SULFUR CYCLING IN THE TERRESTRIAL SUBSURFACE - COMMENSAL INTERACTIONS, SPATIAL SCALES, AND MICROBIAL HETEROGENEITY

Microbiological, geochemical, and isotopic analyses of sediment and wa ter samples from the unconsolidated Yegua formation in east-central Te xas were used to assess microbial processes in the terrestrial subsurf ace. Previous geochemical studies suggested that sulfide oxidation at shallow depths may provide sulfate for sulfate-reducing bacteria (SRB) in deeper aquifer formations. The present study further examines this possibility, and provides a more detailed evaluation of the relations hip between microbial activity, lithology, and the geochemical environ ment on meter-to-millimeter scales. Sediment of varied lithology (sand s, silts, clays, lignite) was collected from two boreholes, to depths of 30 m. Our findings suggest that pyrite oxidation strongly influence s the geochemical environment in shallow sediments (similar to 5 m), a nd produces acidic waters (pH 3.8) that are rich in sulfate (28 mM) an d ferrous iron (0.3 mM). Sulfur and iron-oxidizing bacteria are readil y detected in shallow sediments; they likely play an indirect role in pyrite oxidation. In consistent fashion, there is a relative paucity o f pyrite in shallow sediments and a low S-34/S-32-sulfate ratio (0.2 p arts per thousand) (reflecting contributions from S-34-depleted sulfid es) in shallow regions. Pyrite oxidation likely provides a sulfate sou rce for both oxic and anoxic aquifers in the region. A variety of assa ys and direct-imaging techniques of S-35-sulfide production in sedimen t cores indicates that sulfate reduction occurs in both the oxidizing and reducing portions of the sediment profile, with a high degree of s patial variability. Narrow zones of activity were detected in sands th at were juxtaposed to clay or lignite-rich sediments. The fermentation of organic matter in the lignite-rich laminae provides small molecula r weight organic acids to support sulfate reduction in neighboring san ds. Consequently, sulfur cycling in shallow sediments, and sulfate tra nsport represent important mechanisms for commensal interaction among subsurface microorganisms by providing electron donors for chemoautotr ophic bacteria and electron accepters for SRB. The activity of SRB is linked to the availability of suitable electron donors from spatially distinct zones.