Bacterial diversity and function within an epigenic cave system and implications for other limestone cave systems

There are approximately 48,000 known cave systems in the United States of America, with caves formed in carbonate karst terrains being the most common. Epigenic systems develop from the downward flow of meteoric water through carbonate bedrock and the solutional enlargement of interconnected subsurface conduits. Despite carbonate karst aquifers being globally extensive and important drinking water sources, microbial diversity and function are poorly understood compared to other Earth environments. After several decades of research, studies have shown that microorganisms in caves affect water quality, rates of carbonate dissolution and precipitation, and ecosystem nutrition through organic matter cycling. However, limited prior knowledge exists for the most common system, epigenic caves, regarding microbial taxonomic diversity, their metabolic capabilities, and how community function changes during and following environmental disturbances. To evaluate community development and succession, as well as potential roles in organic matter cycling, bacteria from the Cascade Cave System (CCS) in Kentucky were investigated. From geochemical and metagenomic data collected during a five-month colonization experiment, taxonomically distinct planktonic and sediment-attached bacterial communities formed along the epigenic cave stream. This represents one of the largest metagenomic studies done from any cave. Betaproteobacteria, Gammaproteobacteria, Alphaproteobacteria, and Opitutae were the most abundant groups. Planktonic bacteria pioneered sediment-attached communities, likely attributed to functional differences related to cell motility and attachment. Organic matter cycling affected exogenous heterotrophic community composition and function downstream because of diminished organic matter quality over time. This was reflected in significantly different abundances of genes encoding for carbohydrate and lignin degradation between habitats and depending on cave location. The ubiquity of environmental controls on bacteria functional diversity in karst is unknown because these environments have generally been left out of microbial biogeography research. In a spatial meta-analyses of bacterial diversity data from global cave systems, the ubiquity of some bacteria in karst is evident. Despite evidence for undersampling and difficulties comparing sequencing technologies and strategies, some caves appear to have novel lineages while other caves have taxonomically similar communities despite being 1000s of kilometers apart. The implications are that microbes in karst (i.e., carbonate) caves around the world are functionally comparable.