PROFILING THE FUNCTIONAL DIVERSITY OF MICROBIAL COMMUNITIES TO UNDERSTAND THE DEVELOPMENT AND MAINTENANCE OF THE EDWARDS AQUIFER ECOSYSTEM

The Edwards Aquifer is a regional karst aquifer in central Texas with a distinct freshwater zone juxtaposed with a deep, confined saline and sulfidic water zone. The geochemical interface coincides with a diverse stygobiont community of over 55 known taxa, nearly all of which are endemic to the region. Highly-restricted and dispersal-limited stygobiont assemblages arose from different aquifer invasions over time. Marine-derived groups (i.e., hadziid and sebid amphipods, thermosbaenaceans, cirolanid isopods) date back to the Paleocene-Eocene thermal maximum (~55 mya), but invasions of freshwater species were more recent. Stable isotope ratio analysis of carbon subsidies and stygobionts, including filter-feeders, grazers/foragers, and predators, shows that different proportions of organic matter are derived from photosynthetic activity versus chemolithoautotrophy, depending on proximity to the freshwater-saline water (FW-SW) interface and trophic position. Filter-feeders collected proximal to the FW-SW interface have stable isotope compositions indicative of predominately chemolithoautotrophic organic matter. Based on the phylogeny of 16S rRNA gene sequences, chemolithoautotrophic microbial groups at the FW-SW interface include putative sulfur-oxidizers, methane-oxidizers, ammonium-oxidizers, and sulfate-reducers, as well as heterotrophs and fermenters. Insight into the metabolic potential across the carbon, sulfur, and nitrogen cycles for different chemolithoautotrophic metabolisms comes from high-throughput shotgun sequencing of DNA obtained from the aquifer, followed by annotating metagenomic data and reconstructing draft genomes of environmental microbes. Many of the key bacterial and archaeal groups are not known, or well characterized, from karst aquifer systems. Even after 120 years of study, the Edwards Aquifer ecosystem continues to be at the forefront of hydrogeochemical, geobiological, and ecological investigations.