2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 191-1
Presentation Time: 8:03 AM


WARD, Lewis M., Division of Geological and Planetary Sciences, California Institute of Technology, MC 100-23, 1200 E. California Blvd, Pasadena, CA 91125, KERRIGAN, Zak, Graduate School of Oceanography, University of Rhode Island, 215 South Ferry Road, Narragansett, RI RI 02882-119, AGI?, Heda, Department of Earth Sciences, Uppsala University, Villavägen 16, Uppsala, 752 36, Sweden, JUAREZ RIVERA, Marisol, Geology, University of California-Davis, One Shields Avenue, Davis, CA 95616, PETRYSHYN, Victoria A., Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, STAMPS, Blake W., Department of Microbiology and Plant Biology, University of Oklahoma, Norman, OK 73019, JOHNSON, Hope A., Department of Biological Science, California State University, Fullerton, 800 N. State College Blvd, Fullerton, CA 92831, STEVENSON, Bradley S., Department of Microbiology and Plant Biology, University of Oklahoma, Norman, OK 73072, SPEAR, John R., Division of Environmental Science and Engineering, Colorado School of Mines, Golden, CO 80401-1887 and CORSETTI, Frank A., Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, lward@caltech.edu

Conical stromatolites (Conophyton) are among the best records of microbial life in deep time, as unlike other stromatolites they are not known to form abiogenically. Cones can be found in modern microbial mats, potentially serving as analogs of conical stromatolites. Mechanisms of cone formation are not well understood, though several hypotheses have been proposed including phototaxis and bubble formation. In association with the 2014 International Geobiology Course, we have collected field observations and conducted metagenomic analyses of a modern cone-forming microbial mat from a hot spring in California (Little Hot Creek) to investigate potential mechanisms underlying their formation.

Initial observations of cones in Little Hot Creek revealed surface-normal growth on slopes and under overhangs rather than vertically, suggesting that neither phototaxis nor bubble production are driving cone formation. Bubble production was only observed in flat regions of the mat and not in cones, indicating that there is not a strong association between bubbles and cones. This was further supported by the absence of known phototaxis-associated genes in metagenomic data from both the cones and the surrounding flat mat. Comparison of the metagenomes from the cones and the surrounding mat revealed shared predominant taxa and functional genes between both sample types, differing only in relative abundance and the presence of some low-abundance sequences. Both samples were primarily composed of the phyla Cyanobacteria and Chloroflexi, with the coccoidal cyanobacterium genus Synechococcus and the filamentous, photoheterotrophic chloroflexi genus Roseiflexus as dominant members of both samples. The filamentous cyanobacterium genus Phormidium was highly enriched in the cone samples, suggesting a potential role for abundant filaments in cone formation.

Our study suggests that cone formation can be independent of bubble formation or phototaxis. Alternative mechanisms could be involved, such as differential gene expression, diffusion limitation of nutrients, or the stochastic aggregation of filamentous microbes. These mechanisms could potentially be driven by processes other than oxygenic photosynthesis, and therefore might have implications for interpretation of cones in the rock record.