| Summary: | Branched, columnar stromatolites grew in perennially ice-covered Lake Joyce of the Antarctic McMurdo Dry Valleys during a period of lake level rise. These stromatolites are composed of the remains of filamentous, mat-forming cyanobacteria, sediment, and calcite. Calcite that precipitated within the stromatolites records their morphological evolution from prolate columns with inflexed apices to either branched or irregular columns. Calcite crystals also contain cylindrical molds, which have diameters similar to the trichome widths of cyanobacteria present elsewhere in Lake Joyce. These cyanobacteria have depth-dependent distributions; the shallowest mats from <12 m depth have abundant Phormidium autumnale, Leptolyngbya antarctica, L. fragilis, and Pseudanabaena frigida morphotypes, whereas deeper mats lack P. autumnale . P. autumnale-sized molds are abundant in calcite forming the oldest stromatolite layers, but are absent from younger crystals, suggesting that P. autumnale was present in the first stromatolite layers, but disappeared from the community with successive growth during lake level rise. This loss of P. autumnale-sized molds correlates with a transition from prolate columns to branched and irregular columns. Similar patterns in growth form were present in photosynthetically active mats elsewhere in Lake Joyce. Mats lacking P. autumnale commonly contained small peaks and branch-like bundles of filaments growing away from the mat surface, whereas mats that P. autumnale dominated were characterized by loss of relief with growth. Paired observations of these active mats and columnar stromatolites thus suggest that changes in microbial community composition following lake level rise initiated branched growth.
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