Cyanobacterial mat diversity from freshwater spring Krcic revealed by 16S rRNA amplicon and ITS clone library sequencing

Microbial mats are vertically stratified microbiomes consisting of benthic communities growing on solid substrates. They include communities of functionally distinct groups, largely consisting of prokaryotic Bacteria, Archaea and oxygenic filamentous phototrophic Cyanobacteria capable of thriving in extreme environmental conditions. Krcic is an intermittent karst spring situated in the Dinaric region of Croatia, characterized by high water-level fluctuations and long drought periods. Its physical and chemical properties support growth of specific microbial mat community capable of surviving such extreme hydrological events. We collected samples during different seasons and hydrological regimes in 2014 and 2015 in order to investigate the microbial mat diversity and understand the correlation of its genetic mark- up with environmental conditions. Genotypic diversity of Cyanobacteria was studied using specific primers amplifying ITS region, while Bacteria were studied using 16S rRNA as a phylogenetic marker. Analysis showed that Cyanobacteria was a predominant group in microbial mat (>50%), followed by Proteobacteria (a-Proteobacteria), Planctomycetes (Planctomycetaceae), Bacteroidetes (Cytophagaceae) and Verrucomicrobia (Verrucomicrobiaceae). During stable hydrological conditions a relatively constant microbial community was established, while summer conditions and long drought had a drastic effect on the community structure with enrichment of Firmicutes and Actinobacteria taxa and changes in abundances of dominant taxa. Detailed analysis revealed a switch in the cyanobacterial composition with Phormidium/Microcoleus genera being replaced with species closely related to Chroococcidiopsis, a desiccation- tolerant, endolithic cyanobacterium. Moreover, during stable community periods most of the cyanobacterial sequences related to uncultured Antarctic species, further implying that this ecosystem may comprise a unique assemblage of microbial species.