Biotransformation of phosphogypsum in media containing different forms of nitrogen

Studies on the biotransformation of phosphogypsum (a waste product formed in the course of the production of phosphorous fertilizers) with the use of sulfate reducing bacteria (SRB) demonstrated that it is a good source of sulfates and biogenic elements for these bacteria, though the addition of organic carbon and nitrogen is necessary. The aim of this study was to investigate the form of nitrogen and C:N ratio in the medium on the growth of SRB community in cultures containing phosphogypsum. Batch community cultures of sulfate reducing bacteria were maintained in medium with phosphogypsum (5.0 g/l), different concentrations of sodium lactate (1.6 - 9.4 g/l) and different forms (NH4CI, CO(NH2)2, KNO3) and concentrations (0 - 250 mg/l) of nitrogen. The growth of SRB was studied in the C:N ratio of from 2:1 to 300:1. It was found that: 1 - the best source of nitrogen for SRB is urea, followed by ammonium, the worst were nitrates; 2 - the bacteria were also able to grow in medium without nitrogen but their activity was then by approximately 15% lower than in optimal growth conditions; 3 - in medium with KNO3 inhibition of sulfate reduction by approx. 50% was observed; 4 - the highest reduction of nitrates (removal of nitrate) in media with phosphogypsum and nitrates was at limiting concentrations of sodium lactate. This is probably caused by the selection under these conditions (low concentration of hydrogen sulfide) of denitrifying bacteria or sulfate reducing bacteria capable of using nitrates as an electron acceptor.     

Microbiological Reduction of Sulphates in Salty Environments and Mineralogical Characterization of the Transformation Products

This research was focused on the selection, growth and identification of SRB from soils that were subjected to long-term activity of brine, and an evaluation of mineral phases formed during the biodegradation of organic compounds and sulphate reduction. Isolated communities of anaerobic microorganisms were incubated on Postgate C medium with lactate and/or ethanol as the sole carbon source and were adapted for growth at 4% NaCl. Active reduction of sulphates with simultaneous biodegradation of organic compounds was observed in all cultures. The largest reduction of sulphates was noted in cultures with lactate as the sole carbon source; it reached 1438 mg/L, which corresponds to a 43% reduction of sulphates introduced to the medium. SRB activity in the biodegradation of organic compounds varied between 20 and 80% and depended on the level of salinity of the environment in which the SRB communities were isolated, and on the electron donor applied. The presence of biotransformation products in the post-culture deposits in the form of elemental sulphur reflects the activity of the communities. Additionally, the influence of selected communities on the salinity index was analyzed. Active SRB communities decreased the salinity of the environment by as much as 50%. Sulphate-reducing bacteria are an important group of anaerobic microorganisms, especially considering their participation in such geological processes as mineral precipitation and mineralization of organic matter in extreme environmental conditions, including high salinity.     

Effect of nitrates on biotransformation of phosphogypsum and phenol uptake in cultures of autochthonous sludge microflora from petroleum refining wastewaters

The effect of nitrates on the biotransformation of phosphogypsum at 30 degrees C in stationary cultures of anaerobic, heterogeneous microflora growing in medium with phenol (250-1,000 mg/L) as sole carbon source was studied. The microorganisms used in this study were isolated from sludge in biological petroleum-refining wastewater treatment plant. Phosphogypsum (a waste product in the chemical industry that contains approximately 95% CaSO4) was added in amount of 5 g/L, the source of nitrates was KNO3 in concentration equivalent to that of phenol (250-1,000 mg N-NO3/L). The presence of nitrates in heterogeneous cultures has an inhibitory effect on the process of phosphogypsum biotransformation and stimulates the uptake of phenol. We have found that in cultures in medium containing phenol, phosphogypsum and nitrates at least three physiological groups of microorganisms were present. These were phenol-biodegrading microorganisms not requiring an external electron acceptor, sulfate-reducing bacteria biodegrading phenol or intermediate products of its breakdown and denitrifying bacteria not utilising phenol as a carbon source. On solid medium these bacteria together formed heterogeneous single colonies. In spite of repeated attempts we were unable to isolate pure strains and the only result of these measures was loss of denitrification ability in medium with phenol.     

