Importance of carbon to nitrogen ratio in microbial cement production: Insights through experiments and genome-scale metabolic modelling

Microbial cement production based on microbially induced calcium carbonate precipitation (MICP) is a technology with high potential but is economically limited. Lower medium costs are expected to improve the economic attractiveness of microbial cement production. We report the effect of an important medium parameter, the carbon-to-nitrogen (C/N) ratio (2, 5, 10, 15, and 20) on the intracellular urease activity (IUA) of ureolytic bacteria, Sporosarcina pasteurii (ATCC 11859) through experiments and analysis of a relevant genome-scale metabolic model (GSMM). A positive correlation was observed between IUA and C/N ratio until an optimal C/N ratio of 15. At a C/N ratio of 15, the IUA was 29.5 U/(g dry-cell-weight). Further, the ammonium (nitrogen waste) generation decreased by 7.7-fold at the optimal C/N ratio of 15 compared to that at 2, which is environmentally significant. Despite the decrease in initial urea concentration with an increasing C/N ratio, no decline in the biomass was observed, which is advantageous. Analysis of the GSMM provided the insights that the optimal C/N ratio of 15 also ensures an efficient bio-mineralization process and that increasing the ratio beyond 15 may negatively impact bio-mineralization. The results will be of significance while selecting a low-cost medium for field-scale MICP applications.