Habitat connectivity and environmental controls on the structure of phototrophic microbial mats and bacterioplankton communities in an Antarctic freshwater system

Freshwater ecosystems are considered hotspots of biodiversity in Antarctic polar deserts. Anticipated warming is expected to change the hydrology of these systems, with implications for growth conditions and physical connectivity between habitats. Using 16S rRNA sequencing, we evaluated the structure of microbial mat and planktonic communities within a connected watershed in the McMurdo Wright Valley, Southern Victoria Land, Antarctica to determine the roles of connectivity and habitat in controlling microbial assemblage composition. We examined benthic and planktonic samples from glacial Lake Brownworth, the perennially ice-covered, endorheic Lake Vanda, and the Onyx River, which connects the two. In Lake Vanda, we found distinct microbial communities occupying sub-habitats at different lake depths, and evidence for strong environmental filtering driving these differences. Similarities between communities from Lake Brownworth, Onyx River, and its entry point to Lake Vanda, however, suggest a role for physical connectivity in maintaining microbial diversity in these parts of the system. Functional metagenomics predictions identified genes related to degradation of halogenated aromatic compounds in surface microbial mats exposed to changes in water regimes, which progressively disappeared with increasing depth. Hydrological connectivity and environmental variables appear to work together to shape distinct microbial communities and potential functions in Antarctic freshwater ecosystems.