CHANGES IN MICROBIAL ECOLOGY ASSOCIATED WITH A TRANSITION FROM CONOPHYTON TO JACUTOPHYTON IN MODERN LAKE JOYCE, ANTARCTICA

Microbial mats dominated by cyanobacteria in perennially ice-covered Lake Joyce, McMurdo Dry Valleys, Antarctica, form columnar stromatolites analogous to Neoproterozoic jacutophyton. Eleven intact columnar stromatolites from 20-22m depth were sampled, frozen, and imaged with x-ray computed tomography (CT). CT scans and subsequent dissection demonstrate internal layers defined by calcite-rich and poor intervals. Interior layers define prolate columns with cuspate apices (conophyton), and in some stromatolites, exterior layers transition into divergent branches and petals (jacutophyton). Innermost calcite layers contain bubble casts and tubes, demonstrating that they grew in less than ~9m of water, the maximum observed depth of bubble nucleation. These inner layers also contain cyanobacterial filament molds whose diameters cluster in two groups with a mean of 1.2 and 4.1µm, consistent with extant species of Leptolyngbya and Phormidium, respectively. In contrast, branches and petals contain molds of only the smaller Leptolyngbya, suggesting that an ecological transition correlates with the morphological transition to branching forms. The ecological transition may have been triggered by documented lake level rise (Hawes et al., in review). Phormidium molds support the interpretation that conophyton grew in shallow water as Phormidium are now found only in shallow mats. However, Leptolyngbya is present in both shallow and deeper mat communities, so the presence of only small molds in branches and petals is consistent with growth in deeper water.

The Neoproterozoic Atar Dolomite, Mauritania, hosts conophyton to jacutophyton transitions (Bertrand-Safarti and Moussine-Pouchkine, 1985), which are analogous to the changes in growth form of the Lake Joyce stromatolites. Lake Joyce shares the low-energy and low-sedimentation environment within which the ancient conophyton and jacutophyton stromatolites grew. This Neoproterozoic morphological transition may also be caused by an ecological transition as observed in the Lake Joyce stromatolites.