B21C-0445
Vertical Microbial Community Variability of Carbonate-based Cones may Provide Insight into Formation in the Rock Record

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Caitlin Bojanowski1, Christopher Trivedi2, Leslie Daille3, James Bradley4, Hope Johnson5, Blake W. Stamps6, Bradley S. Stevenson6, William Berelson7, Frank A Corsetti7 and John R Spear2, (1)University of Dayton, Dayton, OH, United States, (2)Colorado School of Mines, Golden, CO, United States, (3)Pontificia Universidad Católica de Chile, Department of Molecular Genetics and Microbiology, Santiago, Chile, (4)University of Bristol, School of Geographical Sciences, Bristol, United Kingdom, (5)California State University Fullerton, Fullerton, CA, United States, (6)University of Oklahoma Norman Campus, Norman, OK, United States, (7)University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States
Abstract:
Stromatolite morphogenesis is poorly understood, and the process by which microbial mats become mineralized is a primary question in microbialite formation. Ancient conical stromatolites are primarily carbonate-based whereas the few modern analogues in hot springs are either non-mineralized or mineralized by silica. A team from the 2015 International GeoBiology Course investigated carbonate-rich microbial cones from near Little Hot Creek (LHC), Long Valley Caldera, California, to investigate how conical stromatolites might form in a hot spring carbonate system.

The cones are up to 3 cm tall and are found in a calm, ~45° C pool near LHC that is 4 times super-saturated with respect to CaCO3. The cones rise from a flat, layered microbial mat at the edge of the pool. Scanning electron microscopy revealed filamentous bacteria associated with calcite crystals within the cone tips.

Preliminary 16S rRNA gene analysis indicated variability of community composition between different vertical levels of the cone. The cone tip had comparatively greater abundance of filamentous cyanobacteria (Leptolyngbya and Phormidium) and fewer heterotrophs (e.g. Chloroflexi) compared to the cone bottom. This supports the hypothesis that cone formation may depend on the differential abundance of the microbial community and their potential functional roles.

Metagenomic analyses of the cones revealed potential genes related to chemotaxis and motility. Specifically, a genomic bin identified as a member of the genus Isosphaera contained an hmp chemotaxis operon implicated in gliding motility in the cyanobacterium Nostoc punctiforme [1]. Isosphaera is a Planctomycete shown to have phototactic capabilities [2], and may play a role in conjunction with cyanobacteria in the vertical formation of the cones. This analysis of actively growing cones indicates a complex interplay of geochemistry and microbiology that form structures which can serve as models for processes that occurred in the past and are preserved in the rock record.

References: [1] Risser, D.D. et al. (2013) Molecular Microbiology, 87(4), 884-893. [2] Giovannoni, S.J. et al. (1987) Archives of Microbiology, 147(3), 276-284.

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