Microbial dissolution of calcite at T = 28 °C and ambient pCO2

This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species (Burkholderia fungorum). Experiments were conducted at T = 28°C and ambient pCO2 over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH4+ as an N source, and H2PO4− as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H2O-CO2-CaCO3 system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH4+ decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H2CO3 generated by dissolution of atmospheric CO2 (H2CO3 + CaCO3 → Ca2+ + 2HCO3−) and H+ released during NH4+ uptake (H+ + CaCO3 → Ca2+ + HCO3−). Reaction with H2CO3 and H+ supplied ∼45% and 55% of the total Ca2+ and ∼60% and 40% of the total HCO3−, respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH4+ was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H2CO3. In lactate bearing reactors, most H+ generated by NH4+ uptake reacted with HCO3− produced by lactate oxidation to yield CO2 and H2O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H2CO3 at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe-calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH4+ appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions.