradiography and fluorescence in situ hybridization to determine carbon metabolism in mixed natural communities of uncultured bacteria from the genus Achromatium

Combined microautoradiography and fluorescence in situ hybridization (FISH) was used to investigate carbon metabolism in uncultured bacteria from the genus Achromatium. All of the Achromatium species identified in a freshwater sediment from Rydal Water, Cumbria, United Kingdom, which were distinguishable only by FISH, assimilated both [(14)C]bicarbonate and [(14)C]acetate. This extends previous findings that Achromatium spp. present at another location could only utilize organic carbon sources. Achromatium spp., therefore, probably exhibit a range of physiologies, i.e., facultative chemolithoautotrophy, mixotrophy, and chemoorganoheterotrophy, similar to other large sulfur bacteria (e.g., Beggiatoa spp.).     

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.     

Heterozygous,Polyploid, Giant Bacterium,Achromatium, Possesses an Identical Functional Inventory Worldwide across Drastically Different Ecosystems

Achromatium is large, hyperpolyploid and the only known heterozygous bacterium. Single cells contain approximately 300 different chromosomes with allelic diversity far exceeding that typically harbored by single bacteria genera. Surveying all publicly available sediment sequence archives, we show that Achromatium is common worldwide, spanning temperature, salinity, pH, and depth ranges normally resulting in bacterial speciation. Although saline and freshwater Achromatium spp. appear phylogenetically separated, the genus Achromatium contains a globally identical, complete functional inventory regardless of habitat. Achromatium spp. cells from differing ecosystems (e.g., from freshwater to saline) are, unexpectedly, equally functionally equipped but differ in gene expression patterns by transcribing only relevant genes. We suggest that environmental adaptation occurs by increasing the copy number of relevant genes across the cell’s hundreds of chromosomes, without losing irrelevant ones, thus maintaining the ability to survive in any ecosystem type. The functional versatility of Achromatium and its genomic features reveal alternative genetic and evolutionary mechanisms, expanding our understanding of the role and evolution of polyploidy in bacteria while challenging the bacterial species concept and drivers of bacterial speciation.     

Major contribution of sulfide-derived sulfur to the benthic food web in a large freshwater lake

In freshwater systems, contributions of chemosynthetic products by sulfur-oxidizing bacteria in sediments as nutritional resources in benthic food webs remain unclear, even though chemosynthetic products might be an important nutritional resource for benthic food webs in deep-sea hydrothermal vents and shallow marine systems. To study geochemical aspects of this trophic pathway, we sampled sediment cores and benthic animals at two sites (90 and 50 m water depths) in the largest freshwater (mesotrophic) lake in Japan: Lake Biwa. Stable carbon, nitrogen, and sulfur isotopes of the sediments and animals were measured to elucidate the sulfur nutritional resources for the benthic food web precisely by calculating the contributions of the incorporation of sulfide-derived sulfur to the biomass and of the biogeochemical sulfur cycle supporting the sulfur nutritional resource. The recovered sediment cores showed increases in ³⁴S-depleted sulfide at 5 cm sediment depth and showed low sulfide concentration with high δ³⁴S in deeper layers, suggesting an association of microbial activities with sulfate reduction and sulfide oxidation in the sediments. The sulfur-oxidizing bacteria may contribute to benthic animal biomass. Calculations based on the biomass, sulfur content, and contribution to sulfide-derived sulfur of each animal comprising the benthic food web revealed that 58%–67% of the total biomass sulfur in the benthic food web of Lake Biwa is occupied by sulfide-derived sulfur. Such a large contribution implies that the chemosynthetic products of sulfur-oxidizing bacteria are important nutritional resources supporting benthic food webs in the lake ecosystems, at least in terms of sulfur. The results present a new trophic pathway for sulfur that has been overlooked in lake ecosystems with low-sulfate concentrations.     

