Massive microbial mats covering up to 4-meter-high carbonate buildups prosper at methane seeps in anoxic waters of the northwestern Black Sea shelf. Strong 13C depletions indicate an incorporation of methane carbon into carbonates, bulk biomass, and specific lipids. The mats mainly consist of densely aggregated archaea (phylogenetic ANME-1 cluster) and sulfate-reducing bacteria (Desulfosarcina/Desulfococcus group). If incubated in vitro, these mats perform anaerobic oxidation of methane coupled to sulfate reduction. Obviously, anaerobic microbial consortia can generate both carbonate precipitation and substantial biomass accumulation, which has implications for our understanding of carbon cycling during earlier periods of Earth's history.

Soda lakes are naturally occurring highly alkaline and saline environments. Although the sulfur cycle is one of the most active element cycles in these lakes, little is known about the sulfate-reducing bacteria (SRB). In this study we investigated the diversity, activity, and abundance of SRB in sediment samples and enrichment cultures from a range of (hyper)saline soda lakes of the Kulunda Steppe in southeastern Siberia in Russia. For this purpose, a polyphasic approach was used, including denaturing gradient gel electrophoresis of dsr gene fragments, sulfate reduction rate measurements, serial dilutions, and quantitative real-time PCR (qPCR). Comparative sequence analysis revealed the presence of several novel clusters of SRB, mostly affiliated with members of the order Desulfovibrionales and family Desulfobacteraceae. We detected sulfate reducers and observed substantial sulfate reducing rates (between 12 and 423 mol/dm 3 day ؊1 ) for most lakes, even at a salinity of 475 g/liter. Enrichments were obtained at salt saturating conditions (4 M Na ؉ ), using H 2 or volatile fatty acids as electron donors, and an extremely halophilic SRB, strain ASO3-1, was isolated. Furthermore, a high dsr gene copy number of 10 8 cells per ml was detected in a hypersaline lake by qPCR. Our results indicate the presence of diverse and active SRB communities in these extreme ecosystems.Soda lakes are extreme environments with pH values up to 11 and salt concentrations up to saturation. These high pH values are maintained due to the high buffering capacity of sodium carbonate/bicarbonate, which are among the major anions in solution. The salinity can vary from 5 to 30 g/liter for hyposaline lakes to up to 500 g/liter for hypersaline lakes. Despite these extreme conditions, soda lakes are highly productive and harbor diverse microbial communities (6,11,25,35) responsible for the element cycling.The sulfur cycle, driven by haloalkaliphilic sulfur-oxidizing bacteria (SOB) and sulfate-reducing bacteria (SRB), is one of the most active element cycles in soda lakes. SOB are represented by members of the class Gammaproteobacteria belonging to the genera Ectothiorodospira, Thioalkalivibrio, Thioalkalimicrobium, and Thioalkalispira. Bacteria belonging to the first genus are phototrophic sulfur purple bacteria, while the other three genera are obligate chemolithoautotrophs (30), utilizing various reduced inorganic sulfur compounds as electron donors. Members of the Ectothiorodospira and Thioalkalivibrio genera are particularly remarkable in their potential to grow in saturated alkaline brines.Although more than 100 SOB strains have been isolated from soda lakes (30), so far only four species of SRB, i.e., Desulfonatronovibrio hydrogenovorans (37), Desulfonatronum lacustre (22), Desulfonatronum thiodismutans (23), and Desulfonatronum cooperativum (38) have been isolated, and they are all low-salt-tolerant alkaliphiles. However, although Desulfonatronovibrio hydrogenovorans is a low-salt organism, it was isolated from Lake Magadi, a hypersalin...

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Here we describe the diversity and activity of sulfate reducing bacteria along a salinity gradient in four different soda lakes from the Kulunda Steppe (South East Siberia, Russia). For this purpose, a combination of culture-dependent and independent techniques was applied. The general bacterial and SRB diversity were analyzed by denaturing gradient gel electrophoresis (DGGE) targeting the 16S rDNA gene. DNA was used to detect the microbial populations that were present in the soda lake sediments, whereas ribosomal RNA was used as a template to obtain information on those that were active. Individual DGGE bands were sequenced and a phylogenetic analysis was performed. In addition, the overall activity of SRB was obtained by measuring the sulfate reduction rates (SRR) and their abundance was estimated by serial dilution. Our results showed the presence of minor, but highly active microbial populations, mostly represented by members of the Proteobacteria. Remarkably high SRR were measured at hypersaline conditions (200 g L @1 ). A relatively high viable count indicated that sulfate reducing bacteria could be highly active in hypersaline soda lakes. Furthermore, the increase of sodium carbonate/bicarbonate seemed to affect the composition of the microbial community in soda lakes, but not the rate of sulfate reduction.

Microbial communities of Kamchatka Peninsula terrestrial hot springs were studied using molecular, radioisotopic and cultural approaches. Analysis of 16S rRNA gene fragments performed by means of high-throughput sequencing revealed that aerobic autotrophic sulfur-oxidizing bacteria of the genus Sulfurihydrogenibium (phylum Aquificae) dominated in a majority of streamers. Another widely distributed and abundant group was that of anaerobic bacteria of the genus Caldimicrobium (phylum Thermodesulfobacteria). Archaea of the genus Vulcanisaeta were abundant in a high-temperature, slightly acidic hot spring, where they were accompanied by numerous Nanoarchaeota, while the domination of uncultured Thermoplasmataceae A10 was characteristic for moderately thermophilic acidic habitats. The highest rates of inorganic carbon assimilation determined by the in situ incubation of samples in the presence of C-labeled bicarbonate were found in oxygen-dependent streamers; in two sediment samples taken from the hottest springs this process, though much weaker, was found to be not dependent on oxygen. The isolation of anaerobic lithoautotrophic prokaryotes from Kamchatka hot springs revealed a wide distribution of the ability for sulfur disproportionation, a new lithoautotrophic process capable to fuel autonomous anaerobic ecosystems.