MICROBIALITES AND MICROBIAL MATS IN ANTARCTIC LAKES AS MODELS FOR PRECAMBRIAN ECOSYSTEMS

Microbial mats are abundant in Antarctic lakes Joyce, Fryxell, Vanda, and Hoare (McMurdo Dry Valleys) and Lake Untersee (Queen Maud Land). During the summer, sufficient light penetrates the ice covers of these lakes to allow photosynthetic biomass accumulation. A paucity of macroscopic metazoans allows these ecosystems to develop much like Precambrian microbial mats. Mat morphology varies both within and between lakes, and in some cases mimics fossil stromatolite morphology. Small amounts of calcite precipitate within mats; lakes Joyce and Fryxell contain lithified microbialites.

Mat biomass is dominated by cyanobacteria. Diatoms and coccoidal algae are present, but cell counts, pigment analyses, and genomic results suggest that they contribute only a small fraction of total biomass. In addition, anoxygenic phototrophs are rare within most shallow mats due to high O₂concentrations to >10 cm depths. In Lake Fryxell, however, an oxycline is present within the water column in the lower photic zone, and anoxygenic phototrophs are an increasingly important component of the mat community with greater depth in the lake.

Mat morphology within these lakes varies with light distribution, sedimentation rate, and microbial community. Pinnacles (mm-cm) are very common in photic zones where sedimentation is low relative to growth rate and a specific cyanobacterial OTU, Phormidium autumnale, is absent. In Lake Joyce, for example, P. autumnale dampens mat roughness, promoting growth of flat mat, and loss of P. autumnale from the community likely induced branching in columnar stromatolites. In Lake Vanda, the morphology of decimeter-scale microbial columns can be modeled using the distribution of cyanbacterial OTUs and light penetration into the columns. In these and other cases, mat/microbialite morphology emerges from feedbacks among environmental and biological factors that are substantially more complex than most models of ancient stromatolite growth.