Transition Metal Catalyst-Assisted Reductive Dechlorination of Perchloroethylene by Anaerobic Aquifer Enrichments

Bioremediation of groundwater contaminated with chlorinated solvents, such as perchloroethylene (PCE) or carbon tetrachloride, can be accomplished by adding nutrients to stimulate a microbial community capable of reductive dechlorination. However, biotransformation of these solvents, especially PCE, typically occurs very slowly or not at all. Experiments were conducted to evaluate whether the addition of transition metal tetrapyrrole catalysts would increase the reductive transformation of PCE to trichloroethylene (TCE) by sulfate-reducing enrichment cultures. Batch assays were used to test vitamin B₁₂ and two synthetic sulfonatophenyl porphine catalysts for the stimulation of reductive dechlorination of PCE by sulfate-reducing bacteria (SRB) enriched from aquifer sediments from two locations at Dover Air Force Base. Cells from the enrichments were concentrated and added to batch assay vials. Vials containing SRB cells amended with vitamin B12 exhibited enhanced transformation of PCE to TCE compared with reactors amended with either synthetic catalysts or reactors containing cells alone. Methane production was observed in reactors that exhibited maximum levels of dechlorination. Storage of aquifer sediments between enrichments led to decreased levels of PCE dechlorination in subsequent assays.     

Studies on the effect of inoculation of activated sludge with bacteria actively degrading hydrocarbons on the biodegradation of petroleum products

Eighteen strains of bacteria were isolated from activated sludge purifying petroleum-refining wastewaters. These strains were plated on solidified mineral medium supplemented with oil fraction in concentration 1000 mg/l. Four of the strains that grew best in the presence of oil were selected for further studies. The strains were identified based on Bonde's scheme and microscopic observations. Three of them belonged to the genus Arthrobacter and one to the genus Micrococcus. Stationary cultures of single strains and their mixtures were set up in mineral medium containing oil (sterile and non-sterile) as sole carbon source in concentration 1000 mg/l. The oils were found to be removed the most efficiently by a mixture of the strains. After 14 days of culture the amount of oil was utilized by from 63 to 95%. In the next stage of the studies the bacteria were used to inoculate activated sludge. Stationary cultures of the activated sludge were set up in mineral medium with oil. The utilisation of petroleum products by non-inoculated activated sludge (control), activated sludge inoculated with a single strain or a mixture of all four strains was examined. In both inoculated activated sludge cultures approximately 80% of the oils were removed, compared to 60% in the control activated sludge. Therefore, inoculated activated sludge showed 20% higher effectiveness of removal of petroleum derivatives.     

Sulfate Reduction Dynamics and Enumeration of Sulfate-Reducing Bacteria in Hypersaline Sediments of the Great Salt Lake (Utah, USA)

Bacterial sulfate reduction activity (SRA) was measured in surface sediments and slurries from three sites in the Great Salt Lake (Utah, USA) using radiolabeled 35S-sulfate. High rates of sulfate reduction (363 ± 103 and 6,131 ± 835 nmol cm-3 d-1) were measured at two sites in the moderately hypersaline southern arm of the lake, whereas significantly lower rates (32 ± 9 nmol cm-3 d-1) were measured in the extremely hypersaline northern arm. Bacterial sulfate reduction was strongly affected by salinity and showed an optimum around 5-6% NaCl in the southern arm and an optimum of around 12% NaCl in the more hypersaline northern arm of the lake. High densities of sulfate-reducing bacteria (SRB) ranging from 2.2 × 107 to 6.7 × 108 cells cm-3 were determined by a newly developed tracer MPN-technique (T-MPN) employing sediment media and 35S-sulfate. Calculation of specific sulfate reduction rates yielded values comparable to those obtained in pure cultures of SRB. However, when using a conventional MPN technique with synthetic media containing high amounts of Fe(II), the numbers of SRB were underestimated by 1-4 orders of magnitude as compared to the T-MPN method. Our results suggest that high densities of slightly to moderately halophilic and extremely halotolerant SRB are responsible for the high rates of sulfate reduction measured in Great Salt Lake sediments.     

