MICROBIAL AND GEOCHEMICAL INVESTIGATION OF DEEP, SULFIDE-RICH CENOTES, MEXICO

Water columns of two deep cenotes (sinkholes) of Yucatan, Mexico provide a unique natural environment in which to study biologically mediated redox reactions of carbon and sulfur and in which to examine isotope fractionation between reactant and product sulfur species. Laboratory experiments hitherto performed to better understand the parameters influencing sulfur isotope fractionation have proved difficult to extrapolate to natural systems; but, in the cenote environment it will be possible to compare specific communities of microorganisms responsible for specific steps in the redox cycle.

A particular advantage of certain Yucatan cenotes for this study of redox processes is that reaction zones are spread out over 10s of meters in the water column, compared to centimeters over which these processes can be observed in soils and sediments. In this research the water columns of two cenotes were investigated using a Datasonde 4a equipped with pH/ORP, electric conductivity, dissolved oxygen and depth sensors. Water samples were collected for major ion analysis and sulfide and sulfate precipitates were collected for isotopic analysis. Carbon (bio-sep) beads were deployed at discrete depths within each cenote for about a month to collect microorganisms. DNA was extracted from the beads and tested against primers for six groups of sulfate reducing bacteria using PCR.

In both cenotes a fresh water lens overlies a saline (modified seawater) intrusion, a stable stratification that is maintained by density differences. Dissolved oxygen is present in the well mixed fresh water lens but is absent below the fresh/saline water interface, and ORP changes abruptly from positive to negative at the interface. Below the interface, sulfate derived from the saltwater is reduced to H₂S (in concentrations ranging from 30 to 140 mg/L). In the zone of H₂S production, sulfur isotope fractionation (Δ³⁴S_SO4-H2S) of 38-55‰ was measured, which clearly indicates active sulfate reduction. Initial PCR results show that the H₂S production is mediated primarily by sulfate reducers of only one group of the six that were tested. This group encompasses microorganisms of the Desulfococcus- Desulfonema-Desulfosarcina species.