Role Of Microbial Carbonic Anhydrase In Calcite Precipitation

Calcite precipitation plays very important roles in a lot of geological processes. Carbonic anhydrase (CA) is a Zn-containing metalloenzyme that catalyzes the interconversion of CO2 and HCO₃-. At present researches on the role of microbial CA on CaCO₃ precipitation have not been reported. The bacterial strain producing extracellular CA, screened and isolated from a karst soil in Southwest China, was used in the paper. The role of microbial CA in calcite precipitation was studied. The results obtained were as follows.(1) In both the experimental systems with and without the microorganisms, the precipitation rate of Ca²⁺ in the CA non-inhibited group was faster than that in the CA inhibited group, and was faster than that in the water and medium control groups. XRD and FESEM analysis indicated that calcite was the dominant mineral phase formed in these two experimental systems. These results suggested that microbial CA could promote calcite precipitation as an activator. On the other hand, the precipitation rate of Ca²⁺ was significantly faster in the experimental system with the microorganisms than that without the microorganisms. FESEM analysis indicated that there were some imprints about 1μm in length on the surface of calcite crystals in both two groups in the experimental system with microorganisms. These results suggested that microorganisms could serve as nucleation sites for calcite precipitation.(2) The comparative study on the calcite precipitation effects among proteins, polysaccharides, and microbial CA extracted from the microbial culture solution was conducted. The results showed that the precipitation of Ca²⁺ in microbial CA group was faster than that in the protein group and the polysaccharide group, and was faster than that in the water control group. Moreover, calcite was the main mineral phase in microbial CA group. These results indicated that CA produced and secreted by microorganisms in the microbial culture solution was a major activator of calcite precipitation.(3) The comparative study on the calcite precipitation effects among microbial CA and other biological factors was conducted. The results showed that the precipitation rate of Ca²⁺ in the microbial CA group was faster than that in 1% bovine serum albumin group, 1% carboxymethyl chitosan group and 1% glutamic acid group, and was faster than that in the water control group. The precipitation rate of Ca²⁺ in the latter three biological factors was lower than that in the water control group. Moreover, XRD and FESEM analysis indicated that calcite was the domainant crystal phase induced by microbial CA and other biological factors. There were great differences in the crystal morphology among microbial CA and other biological factors.In summary, microbial CA plays an significant promoting role in the precipitation of calcite. The results in this paper provided scientific base for enriching the theory of karst dymamics, and also provided scientific base for application of microbial CA in calcite precipitation, and provided a new idea for preparation of CaCO₃ biomieraliztaion materials.