Activity and structure of the sulfate-reducing bacterial community in the sediments of the southern part of Lake Baikal

The rates of sulfate reduction (SR) and the diversity of sulfate-reducing bacteria (SRB) were studied in the sediments of the Posol’skaya Banka elevation in the southern part of Lake Baikal. SR rates varied from 1.2 to 1641 nmol/(dm³ day), with high rates (>600 nmol/(dm³ day)) observed at both deep-water stations and in subsurface silts. Integral SR rates calculated for the uppermost 50 cm of the sediments were higher for gas-saturated and gas hydrate-bearing sediments than in those with low methane content. Enrichment cultures were obtained in Widdel medium for freshwater SRB. Analysis of the 16S rRNA gene fragments from clone libraries obtained from the enrichments revealed the presence of SRB belonged to the genus Desulfosporosinus, with D. lacus as the most closely related member (capable of sulfate, sulfite, and thiosulfate reduction), as well as members of the order Clostridiales.     

Identification of sulfate-reducing bacteria in methylmercury-contaminated mine tailings by analysis of SSU rRNA genes

Sulfate-reducing bacteria (SRB) are often used in bioremediation of acid mine drainage because microbial sulfate reduction increases pH and produces sulfide that binds with metals. Mercury methylation has also been linked with sulfate reduction. Previous geochemical analysis indicated the occurrence of sulfate reduction in mine tailings, but no molecular characterization of the mine tailings-associated microbial community has determined which SRB are present. This study characterizes the bacterial communities of two geochemically contrasting, high-methylmercury mine tailing environments, with emphasis on SRB, by analyzing small subunit (SSU) rRNA genes present in the tailings sediments and in enrichment cultures inoculated with tailings. Novel Deltaproteobacteria and Firmicutes -related sequences were detected in both the pH-neutral gold mine tailings and the acidic high-sulfide base-metal tailings. At the subphylum level, the SRB communities differed between sites, suggesting that the community structure was dependent on local geochemistry. Clones obtained from the gold tailings and enrichment cultures were more similar to previously cultured isolates whereas clones from acidic tailings were more closely related to uncultured lineages identified from other acidic sediments worldwide. This study provides new insights into the novelty and diversity of bacteria colonizing mine tailings, and identifies specific organisms that warrant further investigation with regard to their roles in mercury methylation and sulfur cycling in these environments.     

The Geomicrobiological Role of Sulphate-Reducing Bacteria in Environments Contaminated by Petroleum Products

This paper reviews the geomicrobiological role of sulphate-reducing bacteria (SRB) in environments contaminated with petroleum products and describes the habitats of SRB and their capacity for bioremediation in anaerobic conditions. Moreover, the participation of SRB in biocorrosion and formation of different minerals and sediments is discussed.     

Depth profile of sulfate-reducing bacterial ribosomal RNA and mercury methylation in an estuarine sediment

Abstract: The community structure of complex anaerobic microbial communities has been difficult to elucidate because of an inability to cultivate most of the contributing populations. In this study, the distribution of sulfate-reducing bacteria (SRB) in anaerobic sediments was determined using oligonucleotide probes complementary to the 16S ribosomal RNAs of major phylogenetic groups. Sediment cores were collected from Santa Rosa Sound in northwest Florida, and sectioned by depth into 1 to 2 cm fractions. Nucleic acids were extracted from each fraction and hybridized with the SRB-specific ribosomal RNA probes. SRB ribosomal RNAs accounted for almost 5% of the microbial community ribosomal RNA pool in the 3–4 cm depth fraction and were dominated by Desulfovibrionaceae ribosomal RNA. The SRB ribosomal RNA peak coincided with mercury methylation, an activity attributed to SRB. Profiles of the ribosomal RNAs indicate that SRB populations in sediments are stratified by depth.     

