Microbiological Processes in a High-Temperature Oil Field

Thermophilic sulfate-reducing bacteria (SRB) oxidizing lactate, butyrate, and C₁₂–C₁₆ n-alkanes of oil at a temperature of 90°C were isolated from samples of water and oil originating from oil reservoirs of the White Tiger high-temperature oil field (Vietnam). At the same time, no thermophiles were detected in the injected seawater, which contained mesophilic microorganisms and was the site of low-temperature processes of sulfate reduction and methanogenesis. Thermophilic SRB were also found in samples of liquid taken from various engineering reservoirs used for oil storage, treatment, and transportation. These samples also contained mesophilic SRB, methanogens, aerobic oil-oxidizing bacteria, and heterotrophs. Rates of bacterial production of hydrogen sulfide varied from 0.11 to 2069.63 at 30°C and from 1.18 to 173.86 at 70°C g S/(l day); and those of methane production, varied from 58.4 to 100 629.8 nl CH₄/(l day) (at 30°C). The sulfur isotopic compositions of sulfates contained in reservoir waters and of hydrogen sulfide of the accompanying gas indicate that bacterial sulfate reduction might be effective in the depth of the oil field.     

Identification and Phylogenetic analysis of thermophilic sulfate-reducing bacteria in oil field samples by 16S rDNA gene cloning and sequencing

Thermophilic sulfate-reducing bacteria (SRB) have been recognized as an important source of hydrogen sulfide (H2S) in hydrocarbon reservoirs and in production systems. Four thermophilic SRB enrichment cultures from three different oil field samples (sandstone core, drilling mud, and production water) were investigated using 16S rDNA sequence comparative analysis. In total, 15 different clones were identified. We found spore-forming, low G+C content, thermophilic, sulfate-reducing Desulfotomaculum-related sequences present in all oil field samples, and additionally a clone originating from sandstone core which was assigned to the mesophilic Desulfomicrobium group. Furthermore, three clones related to Gram-positive, non-sulfate-reducing Thermoanaerobacter species and four clones close to Clostridium thermocopriae were found in enrichment cultures from sandstone core and from production water, respectively. In addition, the deeply rooted lineage of two of the clones suggested previously undescribed, Gram-positive, low G+C content, thermophilic, obligately anaerobic bacteria present in production water. Such thermophilic, non-sulfate-reducing microorganisms may play an important ecological role alongside SRB in oil field environments.     

Acetate Production from Oil under Sulfate-Reducing Conditions in Bioreactors Injected with Sulfate and Nitrate

Oil production by water injection can cause souring in which sulfate in the injection water is reduced to sulfide by resident sulfate-reducing bacteria (SRB). Sulfate (2 mM) in medium injected at a rate of 1 pore volume per day into upflow bioreactors containing residual heavy oil from the Medicine Hat Glauconitic C field was nearly completely reduced to sulfide, and this was associated with the generation of 3 to 4 mM acetate. Inclusion of 4 mM nitrate inhibited souring for 60 days, after which complete sulfate reduction and associated acetate production were once again observed. Sulfate reduction was permanently inhibited when 100 mM nitrate was injected by the nitrite formed under these conditions. Pulsed injection of 4 or 100 mM nitrate inhibited sulfate reduction temporarily. Sulfate reduction resumed once nitrate injection was stopped and was associated with the production of acetate in all cases. The stoichiometry of acetate formation (3 to 4 mM formed per 2 mM sulfate reduced) is consistent with a mechanism in which oil alkanes and water are metabolized to acetate and hydrogen by fermentative and syntrophic bacteria (K. Zengler et al., Nature 401:266–269, 1999), with the hydrogen being used by SRB to reduce sulfate to sulfide. In support of this model, microbial community analyses by pyrosequencing indicated SRB of the genus Desulfovibrio, which use hydrogen but not acetate as an electron donor for sulfate reduction, to be a major community component. The model explains the high concentrations of acetate that are sometimes found in waters produced from water-injected oil fields.     

