Cyanobacterial mat diversity from freshwater spring Krcic revealed by 16S rRNA amplicon and ITS clone library sequencing. Cyanomats from Krcic spring (Croatia)

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. Krčić 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 (α-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.

微生物垫（Microbial mats）是一类垂直分层的微生物组，由生长在固体基质上的底栖群落构成。其包含功能各异的类群群落，主要由原核细菌（Bacteria）、古菌（Archaea）以及可在极端环境中存活的产氧丝状光合蓝细菌（Cyanobacteria）组成。克尔契奇（Krčić）是位于克罗地亚迪纳拉地区的间歇喀斯特泉，其特征为高水位波动与长期干旱期。该泉的理化性质可支撑特定微生物垫群落存活以应对这类极端水文事件。研究团队于2014年和2015年在不同季节及水文状况下采集样本，以期探究微生物垫的多样性，并明确其遗传组成与环境条件的关联。研究通过扩增ITS区的特异性引物分析蓝细菌的基因型多样性，同时以16S rRNA作为系统发育标记分析细菌类群。分析结果显示，蓝细菌在微生物垫中为优势类群（占比超50%），其次为变形菌门（Proteobacteria，α-变形菌纲）、浮霉菌门（Planctomycetes，浮霉菌科）、拟杆菌门（Bacteroidetes，噬纤维菌科）以及疣微菌门（Verrucomicrobia，疣微菌科）。在水文稳定条件下，微生物群落结构相对稳定；而夏季环境与长期干旱会对群落结构造成显著影响：厚壁菌门（Firmicutes）与放线菌门（Actinobacteria）类群得以富集，优势类群的丰度也发生改变。详细分析揭示了蓝细菌组成的转变：席藻属（Phormidium）/微鞘藻属（Microcoleus）被与耐旱岩内生蓝细菌拟色球藻属（Chroococcidiopsis）近缘的物种所替代。此外，在群落稳定时期，多数蓝细菌序列与未培养的南极物种近缘，这进一步表明该生态系统可能包含独特的微生物类群组合。