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Impact of the euxinic holomictic conditions on the prokaryotic assemblages in a marine meromictic lake

Milan Čanković*, Jurica Žučko, Ines Petrić, Marija Marguš, Irena Ciglenečki

*Corresponding author:

ABSTRACT: During the year, water column of marine Rogoznica Lake (Croatia) is stratified. However, on rare occasions the lake exhibits complete mixing, primarily driven by meteorological conditions. On such occasion anoxic, sulfide-rich bottom water mixes with oxic surface layer triggering both, oxidation of sulfide predominantly to colloidal sulfur, and establishment of euxinic conditions throughout the water column. Here we investigated prokaryotic communities during euxinic holomictic conditions. High-throughput sequencing of 16S rRNA gene showed that this sulfur-driven community was dominated by Planctomycetales, chemolithotrophic Epsilonbactereota (mainly Arcobacteraceae) and Gammaproteobacteria (Thioglobaceae), followed by Alpha- (Rhodobacteraceae) and Deltaproteobacteria (Desulfobulbaceae and Desulfobacteraceae). The majority of archaeal sequences belonged to Nanoarchaeota, all of which were designated as Woesearchaeia. This class has not been recorded in other meromictic lakes, according to literature. Both, bacterial and archaeal 16S rRNA gene abundance did not vary significantly across the vertical profile of the lake, as shown by qPCR. However, hierarchical clustering showed a significant difference between the microbial assemblage structure in stratified periods and during the holomictic event. Beside the structure, this stressful event had manifold effect on bacterial and archaeal assemblages in terms of diversity and abundance decrease, when compared to usual, stratified conditions.