Oil field souring control by nitrate-reducing Sulfurospirillum spp. that outcompete sulfate-reducing bacteria for organic electron donors

Nitrate injection into oil reservoirs can prevent and remediate souring, the production of hydrogen sulfide by sulfate-reducing bacteria (SRB). Nitrate stimulates nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) and heterotrophic nitrate-reducing bacteria (hNRB) that compete with SRB for degradable oil organics. Up-flow, packed-bed bioreactors inoculated with water produced from an oil field and injected with lactate, sulfate, and nitrate served as sources for isolating several NRB, including Sulfurospirillum and Thauera spp. The former coupled reduction of nitrate to nitrite and ammonia with oxidation of either lactate (hNRB activity) or sulfide (NR-SOB activity). Souring control in a bioreactor receiving 12.5 mM lactate and 6, 2, 0.75, or 0.013 mM sulfate always required injection of 10 mM nitrate, irrespective of the sulfate concentration. Community analysis revealed that at all but the lowest sulfate concentration (0.013 mM), significant SRB were present. At 0.013 mM sulfate, direct hNRB-mediated oxidation of lactate by nitrate appeared to be the dominant mechanism. The absence of significant SRB indicated that sulfur cycling does not occur at such low sulfate concentrations. The metabolically versatile Sulfurospirillum spp. were dominant when nitrate was present in the bioreactor. Analysis of cocultures of Desulfovibrio sp. strain Lac3, Lac6, or Lac15 and Sulfurospirillum sp. strain KW indicated its hNRB activity and ability to produce inhibitory concentrations of nitrite to be key factors for it to successfully outcompete oil field SRB.     

高温油藏生物膜内硫酸盐还原菌的腐蚀机理及防治策略

硫酸盐还原菌(sulfate-reducing bacteria,SRB)广泛分布于高温、高压及高盐的石油油藏中,在油藏硫循环中起主导作用。SRB能在油藏生物膜内生长,有微量低分子有机酸时利用硫酸盐为电子受体并将其还原成硫化氢。硫化氢会腐蚀管道,导致原油泄露等其他安全问题,每年造成的经济损失超过7 000亿元。本文首先总结了油藏生物膜内微生物菌群多样性,分析了生物膜内SRB及其相关菌群的协同腐蚀机理;然后讨论了高温油藏SRB介导的硫氮氢生物地球化学循环过程、胞外电子传递机制及其腐蚀作用,并通过几个高温油藏SRB生物膜内腐蚀的现场案例进一步阐明了SRB的腐蚀机制。在此基础上,提出了应对高温油藏生物膜内SRB腐蚀的生物纳米防治策略,这为高温油藏管道防腐提供了新思路。     

Oil Field Souring Control by Nitrate-Reducing Sulfurospirillum spp. That Outcompete Sulfate-Reducing Bacteria for Organic Electron Donors

Nitrate injection into oil reservoirs can prevent and remediate souring, the production of hydrogen sulfide by sulfate-reducing bacteria (SRB). Nitrate stimulates nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) and heterotrophic nitrate-reducing bacteria (hNRB) that compete with SRB for degradable oil organics. Up-flow, packed-bed bioreactors inoculated with water produced from an oil field and injected with lactate, sulfate, and nitrate served as sources for isolating several NRB, including Sulfurospirillum and Thauera spp. The former coupled reduction of nitrate to nitrite and ammonia with oxidation of either lactate (hNRB activity) or sulfide (NR-SOB activity). Souring control in a bioreactor receiving 12.5 mM lactate and 6, 2, 0.75, or 0.013 mM sulfate always required injection of 10 mM nitrate, irrespective of the sulfate concentration. Community analysis revealed that at all but the lowest sulfate concentration (0.013 mM), significant SRB were present. At 0.013 mM sulfate, direct hNRB-mediated oxidation of lactate by nitrate appeared to be the dominant mechanism. The absence of significant SRB indicated that sulfur cycling does not occur at such low sulfate concentrations. The metabolically versatile Sulfurospirillum spp. were dominant when nitrate was present in the bioreactor. Analysis of cocultures of Desulfovibrio sp. strain Lac3, Lac6, or Lac15 and Sulfurospirillum sp. strain KW indicated its hNRB activity and ability to produce inhibitory concentrations of nitrite to be key factors for it to successfully outcompete oil field SRB.     