Effect of oxygen on viability and substrate utilization in Chromatium

Chromatium D can be exposed to oxygen for prolonged periods without any loss in motility or viability. Oxygen did not affect the rate of thiosulfate disappearance from the media, the oxidation of the inner sulfur atom of thiosulfate to sulfate, or the conversion of the outer sulfur atom of thiosulfate to intracellular sulfur, but it did inhibit the oxidation of intracellular sulfur to sulfate. Oxygen partially inhibited the uptake of pyruvate from the medium, but had little effect on the uptake of acetate. The distribution of label from pyruvate-2-¹⁴C into various cell fractions under aerobic conditions differed only slightly from that obtained under anaerobic conditions. Cells utilizing acetate-2-¹⁴C aerobically converted the majority of the metabolized acetate into a cell fraction with the solubility characteristics of poly-β-hydroxybutyric acid, whereas under anaerobic conditions the acetate was distributed throughout the other cell fractions. Oxygen completely prevented the synthesis of bacteriochlorophyll.     

Chemolithotrophic growth of Desulfovibrio sulfodismutans sp. nov. by disproportionation of inorganic sulfur compounds

A new Desulfovibrio strain ThAc01 was isolated from freshwater mud; the strain conserved energy for growth under strictly anaerobic conditions by disproportionation of thiosulfate or sulfite to sulfate and sulfide according to the following reactions: $$\begin{gathered} S_2 O_3^{2 - } + H_2 O \to SO_4^{2 - } + HS^ - + H^ + \hfill \\ 4SO_3^{2 - } + H^ + {\text{ }} \to 3SO_4^{2 - } + HS^ - \hfill \\ \end{gathered}$$ Strain ThAc01 required acetate as a carbon source, but was unable to utilize acetate as an oxidizable energy source. In a defined medium with acetate and bicarbonate as carbon sources, the growth yields per mol of substrate disproportionated were 2.1 g or 3.2 g dry cell mass on thiosulfate or sulfite, respectively. Strain ThAc01 was also able to grow by dissimilatory sulfate reduction with lactate, ethanol, propanol, or butanol as electron donors and carbon sources which were incompletely oxidized to the corresponding fatty acids. However, growth by sulfate reduction was slower than by disproportionation. Elemental sulfur, nitrate, fumarate, or malate did not serve as electron acceptors. Strain ThAc01 contained desulfoviridin and cytochromes; it required panthothenate and biotin as growth factors and had a DNA base ratio of 64.1 mol% G+C. Disproportionating bacteria similar to strain ThAc01 were enriched with either thiosulfate or sulfite from various freshwater, brackish or marine mud samples. Most probable number enumeration indicated that 2×10⁶ thiosulfate-disproportionating bacteria were present per ml freshwater mud. Of various other sulfate-reducing bacteria tested, only Desulfobacter curvatus (strain AcRM3) was able to disproportionate thiosulfate or sulfite. Desulfovibrio vulgaris (strain Marburg) slowly disproportionated sulfite, but effected only a slight increase in cell density. Strain ThAc01 is proposed as the type strain of a new species, Desulfovibrio sulfodismutans.     

Effect of increased sediment sulfide concentrations on the composition of stable sulfur isotopes (δS) and sulfur accumulation in the seagrasses Zostera marina and Posidonia oceanica

The effect of increased sediment sulfide concentrations on the sulfur isotopic composition (δ³⁴S), total sulfur (TS) and elemental sulfur (S⁰) concentrations in plant tissues was studied for the two seagrasses Zostera marina (3 weeks in laboratory) and Posidonia oceanica (4 months in situ). Porewater sulfide concentrations were experimentally regulated and plants exposed to high sediment sulfide concentrations had δ³⁴S signals closer to the δ³⁴S of sulfide, whereas plants exposed to no / low sulfide concentrations had δ³⁴S signals closer to the δ³⁴S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to high sulfide concentrations. The δ³⁴S varied between the plant tissues in both species with the leaves having more positive δ³⁴S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS and S⁰ concentrations were higher in plants exposed to sulfide in both experiments suggesting that sulfur derived from sediment sulfide accumulated in the plants. The δ³⁴S signal in S⁰ was similar to sediment sulfide verifying that S⁰ found in the seagrasses originated from sediment sulfide. Direct comparisons of δ³⁴S in the two different seagrasses and across the treatments were not possible due to large differences in δ³⁴S of the sulfur sources. F_sulfide adjusted for these differences and may be a useful alternative, when δ³⁴S of the sulfur sources varies between study sites. There were no significant effects of sulfide exposure on plant growth and mortality in Z. marina and P. oceanica after 3 weeks and 8 weeks exposure, respectively, but P. oceanica showed indications of reduced growth and higher mortality after 16 weeks of sulfide exposure probably due to sulfide invasion/toxicity.     

