First observation of microcystin- and anatoxin-a-producing cyanobacteria in the easternmost part of the Gulf of Finland (the Baltic Sea).

The aim of this study was to obtain the first data on the occurrence and distribution of potentially toxic cyanobacteria and cyanotoxins in the Russian Easternmost part of the Gulf of Finland of the Baltic Sea. Studied samples were collected from 2012 to 2017 and three independent approaches - HPLC-HRMS, PCR and light microscopy were applied for cyanotoxins analysis and detection of toxigenic cyanobacteria. Aphanizomenon flos-aquae Ralfs ex Born. et Flah., Planktothrix agardhii (Gom.) Anag. et Kom., Microcystis aeruginosa (Kütz.) Kütz. and Dolichospermum spp. dominated among cyanobacteria in collected samples. In 2012-2013 during research cruises, microcystins concentrations varied from below detection levels to low (0.01-0.6 μg L-1 ) values. In the autumn of 2015 and 2017, during cyanobacterial bloom events very high concentrations of microcystins (dissolved up to 49 μg L-1 , intracellular up to 466 μg g-1 ) and dissolved anatoxin-a (1.4 μg L-1 ) were detected. The evaluated toxin profile was represented by most common arginine-containing variants of microcystins (MC-LR, MC-RR, MC-YR) and their desmethylated forms. Leucine-containing congeners (MC-LF; MC-LY; MC-LW for the biomass sample from the coast of Komarovo, 2015) were found at low concentrations. In environmental DNA from bloom samples, we identified mcy genes regions responsible for MC biosynthesis that are specific for Dolichospermum, Microcystis, and Planktothrix. This study is the first molecular evidence the ability of Microcystis aeruginosa and Planktothrix agardhii from the Gulf of Finland to produce microcystins. On the basis of the obtained data of genus-specific PCR and microscopy, we suppose the presence of anatoxin-a-producing Apanizomenon flos-aquae population in the phytoplankton of Russian part of the Gulf of Finland.