Hydrogenogenic CO conversion in a moderately thermophilic (55 degrees C) sulfate-fed gas lift reactor: competition for CO-derived H(2)

Thermophilic (55 degrees C) sulfate reduction in a gas lift reactor fed with CO gas as the sole electron donor was investigated. The reactor was inoculated with mesophilic granular sludge with a high activity of CO conversion to hydrogen and carbon dioxide at 55 degrees C. Strong competition for H(2) was observed between methanogens and sulfate reducers, while the homoacetogens present consumed only small amounts of H(2). The methanogens appeared to be more sensitive to pH and temperature shocks imposed to the reactor, but could not be completely eliminated. The fast growth rates of the methanogens (generation time of 4.5 h) enabled them to recover fast from shocks, and they rapidly consumed more than 90% of the CO-derived H(2). Nevertheless, steep increases in sulfide production in periods with low methane production suggests that once methanogenesis is eliminated, sulfate reduction with CO-rich gas as electron donor has great potential for thermophilic biodesulfurization.     

The influence of nitrate on microbial processes in oil industry production waters

Sulfide accumulation due to bacterial sulfate reduction is responsible for a number of serious problems in the oil industry. Among the strategies to control the activity of sulfate-reducing bacteria (SRB) is the use of nitrate, which can exhibit a variety of effects. We investigated the relevance of this approach to souring oil fields in Oklahoma and Alberta in which water flooding is used to enhance oil recovery. SRB and nitrate-reducing bacteria (NRB) were enumerated in produced waters from both oil fields. In the Oklahoma field, the rates of sulfate reduction ranged from 0.05 to 0.16 μM S day⁻¹ at the wellheads, and an order of magnitude higher at the oil–water separator. Sulfide production was greatest in the water storage tanks in the Alberta field. Microbial counts alone did not accurately reflect the potential for microbial activities. The majority of the sulfide production appeared to occur after the oil was pumped aboveground, rather than in the reservoir. Laboratory experiments showed that adding 5 and 10 mM nitrate to produced waters from the Oklahoma and Alberta oil fields, respectively, decreased the sulfide content to negligible levels and increased the numbers of NRB. This work suggests that sulfate reduction control measures can be concentrated on aboveground facilities, which will decrease the amount of sulfide reinjected into reservoirs during the disposal of oil field production waters. Journal of Industrial Microbiology & Biotechnology (2001) 27, 80–86. Received 30 January 2001/ Accepted in revised form 30 June 2001     

Thermophilic (55-65 degrees C) and extreme thermophilic (70-80 degrees C) sulfate reduction in methanol and formate-fed UASB reactors

The feasibility of thermophilic (55-65 degrees C) and extreme thermophilic (70-80 degrees C) sulfate-reducing processes was investigated in three lab-scale upflow anaerobic sludge bed (UASB) reactors fed with either methanol or formate as the sole substrates and inoculated with mesophilic granular sludge previously not exposed to high temperatures. Full methanol and formate degradation at temperatures up to, respectively, 70 and 75 degrees C, were achieved when operating UASB reactors fed with sulfate rich (COD/SO4(2-)=0.5) synthetic wastewater. Methane-producing archaea (MPA) outcompeted sulfate-reducing bacteria (SRB) in the formate-fed UASB reactor at all temperatures tested (65-75 degrees C). In contrast, SRB outcompeted MPA in methanol-fed UASB reactors at temperatures equal to or exceeding 65 degrees C, whereas strong competition between SRB and MPA was observed in these reactors at 55 degrees C. A short-term (5 days) temperature increase from 55 to 65 degrees C was an effective strategy to suppress methanogenesis in methanol-fed sulfidogenic UASB reactors operated at 55 degrees C. Methanol was found to be a suitable electron donor for sulfate-reducing processes at a maximal temperature of 70 degrees C, with sulfide as the sole mineralization product of methanol degradation at that temperature.     

