Geomicrobiology of sulfuric acid speleogenesis : microbial diversity, nutrient cycling, and controls on cave formation /

by Engel, Annette Summers.

Filamentous microbial mats from three aphotic sulfidic springs in Lower Kane Cave, Wyoming, were assessed with regard to bacterial diversity, community structure influenced by geochemical changes, and ecosystem function using a 16S rDNA-based phylogenetic approach combined with elemental content and carbon and sulfur stable isotope ratio analyses. The bulk of the microbial mats had carbon isotope values (mean ?13C = -34.7‰, 1? = 3.6) consistent with chemolithoautotrophic carbon fixation from a dissolved inorganic carbon reservoir (cave water, mean ?13C = -7.4‰; n = 8). The most widespread mat morphotype consisted of white filament bundles, with low C:N ratios (3.5–5.4) and high sulfur content (16.1-51.2%). Bacterial diversity was low overall, and the most prevalent taxonomic group was affiliated with the “Epsilonproteobacteria” (68%); six genetically distinct epsilonproteobacterial groups were identified. Other bacterial sequences were affiliated with Gammaproteobacteria (12.2%), Betaproteobacteria (11.7%), Deltaproteobacteria (0.8%), and the Acidobacterium (5.6%) and Bacteriodetes/Chlorobi (1.7%) divisions. Epsilonproteobacterial and bacterial group abundances and overall community structure within the microbial mats 1 A portion of this chapter was used for the publication A.S. Engel, M.L. Porter, L.A. Stern, S. Quinlan, and P.C. Bennett, 2004, Bacterial diversity and ecosystem function of filamentous microbial mats from aphotic (cave) springs dominated by chemolithoautotrophic “Epsilonproteobacteria”, FEMS Microbiology Ecology, accepted. 29 shifted from the spring orifices downstream, corresponding to changes in habitat dissolved sulfide and oxygen concentrations and metabolic requirements of certain bacterial groups. Most of the epsilonproteobacterial groups were identified from high sulfide and low oxygen concentrations were measured, whereas Thiothrix spp. and Thiobacillus spp. had higher abundances where conditions of low sulfide and high oxygen concentrations were observed. Genetic and metabolic diversity among the “Epsilonproteobacteria” maximizes overall cave ecosystem function, and these organisms play a significant role in providing chemolithoautotrophic energy to the otherwise nutrient-poor cave habitat. These results expand the evolutionary and ecological views of “Epsilonproteobacteria” in terrestrial habitats and demonstrate that sulfur cycling supports this subsurface ecosystem through chemolithoautotrophy.