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Sulfur Cycling in Microbial Mats

Two-dimensional mapping of sulfur cycling in microbial mats using SIMS and CARD-FISH. (Post-doc David Fike and graduate student Crystal Gammon)

The metabolic activities of microbial mats have likely regulated biogeochemical cycling over most of Earth's history. However, the relationship between metabolic activity and the establishment of isotopic geochemical gradients in these mats remains poorly constrained. Here we present a parallel microgeochemical and microbiological study of micron-scale sulfur cycling within hypersaline microbial mats from Guerrero Negro, Baja California Sur, Mexico. Dissolved sulfide within the mats was captured on silver discs and analyzed for its abundance and d34S isotopic composition using high-resolution secondary ion mass spectrometry (nanoSIMS). These results were compared to sulfide and oxygen microelectrode profiles. Two-dimensional microgeochemical mapping revealed well-defined laminations in sulfide concentration (on scales from 1 to 200 um), trending toward increased sulfide concentrations at depth. Sulfide d34S decreased from 10% to -20% in the uppermost 3mm and oscillated repeatedly between -10% and -30% down to a depth of 8mm. These variations are attributed to spatially variable bacterial-sulfate reduction within the mat. A parallel examination of the spatial distribution of known sulfate-reducing bacteria within the family Desulfobacteraceae was conducted using catalyzed reporter deposition fluorescence in situ hybridization. Significant concentrations of Desulfobacteraceae were observed in both oxic and anoxic zones of the mat and occurred in several distinct layers, in large aggregates and heterogeneously dispersed as single cells throughout. The spatial distribution of these microorganisms is consistent with the variation in sulfide concentration and isotopic composition we observed. The parallel application of the methodologies developed here can shed light on micron-scale sulfur cycling within microbially dominated sedimentary environments.

 Fike, D. A., C. L. Gammon, W. Ziebis, and V. J. Orphan (2008) Micron-scale mapping of sulfur cycling across the oxycline of a cyanobacterial mat: A paired nanoSIMS and CARD-FISH approach.  ISME Journal 2: 749-759

Further studies of mat-associated sulfide are underway using the silver sulfide capture methodology outlined in Fike et al. (ISME 2008), using the 7F Geo SIMS for analysis of sulfide abundance and isotopic composition. We are focusing on different parts of the diel metabolic cycle: under full light conditions that maximize photosynthetic activity in the uppermost layers of mats and also in the dark to understand the daily changes in sulfur isotope profiles in response to this redox forcing. We are also examining the changes in sulfide isotope fractionation that arise from differing amounts of sulfate in the mats, using field incubations from Guerro Negro, Baja California Sur, Mexico at 80 mM sulfate and laboratory incubations at the NASA Ames Greenhouse facility at 80 mM, 1 mM, and 0.2 mM sulfate concentrations. Understanding the effect of sulfate concentrations on isotope fractionations is necessary in order to apply our knowledge of modern systems to reconstruct the geologic record.

Fike, D.A., N. Finke, G. Blake, J. Zha, T.M. Hoehler, V.J. Orphan (2009) The effect of sulfate concentration on (sub)millimeter-scale sulfide δ34S in hypersaline cyanobacterial mats over the diurnal cycle. Geochem. et Cosmochem. Acta. 73: 6187-6204