Effect of hydrogen sulfide on growth of sulfate reducing bacteria

A culture of sulfate reducing bacteria (SRB) growing on lactate and sulfate was incubated at different pH values in the range of 5.8-7.0. The effect of pH on growth rate was determined in this pH range; the highest growth rate was observed at pH 6.7. Hydrogen sulfide produced from sulfate reduction was found to have a direct and reversible toxicity effect on the SRB. A hydrogen sulfide Concentration of 547 mg/L (16.1 mM) completely inhibited the culture growth. Comparison between acetic acid and hydrogen sulfide inhibition is presented and the concomitant inhibition kinetics are mathematically described. (c) 1992 John Wiley & Sons, Inc.     

Design features of offshore oil production platforms influence their susceptibility to biocorrosion

Offshore oil-producing platforms are designed for efficient and cost-effective separation of oil from water. However, design features and operating practices may create conditions that promote the proliferation and spread of biocorrosive microorganisms. The microbial communities and their potential for metal corrosion were characterized for three oil production platforms that varied in their oil-water separation processes, fluid recycling practices, and history of microbially influenced corrosion (MIC). Microbial diversity was evaluated by 16S rRNA gene sequencing, and numbers of total bacteria, archaea, and sulfate-reducing bacteria (SRB) were estimated by qPCR. The rates of ³⁵S sulfate reduction assay (SRA) were measured as a proxy for metal biocorrosion potential. A variety of microorganisms common to oil production facilities were found, but distinct communities were associated with the design of the platform and varied with different locations in the processing stream. Stagnant, lower temperature (<37 °C) sites in all platforms had more SRB and higher SRA compared to samples from sites with higher temperatures and flow rates. However, high (5 mmol L^?1) levels of hydrogen sulfide and high numbers (10⁷ mL^?1) of SRB were found in only one platform. This platform alone contained large separation tanks with long retention times and recycled fluids from stagnant sites to the beginning of the oil separation train, thus promoting distribution of biocorrosive microorganisms. These findings tell us that tracking microbial sulfate-reducing activity and community composition on off-shore oil production platforms can be used to identify operational practices that inadvertently promote the proliferation, distribution, and activity of biocorrosive microorganisms.     

Biological souring of crude oil under anaerobic conditions

Seawater injection into oil reservoirs for purposes of secondary oil recovery is frequently accompanied by souring (increased sulfide concentrations). Production of hydrogen sulfide causes various problems, such as microbiologically influenced corrosion (MIC) and deterioration of crude oil. Sulfate-reducing bacteria (SRB) are considered to be major players in souring. Volatile fatty acids (VFAs) in oil-field water are believed to be produced by microbial degradation of crude oil. The objective of this research was to investigate mechanisms of souring, focusing specifically on VFA production via crude oil biodegradation. To this end, a microbial consortium collected from an oil–water separator was suspended in seawater; crude oil or liquid n-alkane mixture was added to the culture medium as the sole carbon source, and the culture was incubated under anaerobic conditions for 190 days. Physicochemical analysis showed that preferential toluene degradation and sulfate reduction occurred concomitantly in the culture containing crude oil. Sulfide concentrations were much lower in the alkane-supplemented culture than in the crude oil-supplemented culture. These observations suggest that SRB are related to the toluene activation and VFA consumption steps of crude oil degradation. Therefore, the electron donors for SRB are not only VFA, but many components of crude oil, especially toluene. Alkanes were also degraded by microorganisms, but did not contribute to reservoir souring.     

