Role of Microbes in Hypogenic Karst Development and Modification

Karst surfaces offer reactive, air-water-rock interfaces for microbially-induced and -enhanced reactions. Considering the extent to which carbonate rocks comprise the rock record, and the depths to which karstification occurs in the subsurface, the microbial biomass within karst settings and at karst interfaces is potentially tremendous. Therefore, microbes and microbial processes are central to all of the karst-related geological and ecological sciences. Here, we explore the diversity of microbes from hypogenic karst systems and evaluate the microbial roles in karst development and modification. We consider hypogenic systems to be those where karst formation has been due to recharge from below, rather than from the surface. From a biological perspective, influx of organics and nutrients from the surface is limited in hypogenic systems, and so ecosystems developed within hypogenic karst are based partially or wholly on chemolithoautotrophy. Chemolithoautotrophic reactions couple redox transformations to the fixation of inorganic carbon. In modern hypogenic systems, microbially-induced oxidation and reduction of sulfur compounds, methanogenesis, and metal oxidation (e.g., Fe and Mn) are prevalent. Such transformations directly affect proton balance, and consequently most chemolithoautotrophic oxidation reactions generate acidity that directly influence carbonate dissolution. While sulfur chemistry is not essential for hypogenic speleogenesis, notable sulfidic caves and karst are one type of hypogenic karst setting that has been extensively studied. These systems are dominated by chemolithoautotrophic sulfur-oxidizers, which locally promote carbonate dissolution due to colonization of the rock surfaces within the shallow subsurface. Recent evidence from sulfidic karst aquifers indicates that microbially promoted dissolution also occurs at depth. Ancient hypogenic karst settings have been intensely investigated to understand ferromanganese deposits, as well as oligotrophic metabolism. Our studies reveal known manganese-oxidizers, as well as novel species that are contributing to local carbonate dissolution. These examples illustrate the importance of microbial processes in hypogenic systems.