The Formation Mechanism Of Calcium/Magnesium Carbonate Minerals Induced By Arthrobacter Sp.MF-2 And Curvibacter Sp.HJ-1

Ca-Mg carbonate minerals are widely distributed in nature,and are very common in experimental systems.A large number of researches have shown that bacteria can induce the formation of Ca-Mg carbonate minerals with different crystal species and morphologies via altering the physicochemical property of the microenvironment.The study on formation mechanism of carbonate minerals induced by bacteria is of great significance in enriching soil formation and evolution theory,bioremediation of contaminated soil,immobilization of CO2,and biogeochemical cycling of elements.However,there are still many unknowns about the formation mechanism of Ca-Mg carbonate minerals and the roles of bacteria in biomineralization process.In view of the important significances and deficiencies of current research on the formation mechanism of Ca-Mg carbonate minerals induced by bacteria,a series of mineralization experiments were performed in M-2 culture media with different Mg/Ca molar ratios in the presence of Arthrobacter sp.MF-2,Curvibacter sp.HJ-1 and extracellular polymers secreted by them,respectively.Meanwhile,the corresponding control experiments were carried out.In this paper,we probed the formation mechanism of Ca-Mg carbonate minerals induced by bacteria on the basis of observations on dynamic changes of liquid environment,mineral evolution process and characteristic of isotope fractionation during precipitation.The main findings are as follows:(1)Bacterial species could affect mineral species and morphology.Strain MF-2 tended to form calcite,while strain HJ-1 was more favorable for the formation of metastable minerals(vaterite and aragonite).The evolution tendencies of mineral morphology under the actions of two strains were similar.Morphological evolution orderly developed from rod-shaped,dumbbell-like,cauliflower-like to spherical morphology in the presence of strain MF-2.As for strain HJ-1,morphological evolution orderly developed from rod-shaped,cross-shaped,star-shaped,cauliflower-like to spherical morphology.(2)Bacterial extracellular polymers could affect mineral species and morphology.In the(NH4)2CO3-diffusion experiments,extracellular polymers secreted by bacteria were conductive to the formation of monohydrocalcite and high-Mg calcite.The mineral morphology under the action of bacterial extracellular polymer was mainly irregular and spherical.(3)Mg/Ca molar ratio could affect the species and morphology of carbonate minerals.In the absence of Mg,the formations of vaterite and calcite were observed,while high-Mg calcite and aragonite were the main mineral species under Mg-containing conditions.Mineral morphology had a tendency to transform from rhombohedral to long-columnar and spherical with the increase of Mg/Ca ratio.(4)Bacterial mineralization could affect the isotope fractionations between solid and liquid.The δ13C values of the biogenic carbonate minerals were between-15.7‰and-4.2‰,and the δ18O values were between-10.9‰ and-1.6‰.The temporal changes of δ13C values between different Mg/Ca ratios reflected that the carbonate ions in the mineral were mainly derived from the oxidation of organic substances in the medium.The δ26Mg value of biogenic carbonate minerals was between-2.448‰ and-0.811‰,which was slightly heavier than that of abiogenic minerals.It was affected by inorganic precipitation and biological metabolism.The involving of bacteria and their extracellular polymer contributed to rich heavier Mg in calcite.The C,O and Mg isotope fractionations in systems were significant at the transformation process from ACC to crystalline minerals.(5)The bacterial mineralization mechanism of Ca-Mg carbonate.Bacteria induce the formation of carbonate minerals via creating beneficial physicochemical conditions,providing nucleation sites(bacterial cells and extracellular polymers)and secreting carbonic anhydrase(catalyzing the hydrolysis of CO2 produced by bacterial respiration).Meanwhile,bacterial activities colud affect isotope fractionation between mineral and liquid.The extracellular polymer secreted by bacteria could act as a heterogeneous nucleation site,promoted Mg into the mineral lattice changing the growth orientation,as well as affected mineral crystal species and morphology.Mg2+could also affect the species and morphology of carbonate minerals by changing the crystal growth orientation.