title: Bacterial diversity and ecosystem function of filamentous microbial mats from aphotic (cave) sulfidic springs dominated by chemolithoautotrophic "Epsilonproteobacteria".
abstract: Filamentous microbial mats from three aphotic sulfidic springs in Lower Kane Cave, Wyoming, were assessed with regard to bacterial diversity, community structure, and ecosystem function using a 16S rDNA-based phylogenetic approach combined with elemental content and stable carbon isotope ratio analyses. The most prevalent mat morphotype consisted of white filament bundles, with low C:N ratios (3.5-5.4) and high sulfur content (16.1-51.2%). White filament bundles and two other mat morphotypes had organic carbon isotope values (mean delta13C=-34.7 per thousand, 1sigma=3.6) consistent with chemolithoautotrophic carbon fixation from a dissolved inorganic carbon reservoir (cave water, mean delta13C=-7.4 per thousand for two springs, n=8). Bacterial diversity was low overall in the clone libraries, and the most abundant taxonomic group was affiliated with the "Epsilonproteobacteria" (68%), with other bacterial sequences affiliated with Gammaproteobacteria (12.2%), Betaproteobacteria (11.7%), Deltaproteobacteria (0.8%), and the Acidobacterium (5.6%) and Bacteriodetes/Chlorobi (1.7%) divisions. Six distinct epsilonproteobacterial taxonomic groups were identified from the microbial mats. Epsilonproteobacterial and bacterial group abundances and community structure shifted from the spring orifices downstream, corresponding to changes in dissolved sulfide and oxygen concentrations and metabolic requirements of certain bacterial groups. Most of the clone sequences for epsilonproteobacterial groups were retrieved from areas with high sulfide and low oxygen concentrations, whereas Thiothrix spp. and Thiobacillus spp. had higher retrieved clone abundances where conditions of low sulfide and high oxygen concentrations were measured. 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. Our results demonstrate that sulfur cycling supports subsurface ecosystems through chemolithoautotrophy and expand the evolutionary and ecological views of "Epsilonproteobacteria" in terrestrial habitats.
authors.affiliation: Department of Geological Sciences, Research Group for Microbial Geochemistry, University of Texas at Austin, Austin, TX 78712, USA. aengel@geol.lsu.edu
authors.forename: Annette Summers
authors.initials: AS
authors.lastname: Engel
authors.forename: Megan L
authors.initials: ML
authors.lastname: Porter
authors.forename: Libby A
authors.initials: LA
authors.lastname: Stern
authors.forename: Sarah
authors.initials: S
authors.lastname: Quinlan
authors.forename: Philip C
authors.initials: PC
authors.lastname: Bennett
mesh_terms: D044822:Biodiversity; D018441:Biofilms; D002244:Carbon / Q000378:metabolism*; D002247:Carbon Isotopes / Q000032:analysis; D002965:Classification; D017753:Ecosystem; D055593:Geological Phenomena; D005844:Geology; D010802:Phylogeny; D020560:Proteobacteria / Q000254:growth & development* / Q000502:physiology*; D013440:Sulfides; D014881:Water Supply; D014959:Wyoming
chemical_list: D002247:Carbon Isotopes; D013440:Sulfides; D002244:Carbon