Ecosystem Dynamics of the Microbial Mats in Lake Fryxell, Antarctica

Microbial communities drive biogeochemical cycles on Earth at micron scales, at which microbial ecosystem networks are highly interconnected to each other and their local environment through trophic interactions and the availability of electron acceptors. Feedbacks among microorganisms and environmental conditions structure microbial communities and micron-scale geochemical gradients. We are exploring interactions among microbial community structure and local geochemistry in Lake Fryxell, a perennially ice-covered, meromictic lake in the McMurdo Dry Valleys, Antarctica. In Lake Fryxell, O2 concentration and photosynthetically active radiation (PAR) decline with depth and influence layered benthic mats, which consist of microorganisms from all three domains. These layered mats change pigmentation and morphology with depth in the lake. In 2012, samples were collected for 16S rRNA and metagenomic sequencing in collaboration with the McMurdo Dry Valleys Long Term Ecological Research program. Samples were collected at 9.0, 9.35, and 9.8 m depth with temperature, pressure, O2 concentration, conductivity, PAR, irradiance, and morphology. O2 microelectrode profiles were also collected. At 9.0 and 9.35 m depths, PAR was relatively high, and the water was supersaturated with O2. At 9.8 m depth, PAR was low and lake water was anoxic, but 50 µmol L-1 of O2 was produced by cyanobacteria, creating a mm-thick zone with free O2 in the mat under anoxic water. As in many microbial mats, community-O2 correlations are present in Lake Fryxell: diversity increases with increasing depth into the mat at all lake depths, and diversity decreases with increasing depth in the lake. Microbial communities are less diverse and the dominant phototrophs change at lower PAR and O2 concentrations. Further, O2 generated by cyanobacteria creates a habitat for aerobic and microaerophilic heterotrophs under an anoxic water column. Some lineages, particularly alpha- and gammaproteobacteria and the comamonadaceae, vary significantly in diversity. The cyanobacteria are the dominant lineage contributing to low diversity in oxygen oases. The microbial communities in Lake Fryxell are distinct between depths and mat layers, and affect local environmental conditions, especially redox, in ways that are specific to the community.