A silica budget reveals much about how bacteria might regulate the formation of silica stromatolites in YNP Hot Springs. AGU Meeting 2008.

THANKYAKOOP, C, ZARGAR, K., GOMEZ, F.J., EYDAL, H., BERELSON, W., DAWSON, S., CORSETTI, F., HANSELMANN, K., JOHNSON, H., SPEAR, J., STEVENSON, B.S., DE LA TORRE, J., AND GEOBIO2008.

San Francisco (USA)

Congreso; American Geophysical Union Meeting 2008; 2008

American Geophysical Union

We studied the distribution and morphology of subaqueous stromatolites present in the shallow zones of a hot spring at Yellowstone National Park. The pool water is supersaturated with respect to silica with an average concentration of 5700 +/- 500 uM over a temperature range of 42-72 C. Here we report on the chemical processes that govern stromatolite morphology at the microscale and macroscale level, the concentration and speciation of dissolved silica, and the role of mixing and pond hydrodynamics that influence stromatolite development. The concentration of dissolved silica in shallow shelf zones of OPP can be significantly lower than in the main pond basin. This cannot be attributed to precipitation due to the lower water temperatures alone. The longer residence time of the water in the shelf zone and the presence of stromatolites in these regions might contribute to increased silica precipitation. Calculation of silica budgets in OPP suggests that large amounts of silica are readily available for deposition and that only a small fraction of the available dissolved Si need precipitate in order to grow the ‘field’ of stromatolites observed. Representative ESEM images of stromatolitic laminae indicate the presence of tubular structures which are identified as lithified sheaths of cyanobacterial trichomes. The presence of cyanobacteria is indicated by the green, chlorophyll containing layer, as well as by 16S rRNA gene analysis. The surfaces of the tubes are covered with a very thin coating of mineral Si.  There are regions in these stromatolites where diatoms are abundant, but these zones are primarily the upper dried surface of exposed structures. We propose that (1) careful analysis of the silica mass balance in hot springs could serve as a powerful approach to model silica precipitation and to constrain stromatolite growth in siliceous hot springs, (2) actively photosynthesizing cyanobacteria under oxic conditions in the outer layer of the stromatolites may retard silica precipitation via the local increase in pH, (3) insides and outsides of the tubes may become sites for heavy silica deposition once the cells are inactive or if dead cells are being degraded (4) the spacial distribution of these structures is a function of spring hydrodynamics and the distribution of sheath-forming cyanobacteria