Examination of Microbial Communities through a Freshwater/saltwater Transition Zone in Cenotes, Yucatan, Mexico

Examination of the geochemical and microbial characteristics within a biofilm can be difficult due to small scale transitions between the layers. In the Yucatan, water in cenotes (sinkholes) comprises a unique environment with a large transition zone from aerobic freshwater to underlying anaerobic saltwater. This system is an excellent natural analogue to sewage pipes, where the activity of the sulfur-cycling microbes (reduction of sulfide minerals and oxidation of H₂S) is present. A metabolic capacity and metagenomic analysis of the microbial community was performed through the transition zone. Two cenotes were sampled with Cenote Xcolak having a transition zone between 52 and 56m below the surface, while Cenote 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. Metagenomic analysis examines the genetic makeup of the system which allows for monitoring of the community response to different locations, time, or changing environments. Previous research in these cenotes has identified various groups of sulfate reducing and sulfur oxidizing microbes in the water column as well as in the sediment. Metagenomic libraries are being constructed from the same depths as the phenotypical analysis using universal 16S rDNA and group specific primers (dsr, mcrA, and SRB groups). This research gives insight into the relationship between the geochemistry and microbiology of biofilms which is not easy to observe in situ.