Microbial diversity of hot springs of the Kuril Islands.

The Kuril Islands are located in the Far-East of Russia and enriched with shallow and terrestrial hot springs. Prokaryotic diversity of Kuril geothermal environments has been studied fragmentarily and mainly by culture-dependent methods. We performed the first large-scale investigation of microbial communities, inhabited more than 30 terrestrial hot springs of Kunashir and Iturup Islands, analyzed by 16S rRNA gene fragment amplicon sequencing, together with chemical analysis of thermal waters and sediments. The Circumneutral Bacterial group containing springs with pH 5.7-8.5 and temperature 40-79 °C possessed the highest biodiversity and consisted almost entirely of Bacteria. Cyanobacteriota (the Leptolyngbyaceae and Oculatellaceae families) and phototrophic Chloroflexota dominated in the microbial mats in hot springs with temperatures up to 60 °C. The higher temperature ones were dominated by Aquificota (Sulfurihydrogenibium and Hydrogenobacter species). The Acidic Bacterial group (pH 2.2-3.6, 41-64 °C) inhabited by the genera Acidithiobacillus, Hydrogenobaculum and Thiomonas. Archaea of Acidianus, Metallosphaera, Thermoplasma and Caldisphaera spp. as well as uncultivated lineages ('Ca. Marsarchaeales', 'Ca. Caldiarchaeum', BSLdp215) were abundant in the Acidic Archaeal group (pH 1.5-2.9, 50-94 °C). The microbial composition of the Kuril hot springs strongly correlated with pH and moderately correlated with water chemistry, while degree of correlation between the communities' compositions with temperature and location was low.