Kinetics of calcite precipitation by ureolytic bacteria under aerobic and anaerobic conditions

The kinetics of urea hydrolysis (ureolysis) and induced calcium carbonate (CaCO ₃) precipitation for engineering use in the subsurface was investigated under aerobic conditions using Sporosarcina pasteurii (ATCC strain 11859) as well as Bacillus sphaericus strains 21776 and 21787. All bacterial strains showed ureolytic activity inducing CaCO ₃ precipitation aerobically. Rate constants not normalized to biomass demonstrated slightly higher-rate coefficients for both ureolysis (kurea) and CaCO ₃ precipitation (kprecip) for B. sphaericus 21776 (kurea D 0:10 ± 0:03 h ^-1, kprecip D 0:60 ± 0:34 h ^-1) compared to S. pasteurii (kurea D 0:07 ± 0:02 h-1, kprecip D 0:25 ± 0:02 h ^-1), though these differences were not statistically significantly different. B. sphaericus 21787 showed little ureolytic activity but was still capable of inducing some CaCO ₃ precipitation. Cell growth appeared to be inhibited during the period of CaCO ₃ precipitation. Transmission electron microscopy (TEM) images suggest this is due to the encasement of cells and was reflected in lower kurea values observed in the presence of dissolved Ca. However, biomass regrowth could be observed after CaCO ₃ precipitation ceased, which suggests that ureolysis-induced CaCO ₃ precipitation is not necessarily lethal for the entire population. The kinetics of ureolysis and CaCO ₃ precipitation with S. pasteurii was further analyzed under anaerobic conditions. Rate coefficients obtained in anaerobic environments were comparable to those under aerobic conditions; however, no cell growth was observed under anaerobic conditions with NO ^- ₃, SO _{2 4} ^- or Fe3C as potential terminal electron acceptors. These data suggest that the initial rates of ureolysis and ureolysis-induced CaCO ₃ precipitation are not significantly affected by the absence of oxygen but that long-term ureolytic activity might require the addition of suitable electron acceptors. Variations in the ureolytic capabilities and associated rates of CaCO ₃ precipitation between strains must be fully considered in subsurface engineering strategies that utilize microbial amendments.