Microbial communities of stratified aquatic ecosystems of Kandalaksha Bay (White Sea) shed light on the evolutionary history of green and brown morphotypes of Chlorobiota.

Anoxygenic photoautotrophic metabolism of green sulfur bacteria of the family Chlorobiaceae played a significant role in establishing the Earth's biosphere. Two known major ecological forms of these phototrophs differ in their pigment composition and, therefore, in color: the green and brown forms. The latter form often occurs in low-light environments and is specialized to harvest blue light, which can penetrate to the greatest depth in the water column. In the present work, metagenomic sequencing was used to investigate the natural population of brown Chl. phaeovibrioides ZM in a marine stratified Zeleny Mys lagoon in the Kandalaksha Bay (the White Sea) to supplement the previously obtained genomes of brown Chlorobiaceae. The genomes of brown and green Chlorobiaceae were investigated using comparative genome analysis and phylogenetic and reconciliation analysis to reconstruct the evolution of these ecological forms. Our results support the suggestion that the last common ancestor of Chlorobiaceae belonged to the brown form, i.e. it was adapted to the conditions of low illumination. However, despite the vertical inheritance of these characteristics, among modern Chlorobiaceae populations, the genes responsible for synthesizing the pigments of the brown form are subject to active horizontal transfer.

Genome Sequences of a Green-Colored Chlorobium phaeovibrioides Strain Containing Two Plasmids and a Closely Related Plasmid-Free Brown-Colored Strain.

Here, we report the draft genome sequences of the green sulfur bacterium Chlorobium phaeovibrioides strains GrTcv12 and PhvTcv-s14, isolated from the chemocline zone from meromictic Lake Trekhtzvetnoe, separated from the White Sea, in Russia. This is the first report showing the presence of plasmids containing antiphage systems in the Chlorobium sp. genome.

Genome Sequences of Green- and Brown-Colored Strains of Chlorobium phaeovibrioides with Gas Vesicles.

The draft genomes of green-colored Chlorobium phaeovibrioides GrKhr17 and brown-colored Chlorobium phaeovibrioides BrKhr17, green sulfur bacteria with gas vesicles isolated from Lake Bolshye Khruslomeny, are presented. These sequences contribute to genomic analyses of the Chlorobiaceae family that are part of ongoing research seeking to better understand their ecosystem-specific adaptations.

Genome Sequence of Prosthecochloris sp. Strain ZM and Prosthecochloris sp. Strain ZM-2, Isolated from an Arctic Meromictic Lake.

Draft genome sequences of green-colored and brown-colored green sulfur bacteria (GSB), Prosthecochloris sp. ZM and Prosthecochloris sp. ZM-2, respectively, which were isolated from the Arctic meromictic lake Zeleny Mys, were sequenced. The genomes' differing gene compositions determine the differences in the bacteriochlorophyllic compositions of these bacteria.

Sharp water column stratification with an extremely dense microbial population in a small meromictic lake, Trekhtzvetnoe.

Located on the shore of Kandalaksha Bay (the White Sea, Russia) and previously separated from it, Trekhtzvetnoe Lake (average depth 3.5 m) is one of the shallowest meromictic lakes known. Despite its shallowness, it features completely developed water column stratification with high-density microbial chemocline community (bacterial plate) and high rates of major biogeochemical processes. A sharp halocline stabilizes the stratification. Chlorobium phaeovibrioides dominated the bacterial plate, which reached a density of 2 × 108 cell ml-1 and almost completely intercepts H2 S diffusion from the anoxic monimolimnion. The resulting anoxygenic photosynthesis rate reached 240 µmol C l-1 day-1 , exceeding the oxygenic photosynthesis rate in the mixolimnion. The rates of other processes are also high, reaching 4.5 µmol CH4 l-1 day-1 for methane oxidation and 35 µmol S l-1 day-1 for sulfate reduction. Metagenomic analysis demonstrated that the Chl. phaeovibrioides population in the bacterial plate layer had nearly clonal homogeneity, although some fraction of these cells harbour a plasmid. The Chlorobium population was associated with bacteriophages that share homology with CRISPR spacers in the host. These features make the ecosystem of the Trekhtzvetnoe Lake a valuable model for studying regulation and evolution processes in natural high-density microbial systems.

Microbial processes of the carbon and sulfur cycles in an ice-covered, iron-rich meromictic lake Svetloe (Arkhangelsk region, Russia).

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-1  day-1 ) 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-1  day-1 ) 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-1  day-1 ) below the chemocline zone. The rates of methane oxidation were as high as 1.8 µmol CH4  L-1  day-1 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.