Barite encrustation of benthic sulfur‐oxidizing bacteria at a marine cold seep

Crusts and chimneys composed of authigenic barite are found at methane seeps and hydrothermal vents that expel fluids rich in barium. Microbial processes have not previously been associated with barite precipitation in marine cold seep settings. Here, we report on the precipitation of barite on filaments of sulfide‐oxidizing bacteria at a brine seep in the Gulf of Mexico. Barite‐mineralized bacterial filaments in the interiors of authigenic barite crusts resemble filamentous sulfide‐oxidizing bacteria of the genus Beggiatoa. Clone library and iTag amplicon sequencing of the 16S rRNA gene show that the barite crusts that host these filaments also preserve DNA of Candidatus Maribeggiatoa, as well as sulfate‐reducing bacteria. Isotopic analyses show that the sulfur and oxygen isotope compositions of barite have lower δ³⁴S and δ¹⁸O values than many other marine barite crusts, which is consistent with barite precipitation in an environment in which sulfide oxidation was occurring. Laboratory experiments employing isolates of sulfide‐oxidizing bacteria from Gulf of Mexico seep sediments showed that under low sulfate conditions, such as those encountered in brine fluids, sulfate generated by sulfide‐oxidizing bacteria fosters rapid barite precipitation localized on cell biomass, leading to the encrustation of bacteria in a manner reminiscent of our observations of barite‐mineralized Beggiatoa in the Gulf of Mexico. The precipitation of barite directly on filaments of sulfide‐oxidizing bacteria, and not on other benthic substrates, suggests that sulfide oxidation plays a role in barite formation at certain marine brine seeps where sulfide is oxidized to sulfate in contact with barium‐rich fluids, either prior to, or during, the mixing of those fluids with sulfate‐containing seawater in the vicinity of the sediment/water interface. As with many other geochemical interfaces that foster mineral precipitation, both biological and abiological processes likely contribute to the precipitation of barite at marine brine seeps such as the one studied here.     

Comparison of the Levels of Bacterial Diversity in Freshwater,Intertidal Wetland,and Marine Sediments by Using Millions of Illumina Tags

Sediment, a special realm in aquatic environments, has high microbial diversity. While there are numerous reports about the microbial community in marine sediment, freshwater and intertidal sediment communities have been overlooked. The present study determined millions of Illumina reads for a comparison of bacterial communities in freshwater, intertidal wetland, and marine sediments along Pearl River, China, using a technically consistent approach. Our results show that both taxon richness and evenness were the highest in freshwater sediment, medium in intertidal sediment, and lowest in marine sediment. The high number of sequences allowed the determination of a wide variety of bacterial lineages in all sediments for reliable statistical analyses. Principal component analysis showed that the three types of communities could be well separated from phylum to operational taxonomy unit (OTU) levels, and the OTUs from abundant to rare showed satisfactory resolutions. Statistical analysis (LEfSe) demonstrated that the freshwater sediment was enriched with Acidobacteria, Nitrospira, Verrucomicrobia, Alphaproteobacteria, and Betaproteobacteria. The intertidal sediment had a unique community with diverse primary producers (such as Chloroflexi, Bacillariophyta, Gammaproteobacteria, and Epsilonproteobacteria) as well as saprophytic microbes (such as Actinomycetales, Bacteroidetes, and Firmicutes). The marine sediment had a higher abundance of Gammaproteobacteria and Deltaproteobacteria, which were mainly involved in sulfate reduction in anaerobic conditions. These results are helpful for a systematic understanding of bacterial communities in natural sediment environments.     

