The first high-throughput sequencing of bacterioplankton sheds light on bacterial and cyanobacterial diversity in high-altitude Lake Sevan, Armenia.

Lake Sevan, located in the Armenian Highlands, is one of the world's largest high-altitude freshwater lakes (1900 m a.s.l.). Hydrobiological investigations in the lake have an almost century-long history; however, the bacterial diversity has never been studied. In this study, bacterioplankton composition in Lake Sevan has been characterized for the first time. The samples were collected from different depths at the deepest points of both subbasins, Big and Small Sevan, once per season in 2018 and analyzed genetically by high-throughput sequencing of the V3-V4 region of 16 S rRNA gene. According to the 16 S rRNA gene sequencing, the majority of the bacterioplankton consisted of well-known freshwater microorganisms of the phyla Pseudomonadota, Actinomycetota, Bacteroidota, Cyanobacteriota, and Candidatus Kapabacteria. Representatives from Verrucomicrobiota, Planctomycetota, Bdellovibrionota, Gemmatimonadota, Myxococcota, and other phyla were found sporadically or in minor abundance. Alpha diversity was generally high, except during the summer cyanobacterial bloom. Two types of cyanobacterial occurrence were identified: (1) filamentous, potentially harmful cyanobacteria, such as Dolichospermum flos-aquae, which bloomed in summer, and (2) autotrophic picocyanobacteria, primarily Synechococcus, which dominated the cyanobacterial community in spring, autumn, and winter. In addition, pathogenic bacteria were detected in the lake, including species pathogenic to fish and humans, as well as intracellular parasites.