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Project Progress

We are currently investigating a sulfur spring system in a zone of high deformation in the central coast range of California. This system presents opportunities to study multiple, sustained groundwater discharge sites with environmental gradients affecting extant microbial communities. Understanding microbial community structure at these sites in relation to their environments provides insight into evolutionary mechanisms analogous to past terrestrial habitats of Earth and possibly Mars. These springs also provide access to significant carbonate accretions in which we can search for potential physical and chemical biomarkers from extant biology.

Low member microenvironments allow us to test methods to assess species’ richness, abundance, and ecological roles of community members. Geochemical and isotopic measurements of the system indicate both springs (MS4 and MS11) are primarily fed by deep, sub-surface water with MS4 discharging at lower alkalinity and sulfide levels than MS11. FISH and SEM (see figs. 1 and 2) suggest dominance of similar morphologies and taxa at both springs. Phylogenetic analysis of 16S rRNA gene libraries shows dominance of Gamma- and Epsilonproteobacteria at both springs, while MS4 exhibits greater overall community diversity. Microarray results illustrate substantial richness at both springs, ranging from 251-281 known OTUs as compared to 15-26 from rarefaction data of the libraries. The arrays revealed higher numbers of Alpha- and Beta-proteobacteria at MS4, increased levels of Chloroflexus at MS11, and the significant presence of Deltaproteobacteria at both sites relative to the libraries. Arrays further confirmed higher relative community diversity at MS4, with MS11 maintaining dominance of sulfur-oxidizing species. The novel Epsilon-proteobacteria were underestimated during array analysis due to limitations of chip architecture, whereas clone libraries amplified only dominant members of these sulfur-oxidizing communities. This work demonstrates the importance of integrating multiple molecular methods to overcome their inherent limitations for finer resolution of community structure to assess key ecological roles that are shadowed by dominance of a few members. We also observe the significant role of environmental gradients in selecting for the diversity of metabolic potential along geochemical gradients.

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Physical and chemical biomarkers in this system have been characterized in carbonate cores and biofilms. SEM of the extant biology reveals spherical metal sulfide precipitates s and suggests an origin for those observed in the carbonate thin sections. This prediction is corroborated by ion activity products of less than one for the spring waters. Microprobe analysis has also revealed possible microfossils in the carbonates as seen by sinuous carbonate orientation overlaying lighter elements using back-scattered electron imaging (fig.3). This ‘bio-fabric’ is consistent in length with the observed biology at the springs. Lipid analysis using thin layer chromatography of both the biofilms and core samples indicates pigments and sulfolipids as potential biomarkers.

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