Nitrogen, Carbon, and Sulfur Metabolism in Natural Thioploca Samples

Filamentous sulfur bacteria of the genus Thioploca occur as dense mats on the continental shelf off the coast of Chile and Peru. Since little is known about their nitrogen, sulfur, and carbon metabolism, this study was undertaken to investigate their (eco)physiology. Thioploca is able to store internally high concentrations of sulfur globules and nitrate. It has been previously hypothesized that these large vacuolated bacteria can oxidize sulfide by reducing their internally stored nitrate. We examined this nitrate reduction by incubation experiments of washed Thioploca sheaths with trichomes in combination with ¹⁵N compounds and mass spectrometry and found that these Thioploca samples produce ammonium at a rate of 1 nmol min⁻¹ mg of protein⁻¹. Controls showed no significant activity. Sulfate was shown to be the end product of sulfide oxidation and was observed at a rate of 2 to 3 nmol min⁻¹ mg of protein⁻¹. The ammonium and sulfate production rates were not influenced by the addition of sulfide, suggesting that sulfide is first oxidized to elemental sulfur, and in a second independent step elemental sulfur is oxidized to sulfate. The average sulfide oxidation rate measured was 5 nmol min⁻¹ mg of protein⁻¹ and could be increased to 10.7 nmol min⁻¹ mg of protein⁻¹ after the trichomes were starved for 45 h. Incorporation of ¹⁴CO₂ was at a rate of 0.4 to 0.8 nmol min⁻¹ mg of protein⁻¹, which is half the rate calculated from sulfide oxidation. [2-¹⁴C]acetate incorporation was 0.4 nmol min⁻¹ mg of protein⁻¹, which is equal to the CO₂ fixation rate, and no ¹⁴CO₂ production was detected. These results suggest that Thioploca species are facultative chemolithoautotrophs capable of mixotrophic growth. Microautoradiography confirmed that Thioploca cells assimilated the majority of the radiocarbon from [2-¹⁴C]acetate, with only a minor contribution by epibiontic bacteria present in the samples.     

Pathways and Microbiology of Thiosulfate Transformations and Sulfate Reduction in a Marine Sediment (Kattegat, Denmark)

Reductive and oxidative pathways of the sulfur cycle were studied in a marine sediment by parallel radiotracer experiments with ³⁵SO₄^2-, H₂³⁵S, and ³⁵S₂O₃^2- injected into undisturbed sediment cores. The distributions of viable populations of sulfate- and thiosulfate-reducing bacteria and of thiosulfate-disproportionating bacteria were concurrently determined. Sulfate reduction occurred both in the reducing sediment layers and in oxidized and even oxic surface layers. The population density of sulfate-reducing bacteria was >10⁶ cm^-3 in the oxic layer, high enough that it could possibly account for the measured rates of sulfate reduction. The bacterial numbers counted in the reducing sediment layers were 100-fold lower. The dominant sulfate reducers growing on acetate or H₂ were gas-vacuolated motile rods which were previously undescribed. The products of sulfide oxidation, which took place in both oxidized and reduced sediment layers, were 65 to 85% S₂O₃^2- and 35 to 15% SO₄^2-. Thiosulfate was concurrently oxidized to sulfate, reduced to sulfide, and disproportionated to sulfate and sulfide. There was a gradual shift from predominance of oxidation toward predominance of reduction with depth in the sediment. Disproportionation was the most important pathway overall. Thiosulfate disproportionation occurred only as cometabolism in the marine acetate-utilizing sulfate-reducing bacteria, which could not conserve energy for growth from this process alone. Oxidative and reductive cycling of sulfur thus occurred in all sediment layers with an intermediate “thiosulfate shunt” as an important mechanism regulating the electron flow.     

Reduced inorganic sulfur oxidation supports autotrophic and mixotrophic growth of Magnetospirillum strain J10 and Magnetospirillum gryphiswaldense

Magnetotactic bacteria are present at the oxic–anoxic transition zone where opposing gradients of oxygen and reduced sulfur and iron exist. Growth of non‐magnetotactic lithoautotrophic Magnetospirillum strain J10 and its close relative magnetotactic Magnetospirillum gryphiswaldense was characterized in microaerobic continuous culture. Both strains were able to grow in mixotrophic (acetate + sulfide) and autotrophic (sulfide or thiosulfate) conditions. Autotrophically growing cells completely converted sulfide or thiosulfate to sulfate and produced 7.5 g dry weight per mol substrate at a maximum observed growth rate of 0.09 h^?1 for strain J10 and 0.07 h^?1 for M. gryphiswaldense. The respiratory activity for acetate was repressed in autotrophic and also in mixotrophic cultures, suggesting acetate was used as C‐source in the latter. We have estimated the proportions of substrate used for assimilatory processes and evaluated the biomass yields per mol dissimilated substrate. The yield for lithoheterotrophic growth using acetate as the C‐source was approximately twice the autotrophic growth yield and very similar to the heterotrophic yield, showing the importance of reduced sulfur compounds for growth. In the draft genome sequence of M. gryphiswaldense homologues of genes encoding a partial sulfur‐oxidizing (Sox) enzyme system and reverse dissimilatory sulfite reductase (Dsr) were identified, which may be involved in the oxidation of sulfide and thiosulfate. Magnetospirillum gryphiswaldense is the first freshwater magnetotactic species for which autotrophic growth is shown.     

