An intensive multi-year field study of the modern marine stromatolites atHighborne Cay, Bahamas has identified a variety of microbial communities thatcolonize the stromatolite surfaces. They include both bacterial and diatom dominatedcommunities. The "classic" microbial communities are those described by Reid et al.(2000). They include Schizothrix mats, dominated by S. gebeleinii, which trap and bind ooid sand grains (Type 1 mat); biofilm mats, composed of sulfate reducing bacteria,which form thin crusts of microcrystalline carbonate (Type 2 mat); and Solentia mats,dominated by coccoid endolithic Solentia species, which create cemented layers offused sand grains (Type 3 mat). Another bacterial mat, termed "pudding mat" due toits pudding-like texture, is dominated by thin filaments of Phormidium sp. and singlefilaments of S. gebeleinii, but may also be colonized by a unique species of coccoidcyanobacterium related to Cyanothece. The diatom mats include stalked diatomsand tube diatoms. The stalked diatom mats form as a thin (1-3 mm) surface pink fuzzcomprised of Striatella unipunctata, or a yellow fuzz that may develop into a thick (0.51 cm) yellow fur with Licmophora remulus and Licmophora paradoxa. The tube diatommats, which occur as discrete pustules that may coalesce to create uniform blankets, are formed by naviculid - like tube diatoms. These different mat types recognized based on field descriptions and light microscopy also show distinct differences based on microbial fingerprinting and carbohydrate fractionation. Denaturing gradient gel electrophoresis (DGGE) analyses show similarities between stalked diatom mat types and "classic" mat types 1 and 2; these mats cluster separately from tube diatom mats and pudding mats, which each form distinct clades. In addition, the carbohydrate fractions of classic mat types are composed mostly of structural extracellular polymeric secretions (EPS), whereas stalked diatom and pudding mats contain predominantly non-structural carbohydrates. Although the pudding mats and diatom communities can contribute to the trapping of ooids, the stabilization of the unconsolidated sediment ultimately requires binding by S. gebeleinii. The combination of carbohydrate composition and ability to rapidly rebound after burial result in the high erosion resistance exhibited by the "classic" mats. Conversely, the extremely sensitive nature of diatoms to burial results in the low erosion resistance of the diatom mats. Nevertheless, all may contribute to the biogenesis of the Highborne Cay stromatolites.
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