9:30 AM - 9:45 AM
[MIS06-03] Influences of carbon and sulfur cycle inside the hypersaline microbial mat on the chemical composition of overlying brine water
Keywords:hypersaline environment, microbial mat, biogeochemical cycle
Major ion concentrations, DIC (dissolved inorganic carbon) concentration, and its carbon isotopic composition (δ13CDIC) were measured on 9 brine water samples with different salinity (38-276). Although intensive carbon fixation by photosynthesis occurs in the ponds where microbial mat is formed (salinity 97-159), corresponding DIC concentration showed little decrease (1250-1000 µmol L-1). This may be because mineralization of the fixed carbon by sulfate reduction occurs in the mat, releasing DIC in the brine water (e.g. Des Marais, 2003). This interpretation is supported by the low δ13CDIC value (~-10‰), because sulfate reduction supplies relatively 13C-depleted DIC. Alternative explanation is the chemically enhanced atmospheric invasion of CO2, which can cause considerable depletion in δ13CDIC (Baertschi, 1952; Lazar et al., 1992). By contrast, increase in the δ13CDIC value (~7.2‰) is observed in the ponds with higher salinity (>270) where no microbial mat is developing. This may be attributed to non-equilibrium escape of CO2 due to higher salinity and lower pH (Stiller et al., 1985; Barkan et al., 2001). These results suggest that δ13CDIC of hypersaline environment varies substantially due to both biological and physical processes.
Despite the intense sulfate reduction taking place in the mat, variation in SO42- concentration follows the ideal seawater evaporation path (Timofeeff et al., 2001). This may be because sulfide originating from the sulfate reduction is re-oxidized biotically by sulfur oxidizing bacteria or abiotically by the oxygen produced by cyanobacteria inhabiting the upper layer (e.g. Oren et al., 2009). Taken together, although DIC and SO42- are utilized intensively by the microorganisms in the mat, their content in the brine water are relatively unchanged due to efficient recycling in the mat.