ИСТИНА

The Black Sea is a world’s largest meromictic basin, where deep sulfide-rich waters do not mix with the upper oxic waters. Interestingly, sulfate-reducing bacteria (SRB) were detected there not only in the anoxic zone but also in the oxic water layer. A pure culture of a new SRB species, possessing different systems of antioxidative defense, was isolated for the first time from the Black Sea oxygenated water column. We analyzed the phylogenetic composition of SRB community in the Black Sea oxic waters where representatives of Desulfovibrio-Desulfomicrobium, Desulfococcus-Desulfonema-Desulfosarcina and Desulfotomaculum subgroups were revealed. The pure culture of psychrophilic Gram-negative SRB was isolated from the subsurface oxic waters and described as Desulfofrigus euxinos. This new species possesses different systems of antioxidative defense including O2 reduction and ROS detoxification. FISH, nested and qRT-PCR with primers to 16S rRNA and dsrB genes, DGGE and nucleotide sequencing were used to investigate the SRB community. DNA-DNA hybridization, fatty acids analysis and growth tests with different electron donors/acceptors were performed to characterize the SRB species. Oxygen consumption rates were measured with a Clark-type electrode. Key genes of antioxidative defense were identified by DOP-PCR, and inverse PCR was carried out for the sequence analysis of the entire genes. While some SRB are resistant to aerobic conditions because of multi-component antioxidative mechanisms, their presence in the sea oxygenated waters is likely to depend also on inhabiting anoxic microniches within suspended organic particles.