Microbiological processes in the Lost City vent field, mid-Atlantic ridge

Microbiological and biogeochemical measurements showed that the intensities of CO2 assimilation, methane oxidation, and sulfate reduction in the Lost City vent field (30 degrees N) reach 3.8 microg C/(1 day), 0.06 microg C/(1 day), and 117 microg S/(1 day), respectively. On the surface of the carbonate structures occurring in this field, two varieties of bacterial mats were found. The first variety, which is specific to the Lost City alkaline vent field, represents jelly bacterial mats dominated by slime-producing bacteria of several morphotypes. This mat variety also contains chemolithotrophic and heterotrophic microorganisms, either microaerobic or anaerobic. The intensities of CO2 assimilation, methane oxidation, and sulfate reduction in this variety reach 747 microg C/(dm3 day), 0.02 microg C/(dm3 day), and 28,000 microg S/(dm3 day), respectively. Bacterial mats of the second variety are formed by nonpigmented filamentous sulfur bacteria, which are close morphologically to Thiothrix. The intensities of CO2 assimilation, methane oxidation, and sulfate reduction in the second mat variety reach 8.2 microg C/(dm3 day), 5.8 microg C/(dm3 day), and 17,000 microg S/(dm3 day), respectively. These data suggest the existence of subsurface microflora in the Lost City vent field.     

Microbiological processes at the interface of aerobic and anaerobic waters in the deep-water zone of the Black Sea

Chemical and key microbiological processes (assimilation of carbon dioxide, oxidation and formation of methane, and sulfate reduction) occurring at the boundary between the aerobic-anaerobic interface in the deep-water zone of the Black Sea were investigated. Measurements were taken at depths from 90 to 300 m at intervals of 5-10 m. The integral rate of the dark assimilation of carbon dioxide varied from 120 to 207 mg C/(m2 day) with a maximum at the boundary of cyclonic currents. The organic matter (OM) formed from methane comprised less than 5% of the OM formed from carbon dioxide. A comparison between the rates of methane oxidation and methane production suggests that methane that is oxidized at depths from 100 to 300 m was formed in deeper water horizons. The maximum rate of sulfate reduction (1230 mg S/(m2 day)) was observed in the western halistatic region, and the minimum rate (490 mg S/(m2 day)), in the eastern halistatic region. The average rate of hydrogen sulfide production measured at three deep-sea stations amounted to 755 mg S/(m2 day), or 276 g S/(m2 year).     

Microbial prevalence in domestic humidifiers.

The prevalence of viable thermophilic bacteria and actinomycetes and mesophilic fungi was examined in 145 samples from 110 domestic humidifiers. A total of 72 and 43% of furnace and console humidifier samples, respectively, contained viable thermophilic bacteria, whereas 60 and 72% of these samples produced mesophilic fungal growth. Thermophilic actinomycetes were recovered from seven humidifier samples. Efforts to detect thermophilic actinomycete antigens in 15 humidifier fluid samples were not successful. Antifoulants added to humidifier fluid reservoirs had no apparent effect on microbial frequency. Airborne microbial recoveries did not reflect patterns of humidifier contamination with respect to either kinds or numbers of microorganisms in 20 homes in which volumetric air samples were obtained during humidifier operation.     

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.     

Sulfidogenic fluidized-bed treatment of metal-containing wastewater at low and high temperatures

The applicability of a fluidized-bed reactor (FBR)-based sulfate reducing bioprocess was investigated for the treatment of iron-containing (40-90 mg/L) acidic wastewater at low (8 degrees C) and high (65 degrees C) temperatures. The FBRs operated at low and high temperatures were inoculated with cultures of sulfate-reducing bacteria (SRB) originally enriched from arctic and hot mining environments, respectively. Ethanol was supplemented as carbon and electron source for SRB. At 8 degrees C, ethanol oxidation and sulfate reduction rates increased steadily and reached 320 and 265 mg/L.day, respectively, after 1 month of operation. After this point, the rates did not change significantly during 130 days of operation. Despite the complete ethanol oxidation and iron precipitation, the average sulfate reduction efficiency was 35 +/- 4% between days 30 and 130 due to the accumulation of acetate. At 65 degrees C, a rapid startup was observed as 99.9, 46, and 29% ethanol, sulfate, acetate removals, in respective order, were observed after 6 days. The feed pH was decreased gradually from its initial value of 6 to around 3.7 during 100 days of operation. The wastewater pH of 4.3-4.4 was neutralized by the alkalinity produced in acetate oxidation and the average effluent pH was 7.8 +/- 0.8. As in the low temperature FBR, acetate accumulated. Hence, the oxidation of acetate is the rate-limiting step in the sulfidogenic ethanol oxidation by thermophilic and psychrotrophic SRB. The sulfate reduction rate is three times and acetate oxidation rate is four times higher at 65 degrees C than at 8 degrees C.     

