Biogeochemical, isotope geochemical and microbiological investigation of Lake Svetloe (White Sea basin), a meromictic freshwater was carried out in April 2014, when ice thickness was ∼0.5 m, and the ice-covered water column contained oxygen to 23 m depth. Below, the anoxic water column contained ferrous iron (up to 240 μμM), manganese (60 μM), sulfide (up to 2 μM) and dissolved methane (960 μM). The highest abundance of microbial cells revealed by epifluorescence microscopy was found in the chemocline (redox zone) at 23-24.5 m. Oxygenic photosynthesis exhibited two peaks: the major one (0.43 μmol C L day ) below the ice and the minor one in the chemocline zone, where cyanobacteria related to Synechococcus rubescens were detected. The maximum of anoxygenic photosynthesis (0.69 μmol C L day ) at the oxic/anoxic interface, for which green sulfur bacteria Chlorobium phaeoclathratiforme were probably responsible, exceeded the value for oxygenic photosynthesis. Bacterial sulfate reduction peaked (1.5 μmol S L day ) below the chemocline zone. The rates of methane oxidation were as high as 1.8 μmol CH L day at the oxi/anoxic interface and much lower in the oxic zone. Small phycoerythrin-containing Synechococcus-related cyanobacteria were probably involved in accumulation of metal oxides in the redox zone.

So far, anaerobic sulfate-dependent acetate oxidation at high pH has only been demonstrated for a low-salt-tolerant syntrophic association of a clostridium 'Candidatus Contubernalis alkalaceticum' and its hydrogenotrophic sulfate-reducing partner Desulfonatronum cooperativum. Anaerobic enrichments at pH 10 inoculated with sediments from hypersaline soda lakes of the Kulunda Steppe (Altai, Russia) demonstrated the possibility of sulfate-dependent acetate oxidation at much higher salt concentrations (up to 3.5 M total Na + ). The most salt-tolerant purified cultures contained two major components apparently working in syntrophy. The primary acetate-fermenting component was identified as a member of the order Clostridiales forming, together with 'Ca. Contubernalis alkalaceticum', an independent branch within the family Syntrophomonadaceae. A provisional name, 'Ca. Syntrophonatronum acetioxidans', is suggested for the novel haloalkaliphilic clostridium. Two phylotypes of extremely haloalkaliphilic sulfatereducing bacteria of the genus Desulfonatronospira were identified as sulfate-reducing partners in the acetate-oxidizing cultures under extreme salinity. The dominant phylotype differed from the two species of Desulfonatronospira described so far, whilst a minor component belonged to Desulfonatronum thiodismutans. The results proved that, contrary to previous beliefs, sulfatedependent acetate oxidation is possible, albeit very slowly, in nearly saturated soda brines.

The diversity of soxB gene encoding a key enzyme of the Sox pathway sulfate thiohydrolase has been investigated in pure cultures of various halophilic and haloalkaliphilic sulfur-oxidizing bacteria (SOB) and in salt and soda lakes in southwestern Siberia and Egypt. The gene was detected in the majority of strains belonging to eleven SOB genera excluding members of genera Thiohalospira and Thioalkalimicrobium. The uncultured diversity of soxB in salt and soda lakes was low with a majority of detected sequences belonging to autotrophic SOB from the Gammaproteobacteria. In addition, the soxB analysis allowed detection of putative heterotrophic Gamma- and Alphaproteobacterial SOB yet unknown in culture. All clone libraries obtained from soda lakes contained soxB belonging to the genus Thioalkalivibrio in agreement with the cultivation results. Besides, representatives of the genera Halothiobacillus, Marinobacter, and Halochromatium and of the family Rhodobacteraceae have been detected in both type of saline lakes.

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Oscillochloris trichoides is a mesophilic, filamentous, photoautotrophic, nonsulfur, diazotrophic bacterium which is capable of carbon dioxide fixation via the reductive pentose phosphate cycle and possesses no assimilative sulfate reduction. Here, we present the draft genome sequence of Oscillochloris trichoides subsp. DG-6, the type strain of the species, which has permitted the prediction of genes for carbon and nitrogen metabolism and for the light-harvesting apparatus.The anoxygenic filamentous phototrophic bacteria (AFPB) group belongs to the phylum Chloroflexi, which contains two families: Chloroflexaceae and Oscillochloridaceae. Oscillochloridaceae consists of single Oscillochloris genus, to which the investigated bacterium Oscillochloris trichoides subsp. DG-6 belongs. The representatives of the Chloroflexaceae family are in general thermophilic photoorganotrophs (5,11,12), while Oscillochloridaceae bacteria are mesophilic photolithoautotrophs (9). The strain O. trichoides subsp. DG-6 was isolated from microbial mats of warm hydrogen sulfide springs in the Caucasus region of southeast Europe (7,8).The genomic DNA sequencing was performed using the combined approach which included the Sanger (ABI 3730) sequencing of the genomic DNA library with the insert lengths of 2.5 to 3.0 kbp combined with whole-genome shotgun and paired-end sequencing using a 454 FLX apparatus. The draft assembly of the obtained sequences was carried out using the Phred/Phrap/Consed software (2, 4). The identification and annotation of the found open reading frames (ORFs) were performed using the Glimmer software package (3) and by comparison with the corresponding data from the UniProt, KEGG, and NR-NCBI databases.The resulting genomic DNA sequence (4,370,273 nucleotides) of O. trichoides subsp. DG-6 included 65 contigs, united into 5 scaffolds separated by ribosomal operons which were placed in 2 separate contigs-16S and 23S-5S. The number of complete (16S-23S-5S) ribosomal operons was four, and one other ribosomal operon was reduced to 23S-5S genes. The mean GC content of the obtained sequence was 59.26%, thus corresponding to the previously published data (9). In the obtained sequence, 3,853 ORFs were found, and between them 44 genes encoding tRNAs were revealed.In the results of the analysis of the Oscillochloris trichoides subsp. DG-6 genomic DNA sequence, the set of genes corresponding to the photosystem II genes was revealed. Among these genes were those encoding the proteins of light-harvesting antennas together with proteins participating in the synthesis of bacteriochlorophylls and carotenoids. This set was spread between five regions of the genomic DNA.As it was shown previously (6), the bacteria belonging to the Oscillochloridaceae family are capable in autotrophic assimilation of carbon dioxide via the Calvin-BensonBassham cycle. In the results of the analysis of the O. trichoides subsp. DG-6 genomic DNA, the set of genes encoding the enzymes of this metabolic pathway was revealed. The gene order in the set (cbbL-cbbS...