Sulfate-reducing bacteria methylate mercury at variable rates in pure culture and in marine sediments

Differences in methylmercury (CH(3)Hg) production normalized to the sulfate reduction rate (SRR) in various species of sulfate-reducing bacteria (SRB) were quantified in pure cultures and in marine sediment slurries in order to determine if SRB strains which differ phylogenetically methylate mercury (Hg) at similar rates. Cultures representing five genera of the SRB (Desulfovibrio desulfuricans, Desulfobulbus propionicus, Desulfococcus multivorans, Desulfobacter sp. strain BG-8, and Desulfobacterium sp. strain BG-33) were grown in a strictly anoxic, minimal medium that received a dose of inorganic Hg 120 h after inoculation. The mercury methylation rates (MMR) normalized per cell were up to 3 orders of magnitude higher in pure cultures of members of SRB groups capable of acetate utilization (e.g., the family Desulfobacteriaceae) than in pure cultures of members of groups that are not able to use acetate (e.g., the family Desulfovibrionaceae). Little or no Hg methylation was observed in cultures of Desulfobacterium or Desulfovibrio strains in the absence of sulfate, indicating that Hg methylation was coupled to respiration in these strains. Mercury methylation, sulfate reduction, and the identities of sulfate-reducing bacteria in marine sediment slurries were also studied. Sulfate-reducing consortia were identified by using group-specific oligonucleotide probes that targeted the 16S rRNA molecule. Acetate-amended slurries, which were dominated by members of the Desulfobacterium and Desulfobacter groups, exhibited a pronounced ability to methylate Hg when the MMR were normalized to the SRR, while lactate-amended and control slurries had normalized MMR that were not statistically different. Collectively, the results of pure-culture and amended-sediment experiments suggest that members of the family Desulfobacteriaceae have a greater potential to methylate Hg than members of the family Desulfovibrionaceae have when the MMR are normalized to the SRR. Hg methylation potential may be related to genetic composition and/or carbon metabolism in the SRB. Furthermore, we found that in marine sediments that are rich in organic matter and dissolved sulfide rapid CH(3)Hg accumulation is coupled to rapid sulfate reduction. The observations described above have broad implications for understanding the control of CH(3)Hg formation and for developing remediation strategies for Hg-contaminated sediments.     

Diversity of bacteria and Archaea in sulphate-reducing enrichment cultures inoculated from serial dilution of Zostera noltii rhizosphere samples

We have analysed the diversity of culturable sulphate-reducing bacteria (SRB) in Zostera noltii colonized sediments from Bassin d'Arcachon (France). Four organic substrates have been tested as well as the combination of H2 and CO2 to select for lithotrophic SRB. All energy sources were supplied in parallel cultures that were amended with yeast extract plus NH4+ and prepared without a source of combined nitrogen, the latter to select for diazotrophic SRB. The 10 different enrichment media were inoculated from serial dilution of rhizosphere samples. The highest dilution cultures yielding positive growth (i.e. 10-7) were studied by molecular techniques (16S rDNA clone libraries, RISA and ARDRA). Lactate as a single organic substrate in combination with a source of combined nitrogen resulted in selection of members of the Desulfovibrionaceae. Surprisingly, when lactate was added without a source of combined nitrogen, Desulfobacteriaceae were selected. A strong influence of the presence or absence of combined nitrogen was also observed for the substrates sucrose and fructose. Whereas the liquid culture growing on sucrose and NH4+ systematically yielded 16S rDNA clones related to an environmental unidentified green sulphur bacterium (OPS185), on plates we were able to isolate a SRB related to Desulfovibrio dechloracetivorans, which likely represents a non-described species. Under diazotrophic conditions, sucrose selected for SRB clones related to the cluster formed by Desulfovibrio zosterae, Desulfovibrio salexigens and Desulfovibrio bastinii. The corresponding isolate obtained on plates showed only low sequence similarity with this closest neighbour (93.8%), and we suggest that it also represents a non-described species. Surprisingly, a 16S rDNA sequence corresponding to an archaeon, i.e. a non-extremophile Crenoarchaeota, was retrieved from several of the SRB enrichment cultures even after subsequent transfers.     