Effect of nitrate and nitrite on sulfide production by two thermophilic,sulfate-reducing enrichments from an oil field in the North Sea

Thermophilic sulfate-reducing bacteria (tSRB) can be major contributors to the production of H₂S (souring) in oil reservoirs. Two tSRB enrichments from a North Sea oil field, NS-tSRB1 and NS-tSRB2, were obtained at 58°C with acetate-propionate-butyrate and with lactate as the electron donor, respectively. Analysis by rDNA sequencing indicated the presence of Thermodesulforhabdus norvegicus in NS-tSRB1 and of Archaeoglobus fulgidus in NS-tSRB2. Nitrate (10 mM) had no effect on H₂S production by mid-log phase cultures of NS-tSRB1 and NS-tSRB2, whereas nitrite (0.25 mM or higher) inhibited sulfate reduction. NS-tSRB1 did not recover from inhibition, whereas sulfate reduction activity of NS-tSRB2 recovered after 500 h. Nitrite was also effective in souring inhibition and H₂S removal in upflow bioreactors, whereas nitrate was similarly ineffective. Hence, nitrite may be preferable for souring prevention in some high-temperature oil fields because it reacts directly with sulfide and provides long-lasting inhibition of sulfate reduction.     

Advances In Biotreatment of Acid Mine Drainage and Biorecovery of Metals: 2. Membrane Bioreactor System for Sulfate Reduction

Several biotreatmemt techniques for sulfate conversion by the sulfate reducing bacteria (SRB) have been proposed in the past, however few of them have been practically applied to treat sulfate containing acid mine drainage (AMD). This research deals with development of an innovative polypropylene hollow fiber membrane bioreactor system for the treatment of acid mine water from the Berkeley Pit, Butte, MT, using hydrogen consuming SRB biofilms. The advantages of using the membrane bioreactor over the conventional tall liquid phase sparged gas bioreactor systems are: large microporous membrane surface to the liquid phase; formation of hydrogen sulfide outside the membrane, preventing the mixing with the pressurized hydrogen gas inside the membrane; no requirement of gas recycle compressor; membrane surface is suitable for immobilization of active SRB, resulting in the formation of biofilms, thus preventing washout problems associated with suspended culture reactors; and lower operating costs in membrane bioreactors, eliminating gas recompression and gas recycle costs. Information is provided on sulfate reduction rate studies and on biokinetic tests with suspended SRB in anaerobic digester sludge and sediment master culture reactors and with SRB biofilms in bench-scale SRB membrane bioreactors. Biokinetic parameters have been determined using biokinetic models for the master culture and membrane bioreactor systems. Data are presented on the effect of acid mine water sulfate loading at 25, 50, 75 and 100 ml/min in scale-up SRB membrane units, under varied temperatures (25, 35 and 40 °C) to determine and optimize sulfate conversions for an effective AMD biotreatment. Pilot-scale studies have generated data on the effect of flow rates of acid mine water (MGD) and varied inlet sulfate concentrations in the influents on the resultant outlet sulfate concentration in the effluents and on the number of SRB membrane modules needed for the desired sulfate conversion in those systems. The pilot-scale data indicate that the SRB membrane bioreactors systems can be applied toward field-scale biotreatment of AMD and for recovery of high purity metals and an agriculturally usable water.     

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.