Modern stromatolite microbial diversity in Yellowstone National Park, Wyoming in the context of microbial alpha and beta diversity along Yellowstone geothermal outfalls.

Abstract

To better understand the diversity of mechanisms for stromatolite morphogenesis as well as the diversity of microorganisms and microbial metabolisms associated with stromatolites it is imperative to describe the biological attributes of the full diversity of "living" stromatolite-like geobiological structures worldwide. It is of additional interest to describe geographically and geochemically comparable ecosystems but lacking stromatolites to stromatolite-forming environments to unravel the geochemical and microbiological aspects of stromatolite morphogenesis. Here we present a living stromatolite system in a Yellowstone National Park (YNP) hot spring that exhibits features in contrast to many popularly studied modern stromatolite analogs. Most notably, the YNP stromatolites are more finely laminated than living marine stromatolites and may be a more suitable textural analog to finely laminated stromatolites found in the rock record. The YNP stromatolites are composed of silica-encrusted cyanobacterial mats. The predominant lithofacies of the YNP stromatolite is comprised of silica-encrusted filaments and is distinctly laminated. The laminated quality of the main lithofacies is due to an alternating-possibly on a diurnal cycle-growth orientation of filamentous cyanobacteria. Two cyanobacterial mat types grow on the stromatolite surfaces and are preserved as two distinct lithofacies. One mat is present when the stromatolites are submerged or at the water-atmosphere interface and the other when stromatolites protrude from the hot spring. The lithofacies created by the encrustation of submerged mats constitutes the bulk of the stromatolites, is comprised of silica-encrusted filaments, and is distinctly laminated. To better understand the cyanobacterial membership and community structure differences between the mats, we collected mat samples from each type. Molecular methods revealed that submerged mat cyanobacteria were predominantly one novel phylotype while the exposed mats were predominantly heterocystous phylotypes (Chlorogloeopsis HTF and Fischerella). The cyanobacterium dominating the submerged mat type does not belong in any of the subphylum groups of cyanobacteria recognized by the Ribosomal Database Project and has also been found in association with travertine stromatolites in a Southwest Japan hot spring. Cyanobacterial membership profiles indicate that the heterocystous phylotypes are 'rare biosphere' members of the submerged mats. The heterocystous phylotypes likely emerge when the water level of the hot spring drops. Environmental pressures tied to water level such as sulfide exposure and possibly oxygen tension may inhibit the heterocystous types in submerged mats. In contrast to living marine examples where the interplay of pCO2 and [Ca2+] is the main influence on microbial lithification, the lithifcation in the YNP system is driven by a rapid decrease in silica solubility between high-temperature subsurface water emitted from the hot spring vent and lower-temperature surface water. The continuously favorable conditions for rapid lithification in the hot spring system coupled to a cyanobacterial diurnal growth cycle that manifests itself as fine laminations in the stromatolite lithofacies leads to a growth rate for the YNP stromatolite on the order of centimeters of deposition in several months. The YNP system displays a perhaps overlooked mechanism for stromatolite morphogenesis and is compelling both for the presence of a seemingly unstudied cyanobacterium as well as a finely-laminated modern stromatolite analog of which there are few other examples.