Selection of Mycobacterium sp. strains with capacity to biotransform high concentrations of β-sitosterol

In this work, phytosterol-biotransforming strains were selected from Mycobacterium sp., using a high concentration of beta-sitosterol. The selection was made by culturing the strains in a medium enriched with 14 g beta-sitosterol/l as the unique source of carbon. During 2 months, the bacterial cultures were transferred successively. The extraction of the biotransformation products was made with methanol and ethyl acetate. The qualitative and quantitative analysis was made by means of thin-layer chromatography, gas-liquid chromatography (GLC) and GLC-mass spectrometry. Under these conditions, it was observed that after seven transfers, the strains MYcobacterium sp. MB-3683 and the Mycobacterium fortuitum B-11045 increased their biotransformation capacity from 20% to 64% and from 34% to 55%, respectively. The products in the highest proportion identified for each trial were androstenedione and androstadienedione. The results suggest that the high substrate concentration could be a selective mechanism to obtain strains more efficient in the biotransformation of beta-sitosterol into steroidal bases.     

Detection and Enumeration of Sulphate-Reducing Bacteria in Estuarine Sediments by Competitive PCR

The distribution of sulphate-reducing bacteria (SRB) in the sediments of the Colne River estuary, Essex, UK covering different saline concentrations of sediment porewater was investigated by the use of quantitative competitive PCR. Here, we show that a new PCR primer set and a new quantitative method using PCR are useful tools for the detection and the enumeration of SRB in natural environments. A PCR primer set selective for the dissimilatory sulphite reductase gene (dsr) of SRB was designed. PCR amplification using the single set of dsr-specific primers resulted in PCR products of the expected size from all 27 SRB strains tested, including Gram-negative and positive species. Sixty clones derived from sediment DNA using the primers were sequenced and all were closely related with the predicted dsr of SRB. These results indicate that PCR using the newly designed primer set are useful for the selective detection of SRB from a natural sample. This primer set was used to estimate cell numbers by dsr selective competitive PCR using a competitor, which was about 20% shorter than the targeted region of dsr. This procedure was applied to sediment samples from the River Colne estuary, Essex, UK together with simultaneous measurement of in situ rates of sulphate reduction. High densities of SRB ranging from 0.2 ? 5.7 × 10⁸ cells ml^{? 1} wet sediment were estimated by the competitive PCR assuming that all SRB have a single copy of dsr. Using these estimates cell specific sulphate reduction rates of 10^{? 17} to 10^{? 15} mol of SO₄ ^{2 ?} cell^{? 1} day^{? 1} were calculated, which is within the range of, or lower than, those previously reported for pure cultures of SRB. Our results show that the newly developed competitive PCR technique targeted to dsr is a powerful tool for rapid and reproducible estimation of SRB numbers in situ and is superior to the use of culture-dependent techniques.     

Methylmercury and methane production potentials in North Carolina Piedmont stream sediments

Methylated mercury (MeHg) can be produced by all microbes possessing the genes hgcA and hgcB, which can include sulfate-reducing bacteria (SRB), iron-reducing bacteria (FeRB), methane-producing archaea (MPA), and other anaerobic microbes. These microbial groups compete for substrates, including hydrogen and acetate. When sulfate is in excess, SRB can outcompete other anaerobic microbes. However, low concentrations of sulfate, which often occur in stream sediments, are thought to reduce the relative importance of SRB. Although SRB are regarded as the primary contributors of MeHg in many aquatic environments, their significance may not be universal, and stream sediments are poorly studied with respect to microbial Hg methylation. We evaluated suppression of methanogenesis by SRB and the potential contributions from SRB, MPA and other MeHg producing microbes (including FeRB) to the production of MeHg in stream sediments from the North Carolina Piedmont region. Lower methanogenesis rates were observed when SRB were not inhibited, however, application of a sulfate-reduction inhibitor stimulated methanogenesis. Greater MeHg production occurred when SRB were active. Other MeHg producing microbes (i.e., FeRB) contributed significantly less MeHg production than SRB. MPA produced MeHg in negligible amounts. Our results suggest that SRB are responsible for the majority of MeHg production and suppress methanogenesis in mid-order stream sediments, similar to other freshwater sediments. Further investigation is needed to evaluate the generality of these findings to streams in other regions, and to determine the mechanisms regulating sulfate and electron acceptor availability and other potential factors governing Hg methylation and methane production in stream sediments.     

