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.

Niche differentiation among sulfur-oxidizing bacterial populations in cave waters

“…Filamentous epsilonproteobacteria dominate in waters with high sulfide and low oxygen, while Thiothrix dominate in waters with low sulfide and high oxygen. A similar pattern was suggested by 16S rDNA clone frequencies in a study of LKC (Engel et al, 2004), but has not been demonstrated until now. Figure 2 also shows that either sulfide or oxygen concentrations alone are poor predictors of biofilm compositions.…”

An oligarchic microbial assemblage in the anoxic bottom waters of a volcanic subglacial lake

“…In the sample, detectable (X2 mM) O 2 was absent and bottom water would have evolved to sulfide/oxygen ratios b12 within a few days by abiotic oxidation alone (Millero, 2005). At sulfide/ oxygen ratios4150, biofilms of filamentous e-proteobacteria replace the g-proteobacteria groups Beggiatoa and Thiothrix in cave waters (Engel et al, 2004;Macalady et al, 2008), and many e-proteobacteria such as S. kujiense are capable of microaerophilic growth at micromolar O 2 (Kodama and Watanabe, 2004). Sulfide-oxidizing bacteria activity also explains the absence of detectable thiosulfate (S 2 O 3 2À ) and sulfite (SO 3 2À ), which would otherwise accumulate during abiotic sulfide oxidation (Millero, 2005).…”

Tackling the minority: sulfate-reducing bacteria in an archaea-dominated subsurface biofilm

“…Both of these sites are characterized by a main, sulfidic spring, emanating into a streamlet where whitish mats of sulfide-oxidizing bacteria cover the submerged surfaces. These aquifers are very similar to sulfidic cave springs that are rich in sulfide, ammonia and sulfate (Engel et al, 2004) but poor in dissolved organic carbon, suggesting that the major microbial community of the biotopes are chemolithoautothrophs (Engel et al, 2003;Kodama and Watanabe, 2004). Although, sulfidic springs represent o10% of terrestrial fresh water springs (Palmer, 1991), they are believed to have an important role in global sulfur-cycling (Engel et al, 2003), as they can spawn huge amounts of microbial biomass mainly consisting of sulfur-oxidizing bacteria such as Thiothrix, Beggiatoa and Sulfuricuvum.…”