To Eat or Not to Eat: A Thermodynamic Model of Free and Calcite-Bound Organic Matter Respiration by Karst Microbes

In the subsurface, heterotrophic microorganisms require organic matter not only as a carbon source but also as an electron acceptor to be used during respiration. This organic matter is characteristically found adsorbed to mineral surfaces in dilute concentrations. While the utilization of free, simple organic molecules is most likely energetically favorable, molecules that are sorbed to mineral surfaces must first be desorbed, resulting in a decrease in the overall Gibbs energy of the organic molecule thus making its utilization less energetically favorable for a microbe. Therefore, the goal of this study was to calculate the energy yield of the oxidation of simple, organic molecules of varying concentrations (free aqueous versus bound citric acid and benzoic acid) and then model the environmental conditions that constrain their use in karst vadose zone-like systems. We assemble organic compound/O2/CO2 stability fields that make it possible to predict the environmental conditions under which complete oxidation of the substrates may occur. Our results suggest that under certain conditions likely to be found in the subsurface, the utilization of compounds such as benzoic acid which are more strongly bound to calcite are energetically unfavorable and, thus, cannot be exploited as a source of organic carbon to cave-dwelling microbes. These results have implications for the possible role of microbes in calcite dissolution/precipitation reactions in the subsurface as well as the mobilization and fate of some groundwater contaminants.