Spatial variability of sulfate reduction in a shallow aquifer

The distribution and metabolic activity of sulfate-reducing bacteria (SRB) in a shallow, suboxic aquifer were studied. A radioimaging technique was used to visualize and quantify the activity of sulfate reducers in sediments at a centimetre-level scale. The distribution of SRB metabolic activity was heterogeneous with areas showing little activity far outnumbering areas with high activity. Variation in sulfate-reducing activity was not statistically correlated with variation in depth, bacterial numbers, or the following sediment properties: sediment type (sand, peat or silt), grain size, permeability and hydraulic conductivity. Sulfate-reducing bacteria activity did vary significantly with sediment porosity (multivariate analysis, r = 0.48). We hypothesized that the small pore sizes associated with sediments with low porosity restricted the ability of SRB to grow to high numbers as well as their access to nutrients. To further explore the relationship between pore size and microbial metabolic activity, columns with varying pore diameters were constructed. Sulfate-reducing bacteria in the columns with the smallest pore diameters had the lowest rates of metabolism and SRB metabolic rates increased as the pore diameter increased. For the aquifer studied, sediment porosities and pore sizes were the main factor controlling SRB activity.     

Diversity of sulfate-reducing bacteria from an extreme hypersaline sediment, Great Salt Lake (Utah)

The diversity of sulfate-reducing bacteria (SRB) inhabiting the extreme hypersaline sediment (270 g L(-1) NaCl) of the northern arm of Great Salt Lake was studied by integrating cultivation and genotypic identification approaches involving PCR-based retrieval of 16S rRNA and dsrAB genes, the latter encoding major subunits of dissimilatory (bi) sulfite reductase. The majority (85%) of dsrAB sequences retrieved directly from the sediment formed a lineage of high (micro) diversity affiliated with the genus Desulfohalobium, while others represented novel lineages within the families Desulfohalobiaceae and Desulfobacteraceae or among Gram-positive SRB. Using the same sediment, SRB enrichment cultures were established in parallel at 100 and at 190 g L(-1) NaCl using different electron donors. After 5-6 transfers, dsrAB and 16S rRNA gene-based profiling of these enrichment cultures recovered a SRB community composition congruent with the cultivation-independent profiling of the sediment. Pure culture representatives of the predominant Desulfohalobium-related lineage and of one of the Desulfobacteraceae-affilated lineages were successfully obtained. The growth performance of these isolates and of the enrichment cultures suggests that the sediment SRB community of the northern arm of Great Salt Lake consists of moderate halophiles, which are salt-stressed at the in situ salinity of 27%.     

Biotransformation of geraniol, nerol and citral by sporulated surface cultures of Aspergillus niger and Penicillium sp

The biotransformation of geraniol, nerol and citral by Aspergillus niger was studied. A comparison was made between submerged liquid, sporulated surface cultures and spore suspensions. This bioconversion was also carried out with surface cultures of Penicillium sp. The main bioconversion products obtained from geraniol and nerol by liquid cultures of A. niger were linalool and alpha-terpineol. Linalool, alpha-terpineol and limonene were the main products obtained from nerol and citral by sporulated surface cultures, whereas geraniol was converted predominantly to linalool, also resulting in higher yields. Bioconversion of nerol with Penicillium chrysogenum yielded mainly alpha-terpineol and some unidentified compounds. With P. rugulosum the major bioconversion product from nerol and citral was linalool. The bioconversion of nerol to alpha-terpineol and linalool by spore suspensions of A. niger was also investigated. Finally the biotransformation with sporulated surface cultures was also monitored by solid phase microextraction (SPME). It was found that SPME is a very fast and efficient screening technique for biotransformation experiments.     

