PHYLOGENETIC DIVERSITY OF THE MICROBIAL MATS IN LAKE FRYXELL, ANTARCTICA

Diversity is a key ecosystem component in microbial communities, but mechanisms that promote diversity or lack thereof remain underexplored. One valuable place to explore diversity is Lake Fryxell, a perennially ice-covered, meromictic lake in the McMurdo Dry Valleys of Antarctica. In Lake Fryxell, O₂ concentration and photosynthetically active radiation (PAR) decline with depth, coincident with changes in pigmentation and morphology of thick, layered benthic microbial mats. This ecosystem offers the opportunity to study environmental constraints on microbial diversity in the absence of macroscopic organisms.

Divers with the McMurdo Dry Valleys LTER collected triplicate DNA samples from 9.0, 9.35, and 9.8 m depth and measured temperature, O2 concentration, conductivity, PAR, and mat morphology. Oxygen microelectrode profiles were collected in the mats. At 9.0 and 9.35 m depths, PAR was relatively high and water was O2 supersaturated. At 9.8 m depth, PAR was low and water was anoxic, but traces of O₂ were produced by cyanobacteria, creating a mm-thick O₂-rich zone in the mat. Two distinct phylogenetic diversity patterns emerged in mats: 1) diversity increased into the mat layers at all lake depths; and 2) diversity decreased with increasing depth in the lake. Some microbial lineages, particularly alpha- and gammaproteobacteria and comamonadaceae, contribute the most to these trends.

In many microbial mats, diversity decreases as O₂ declines. This relationship is seen in Lake Fryxell, where surface communities were less diverse at lower O₂ in deeper water, despite a greater variety of electron acceptors. In contrast, at 9.0 m, O₂ was lower but present in the bottom microbial mat layer, where diversity was highest. Therefore, redox state alone does not determine diversity. Higher PAR at the mat surface may increase diversity of all layers, as diversity is generally greater at shallower lake depths. These results provide an opportunity to refine models of microbial diversity in response to redox state and PAR.