Microbial kinetic controls on calcite morphology in supersaturated solutions

Recognizing microbial imprints in the morphology of calcium carbonate minerals that form in very supersaturated solutions containing a high level of dissolved inorganic carbon (DIC) is challenging. To better define criteria for this purpose, we have analyzed the influence of sulfate-reducing bacterium Desulfovibrio desulfuricans strain G20 on the morphology of calcite in such solutions. G20 does not induce large shifts of pH or alkalinity under these conditions, but its uptake of millimolar sulfate and lactate decreases the number of anhedral crystals and stimulates growth of subhedral spar crystals relative to the abiotic controls. In addition, organic compounds associated with the basal growth medium, purified exopolymeric substances produced by G20 and lypopolysaccharide, stimulate the growth of anhedral crystals and crystals with rounded edges at low supersaturation index (SI) of calcite. The effect of organic compounds is reduced at higher SI, where rhombohedral habits dominate. Our results suggest that the local production and uptake of kinetic inhibitors within microbial biofilms may be an important control on calcite morphology in supersaturated solutions.