Identity and abundance of active sulfate-reducing bacteria in deep tidal flat sediments determined by directed cultivation and CARD-FISH analysis

The identity and abundance of potentially active sulfate-reducing bacteria (SRB) in several metre deep sediments of a tidal sand flat in the German Wadden Sea were assessed by directed cultivation and cultivation-independent CARD-FISH analysis (catalysed reporter deposition fluorescence in situ hybridization). Presumably abundant SRB from different sediment layers between 0.5 and 4 m depth were selectively enriched in up to million-fold diluted cultures supplemented with lactate, acetate or hydrogen. Partial 16S rRNA gene sequences obtained from highest dilution steps showing sulfide formation indicated growth of deltaproteobacterial SRB belonging to the Desulfobulbaceae and the Desulfobacteraceae as well as of members of the Firmicutes. Subsequent isolation resulted in 10 novel phylotypes of both litho- and organotrophic sulfate-reducing Deltaproteobacteria. Molecular pre-screening identified six isolates as members of the Desulfobulbaceae, sharing highest identities with either candidatus 'Desulfobacterium corrodens' (95-97%) or Desulfobacterium catecholicum (98%), and four isolates as members of Desulfobacteraceae, being related to either Desulfobacter psychrotolerans (98%) or Desulfobacula phenolica (95-97%). Relatives of D. phenolica were exlusively isolated from 50 and 100 cm deep sediments with 10 and 2 mM of pore water sulfate respectively. In contrast, relatives of D. corrodens, D. psychrotolerans and D. catecholicum were also obtained from layers deeper than 100 cm and with less than 2 mM sulfate. The high in situ abundance of members of both families in sediment layers beneath 50 cm could be confirmed via CARD-FISH analysis performed with a set of six SRB-specific oligonucleotide probes. Moreover, SRB represented a numerically significant fraction of the microbial community throughout the sediment (up to 7%) and reached even higher cell numbers in deep, sulfate-poor layers than in the sulfate-rich surface sediment. This relatively large community size of potentially active SRB in deep sandy sediments might on the one hand be a result of their syntrophic association with other anaerobes. Our results furthermore support the hypothesis that enhanced advective pore water transport might supply nutrients to microbial communities in deep sandy sediments and point to their so far unrecognized contribution to biogeochemical processes in Wadden Sea sediments.     

Diversity of dimethylsulfide-degrading methanogens and sulfate-reducing bacteria in anoxic sediments along the Medway Estuary,UK

Methane is a powerful greenhouse gas but the microbial diversity mediating methylotrophic methanogenesis is not well-characterized. One overlooked route to methane is via the degradation of dimethylsulfide (DMS), an abundant organosulfur compound in the environment. Methanogens and sulfate-reducing bacteria (SRB) can degrade DMS in anoxic sediments depending on sulfate availability. However, we know little about the underlying microbial community and how sulfate availability affects DMS degradation in anoxic sediments. We studied DMS-dependent methane production along the salinity gradient of the Medway Estuary (UK) and characterized, for the first time, the DMS-degrading methanogens and SRB using cultivation-independent tools. DMS metabolism resulted in high methane yield (39%–42% of the theoretical methane yield) in anoxic sediments regardless of their sulfate content. Methanomethylovorans, Methanolobus and Methanococcoides were dominant methanogens in freshwater, brackish and marine incubations respectively, suggesting niche-partitioning of the methanogens likely driven by DMS amendment and sulfate concentrations. Adding DMS also led to significant changes in SRB composition and abundance in the sediments. Increases in the abundance of Sulfurimonas and SRB suggest cryptic sulfur cycling coupled to DMS degradation. Our study highlights a potentially important pathway to methane production in sediments with contrasting sulfate content and sheds light on the diversity of DMS degraders.