The Catalytic Effect of the Heterotrophic Bacterium Virgibacillus marismortui on Basaltic Rock Dissolution Under Excess Nutrient Conditions

Biotic and abiotic batch experiments were performed in the presence of excess nutrients (N and P) and basaltic rocks in the medium under various conditions (NA, non-agitated; A, agitated). Changes in solution chemistry were monitored to characterize the influence of aerobic heterotrophic bacteria (Virgibacillus marismortui) on the dissolution rates and mechanisms of two basaltic rocks collected from King George (BAn; basaltic andesine) and Deception Island (BD, basalt), Antarctica. The presence of bacteria in the medium accelerated dissolution of basaltic rocks by a factor of 1.5 to 3 depending on both the rock composition and experimental conditions. The calculated linear element release rates in the biotic experiments (B, biotic) are enhanced in comparison with abiotic systems (A, abiotic) in the order: BD-B-NA > BD-B-A > BAn-B-NA. Unlike the abiotic reference experiments (BD-A-NA; BD-A-A, BAn-A-NA) with pH shifts of less than 0.5 units an accelerated release of Si, Ca, Mg, Fe and Al coincided with an increase in protein concentrations and a marked decrease in pH in the biotic experiments. As indicated by solution acidity, element release, trends of the reactive fluids and surface chemistry of the reacted rock samples, the catalytic effect of bacteria on mineral dissolution reactions was predominantly acidification due to bacterial metabolism. The major catalytic effect of acidification on mineral dissolution was likely suppressed by ammonification, resulting in a pH increase in the reactive fluid. The absence of secondary phases (e.g. Fe and Al oxyhydroxides) on the surface of biotically reacted rock particles and the presence of divalent cations (i.e. Ca and Mg) on the cell surfaces further show the role of the bacteria and associated organic ligands for metal-complexation reactions during basalt dissolution. This study shows that aerobic heterotrophic bacteria may play a critical role for acidity driven silicate dissolution in environments rich in nitrogenated organic compounds and may even influence the amount of Ca and Mg being released from Ca and Mg rich silicates to environments.