The effect of enrichment media on the stimulation of native ureolytic bacteria in calcareous sand

Microbial-induced carbonate precipitation (MICP) is a naturally occurring bio-mineralization process. The most widely explored MICP pathway is ureolysis. The most existing studies focus on introducing exogenous ureolytic bacteria. Stimulating native ureolytic bacteria in soil (bio-stimulation) is an alternative approach, which is more cost-saving and eco-friendly. Currently, there are only a few studies regarding bio-stimulation in calcareous beach sand and the rationale of selecting enrichment media has not yet been fully established. In the present study, a batch-type experiment was conducted to investigate the effect of enrichment medium type on bio-stimulation efficiency. Calcareous beach sand sampled from a beach in Hawaii was used in the experiment. Three enrichment media—yeast extract, malt extract and nutrient broth—each amended with four urea concentrations (i.e., 0, 50, 100 and 170 mM) were adopted to enrich native ureolytic bacteria in the beach sand. Ureolytic activity, electric conductivity, pH value and viable cell number were measured during the 72-h experimental period. The results showed that the selection of effective medium type and urea concentration depends on whether a particular enrichment medium can: (1) achieve the optimal pH range; (2) establish the dominance of ureolytic activity over other metabolic activities; and (3) stimulate sufficient amount of native soil bacteria (thus sufficient amount of ureolytic bacteria). Moreover, it is found that excessive nutrients and the addition of ammonium in the enrichment solution are beneficial to the bio-stimulation.