A MICROBIAL ANALYSIS OF WATER THROUGH A TURBID FRESHWATER/SALTWATER TRANSITION ZONE IN CENOTES, YUCATAN, MEXICO

Yucatan cenotes comprise a unique environment with a large transition zone from aerobic freshwater to underlying anaerobic saltwater, thus providing a place to study biologically mediated redox reactions of carbon and sulfur. In these cenotes, the saltwater intrusion is highly sulfidic with a sulfate-sulfide isotopic fractionation of 63‰, well above 42‰ produced by pure cultures of sulfate reducing bacteria in laboratories. Previous research indicated that H₂S production is mediated primarily by sulfate reducers of the Desulfococcus-Desulfonema-Desulfosarcina species. However, these bacteria make up 0.1% of the whole microbial community; this consequently has led to an analysis of the whole microbial ecology. Cenotes Xcolak and Calica were sampled. Xcolak has a transition zone that is between 52 and 56m below the surface, while Calica's occurs between 14 and 16m. Microbial community characterization from ten depths in Xcolak and six depths in Calica were measured by sole carbon source utilization in BIOLOG EcoPlates (BIOLOG, Inc.) under aerobic and anaerobic conditions. Preliminary results indicate that the microbial community structures are statistically different between the two cenotes, presumably resulting from differences in depth to the transition zone and variation of geochemical conditions.

At the transition zone (56m) in Xcolak a layer of high turbidity was discovered. This layer could either be due to organic matter accumulating above the density transition or accumulation of bacteria containing elemental sulfur “globules” such as Thiomargarita or Allochromatium below the density transition. Carbon consumption patterns on the BIOLOG plates negate the likelihood that the turbid layer is organic matter accumulating, because at the interface there is not increased utilization of carbon sources compared to samples from depths around the interface. It is hypothesized that the bacteria producing “globules” accumulate at the interface where they are oxidized. Because S has an intermediate isotopic composition it does not change the sulfur isotope composition of the fresh water as much as H₂S would. This hypothesis may explain the perplexing loss of ³⁴S-depleted sulfur in the saline intrusion. Future work will focus on molecular investigation of the organisms associated with this hypothesis.