A WINDOW INTO THE MADISON AQUIFER: MANGANESE AUTOTROPHY IN THE LAKES OF WIND CAVE, WIND CAVE NATIONAL PARK, SOUTH DAKOTA (Invited Presentation)

Subsurface aquifers are an important source of freshwater; however, our understanding of the biogeochemical processes that support microbial ecosystem dynamics remains limited due to the difficulty of accessing these environments. The lakes in Wind Cave provide access for directly sampling the Madison aquifer, a regionally significant aquifer that contains one of the lowest cell numbers of any water on Earth. Despite this low biomass, the microbial community found there is remarkably stable and diverse. The low number of microbial cells in this environment has made sampling challenging; however, using tangential flow filtration, we were able to collect enough cells for metagenomics to study the community interactions and processes that supported life under the ultraoligotrophic conditions (0.29 mg/L TOC). Using comparative filtration, our data demonstrate that the lakes community is enriched in ultra-small cells, particularly within the Patescibacteria and Nitrospirota. Metagenomic analyses suggest an increase in genotypes associated with an oligotrophic lifestyle, including carbohydrate metabolism and amino acid scavenging. A reduction in evident prophage, along with a significant increase in integron density, suggests that integrons provide the genetic plasticity and metabolic flexibility necessary for adaptive evolution and survival. Finally, primary productivity in the ecosystem appears to be driven by chemolithoautotrophic manganese-oxidation, while evidence of redox active Mn-oxides may in-turn aid in the oxidation of recalcitrant organic carbon to release low molecular weight organics for growth. These suggest that due to the unique geochemical conditions of karst aquifers, the Mn geochemical cycle may play a principle role in subsurface primary productivity, while subsurface Mn-reduction may play an important, but as yet under-appreciated role in the carbon cycle.