Halotolerant, alkaliphilic urease-producing bacteria from different climate zones and their application for biocementation of sand

Microbially induced calcium carbonate precipitation (MICP) is a phenomenon based on urease activity of halotolerant and alkaliphilic microorganisms that can be used for the soil bioclogging and biocementation in geotechnical engineering. However, enrichment cultures produced from indigenous soil bacteria cannot be used for large-scale MICP because their urease activity decreased with the rate about 5 % per one generation. To ensure stability of urease activity in biocement, halotolerant and alkaliphilic strains of urease-producing bacteria for soil biocementation were isolated from either sandy soil or high salinity water in different climate zones. The strain Bacillus sp. VUK5, isolated from soil in Ukraine (continental climate), was phylogenetically close in identity (99 % of 16S rRNA gene sequence) to the strain of Bacillus sp. VS1 isolated from beach sand in Singapore (tropical rainforest climate), as well as to the strains of Bacillus sp. isolated by other researchers in Ghent, Belgium (maritime temperate climate) and Yogyakarta, Indonesia (tropical rainforest climate). Both strains Bacillus sp. VS1 and VUK5 had maximum specific growth rate of 0.09/h and maximum urease activities of 6.2 and 8.8 mM of hydrolysed urea/min, respectively. The halotolerant and alkaliphilic strain of urease-producing bacteria isolated from water of the saline lake Dead Sea in Jordan was presented by Gram-positive cocci close to the species Staphylococcus succinus. However, the strains of this species could be hemolytic and toxigenic, therefore only representatives of alkaliphilic Bacillus sp. were used for the biocementation studies. Unconfined compressive strengths for dry biocemented sand samples after six batch treatments with strains VS1and VUK5 were 765 and 845 kPa, respectively. The content of precipitated calcium and the strength of dry biocemented sand at permeability equals to 1 % of initial value were 12.4 g Ca/kg of dry sand and 454 kPa, respectively, in case of biocementation by the strain VS1. So, halotolerant, alkaliphilic, urease-producing bacteria isolated from different climate zones have similar properties and can be used for biocementation of soil.