红树林沉积物中微生物驱动硫循环研究进展

红树林滨海湿地是在周期性咸水、淡水作用下形成的特殊生态系统,其沉积物中有机质含量丰富,微生物驱动的营养物质循环活跃。由于红树林沉积物中硫酸盐含量高、硫化物种类多,因此红树林是研究硫元素生物地球化学循环过程和机制的理想系统。本文综述了红树林生态系统中主要的硫元素循环过程,重点总结了硫氧化和硫酸盐还原过程及其功能微生物,分析了影响硫氧化和硫酸盐还原的主要环境因素,并对红树林沉积物中微生物驱动硫循环的重点研究方向进行了展望。鉴于微生物驱动的硫循环过程耦合碳、氮和金属元素循环,本文可为深入探究微生物驱动的生物地球化学元素循环耦合机制提供参考。     

Reduction of Sulfur Compounds in the Sediments of a Eutrophic Lake Basin

Concentrations of various sulfur compounds (SO₄²⁻, H₂S, S⁰, acid-volatile sulfide, and total sulfur) were determined in the profundal sediments and overlying water column of a shallow eutrophic lake. Low concentrations of sulfate relative to those of acid-volatile sulfide and total sulfur and a decrease in total sulfur with sediment depth implied that the contribution of dissimilatory sulfur reduction to H₂S production was relatively minor. Addition of 1.0 mM Na₂³⁵SO₄ to upper sediments in laboratory experiments resulted in the production of H₂³⁵S with no apparent lag. Kinetic experiments with ³⁵S demonstrated an apparent K_m of 0.068 mmol of SO₄²⁻ reduced per liter of sediment per day, whereas tracer experiments with ³⁵S indicated an average turnover time of the sediment sulfate pool of 1.5 h. Total sulfate reduction in a sediment depth profile to 15 cm was 15.3 mmol of sulfate reduced per m² per day, which corresponds to a mineralization of 30% of the particulate organic matter entering the sediment. Reduction of ³⁵S⁰ occurred at a slower rate. These results demonstrated that high rates of sulfate reduction occur in these sediments despite low concentrations of oxidized inorganic compounds and that this reduction can be important in the anaerobic mineralization of organic carbon.     

Diversity of sulfur-cycle prokaryotes in freshwater lake sediments investigated using aprA as the functional marker gene

The diversity of sulfate-reducing prokaryotes (SRPs) and sulfur-oxidizing prokaryotes (SOPs) in freshwater lake ecosystems was investigated by cloning and sequencing of the aprA gene, which encodes for a key enzyme in dissimilatory sulfate reduction and sulfur oxidation. To understand their diversity better, the spatial distribution of aprA genes was investigated in sediments collected from six geographically distant lakes in Antarctica and Japan, including a hypersaline lake for comparison. The microbial community compositions of freshwater sediments and a hypersaline sediment showed notable differences. The clones affiliated with Desulfobacteraceae and Desulfobulbaceae were frequently detected in all freshwater lake sediments. The SOP community was mainly composed of four major phylogenetic groups. One of them formed a monophyletic cluster with a sulfur-oxidizing betaproteobacterium, Sulfuricella denitrificans, but the others were not assigned to specific genera. In addition, the AprA sequences, which were not clearly affiliated to either SRP or SOP lineages, dominated the libraries from four freshwater lake sediments. The results showed the wide distribution of some sulfur-cycle prokaryotes across geographical distances and supported the idea that metabolic flexibility is an important feature for SRP survival in low-sulfate environments.