Experimental Study On The Biomineralization Mechanism Of Ca And Mg Ions Induced By Microorganisms

Microbial-mineralization could change the microenvironment and leading to the precipitation of minerals.The Ca-Mg carbonate minerals produced by microorganisms account for more than 70%of the total carbonate sedimentary rocks,forming excellent oil and gas reservoirs and mineral resources.Microorganisms in sedimentary environment are complex and diverse.However,the mechanism of microenvironment change during the biomineralization of calcium and magnesium,the characterization and nucleation sites of mineralized products,the source and function of organic components in minerals,intracellular mineralization phenomena and other issues are still controversial.In present study,biomineralization of calcium and magnesium induced by facultative anaerobic bacteria LH1 and Enterobacter ludwigii SYB1)were used except for cyanobacteria(Synechocystis sp.PCC6803),sulfate-reducing bacteria(Acinetobacter calcoaceticus SRB4)and halophilic bacteria(Bacillus licheniformis SRB2)were carried out in fresh water,salt water,aerobic and facultative anaerobic environments containing different Mg/Ca(Ca2+=0.01 M)molar ratios.In the biomineralization experiments,it was found that carbonic anhydrase plays an important role in the increasing the pH and saturation index.S.PCC6803 does not produce ammonia,a large amount of carbonate minerals can also be produced by the catalytic of CA.Bacteria(SRB2,LH1,SYB1)can produce ammonia gas,and the pH value will increase through hydration,but the calculation results based on chemical reactions show that the amount of OH-generated by ammonia gas production can only increase the pH value to about 8.3.Alkaline CA can catalyze the hydration of carbon dioxide to generate carbonate and bicarbonate.In order to verify the role of CA in increasing the pH value,a mixed solution of sodium carbonate and sodium bicarbonate was used to simulate the real-time carbonate and bicarbonate ion concentrations in the medium,and the pH value of the simulated solution was found to be about 9.5.Therefore,CA played a key role in the alkaline environment formation.The large amount of carbonate and bicarbonate produced by CA also increase the carbonate mineral saturation index,resulting in rapid nucleation and precipitation of minerals.Compared with the "early occurrence" and "early end"characteristics of ammonia production,CA activity appears late,but the duration can last until the recession period of microbial growth.It was found that Ca-Mg carbonate and phosphate minerals induced by bacteria are significantly affected by Mg/Ca molar ratios,they usually have unique characteristics including various morphologies,preferred orientation,and higher thermodynamic stability,etc.The minerals induced by S.PCC6803 are calcite(Mg/Ca=0)and low-Mg calcite(Mg/Ca=0.2,0.4 and 0.6).The magnesium content in minerals were dramatically influenced by the hydro-chemistry.Based on the fact that S.PCC6803 promote the entrance of magnesium into the calcite structure,providing a theoretical reference for the mechanism of dolomization in nature.Sulfate-reducing bacteria SRB4 induced struvite under the Mg/Ca ratio of 9,whose crystals are different from struvite of chemical origin,and the cell volume increases,fibrous whiskers appear,and the ratio of phosphorus and magnesium elements is 1.65:1(standard ratio is 1.46:1),showing a higher content of phosphorus elements.In the medium with similar salinity to seawater,halophilic bacteria SRB2 induced minerals including calcite,vaterite,monohydrocalcite(MHC)and nesquehonite(NES).The lattice structure of calcite has crystal plane dislocation,the(002)crystal plane of NES was preferentially oriented.LH1 induced calcite,MHC,dypingite(DYP)and NES.SYB1 induced calcite,MHC,DYP and NES.The(222)crystal plane of MHC is preferentially oriented,and its activation energy(171.60 kJ·mol-1)is higher than chemical minerals(166.33 kJ-mol-1),so biominerals have higher thermal stability.From the above results,it can be seen that there were four strains can induce Mg to participate in biomineralization,and the products are struvite and NES(some are also doped with a small amount of DYP).Microbial minerals contain 17kinds of amino acids with organic functional groups including C=O,O-H,S-H,N-H,and P-O.It is indicated that organic functional groups can attract calcium and magnesium ions to aggregate on the EPS and nucleate to form minerals.Based on molecular dynamics simulations,it was found that there are differences in the adsorption capacity of 17 amino acid molecules on different crystal surfaces of MHC.Various types of amino acids tend to adsorb on the[111]crystal plane,resulting in the change of the growth rate of the crystal plane.Aspartic acid and glutamic acid have the lowest adsorption energy on the(222)crystal plane(-192.3654 and-186.4257 kcal·mol-1),so they are the highest amino acids in microbial MHC.There were poor or well crystalline particles such as struvite and monohydrocalcite,combined with the phenomenon that the surface of microbial cells can form a mineralized shell,which further confirmed that EPS is the nucleation site.For the study of intracellular mineralization,it was found that the spherical inclusions produced by bacteria may be the result of extracellular stress resistance.The phase combination and mineralization mechanism of calcium-magnesium minerals induced by microorganisms summarized in this study are expected to provide a reference for the microbial mineralization mechanism and mineral nucleation sites in geological history and modern natural environment.