KARST GROUNDWATER IN YUCATAN PENINSULA, MEXICO, AND MICROBES: WHICH ENVIRONMENTAL CONDITIONS INFLUENCE THE MICROBIAL COMMUNITIES MOST?

Yucatan Peninsula, Mexico, has encountered groundwater problems throughout its history due to its highly permeable limestone karst terrain and dearth of soil. In the past 40 years, the explosively increasing population and inadequate infrastructure have deteriorated the groundwater quality even further. One major problem is wastewater treatment and disposal: currently there is no unified strategy for handling the wastewater or its effluent. One option is to inject the effluent into saline groundwater below the freshwater boundary, with an expectation that indigenous microbes in the sediment and deep saline groundwater will handle the organic carbon load. However, the composition of these microbial populations has not been characterized. In this study, Illumina sequencing of the 16S rRNA gene was used to look at the microbial community compositions of four groundwater sites to determine which factors (salinity, season, vegetation carbon and light input, location, and environment) contribute most to the differences between the microbial communities. Shannon entropy, Chao1 index and phylogenetic difference were used to assess the alpha diversity of the samples, with the sediment samples and the groundwater site at a busy tourist resort showing significantly higher diversity than other sites. Beta diversity was evaluated using UniFrac method, showing the greatest difference between water and sediment environments, and between the water at the coastal and inland locations. Of the specific groups of microbes, bacteria associated with sulfur cycling are expectedly abundant at the interface and saline water. Of sulfur oxidizing bacteria, the ratio of purple non-sulfur bacteria to purple sulfur bacteria is high, which is atypical to most aquatic environment, indicating organic pollution. Of sulfate reducing bacteria, there is a notable absence of all Gram-positive sulfate reducers from almost all sites. Many samples also show a high abundance of bacteria belonging to genera Sphingomonas and Pseudomonas, which in the aquatic environment are generally associated with organic pollution, and genus Novosphingobium that is a genus notable for its ability to degrade aromatic compounds.