Gaisin, Vasil A

Abstract A new filamentous phototrophic bacterium Khr17 was isolated as an enrichment culture from the brackish polar lake Bol'shie Khruslomeny. The organism was a halotolerant, strictly anaerobic phototroph possessing photosystem II. Sulfide was required for phototrophic growth. The cells of bacterium Khr17 formed nonmotile, wavy trichomes surrounded by a sheath. The cells contained chlorosomes, gas vesicles, and storage granules. The antenna pigments of bacterium Khr17 were bacteriochlorophyll c and β- and γ-carotenes. The genome of Khr17 bacterium carries all the genes responsible for CO2 fixation via the 3-hydroxypropionate pathway. The genes encoding the proteins of the nitrogenase complex were not found. The DNA G + C content was 59.9%. The 16S rRNA gene sequence of isolate Khr17 exhibited 99.4% similarity to related species. The average nucleotide identity and digital DNA–DNA hybridization values for the isolate showed 91.9% and 46.9% similarity, respectively, to other ‘Ca. Chloroploca’ species. Based on its phenotypic and phylogenetic characteristics, classification of Khr17 as member of a new species, ‘Ca. Chloroploca septentrionalis’ sp. nov., was proposed. Members of the genus ‘Ca. Chloroploca’ have previously not been found in Arctic areas and in the plankton of meromictic lakes.

ABSTRACT Chloroflexales bacteria are mostly known as filamentous anoxygenic phototrophs that thrive as members of the microbial communities of hot spring cyanobacterial mats. Recently, we described many new Chloroflexales species from non-thermal environments and showed that mesophilic Chloroflexales are more diverse than previously expected. Most of these species were isolated from aquatic environments of mid-latitudes. Here, we present the comprehensive characterization of a new filamentous multicellular anoxygenic phototrophic Chloroflexales bacterium from an Arctic coastal environment (Kandalaksha Gulf, the White Sea). Phylogenomic analysis and 16S rRNA phylogeny indicated that this bacterium belongs to the Oscillochloridaceae family as a new species. We propose that this species be named ‘Candidatus Oscillochloris kuznetsovii’. The genomes of this species possessed genes encoding sulfide:quinone reductase, the nitrogenase complex and the Calvin cycle, which indicate potential for photoautotrophic metabolism. We observed only mesophilic anaerobic anoxygenic phototrophic growth of this novel bacterium. Electron microphotography showed the presence of chlorosomes, polyhydroxyalkanoate-like granules and polyphosphate-like granules in the cells. High-performance liquid chromatography also revealed the presence of bacteriochlorophylls a, c and d as well as carotenoids. In addition, we found that this bacterium is present in benthic microbial communities of various coastal environments of the Kandalaksha Gulf.