| Study on the formation mechanism of carbonate minerals may provide a scientific basis for the mineral capture technology to fix atmospheric CO2 and is likely to provide important clues in discovering extraterrestrial life.The co-precipitation of carbonate minerals with heavy metals is likely to play a significant role in the treatment of water and soil polluted with heavy metals.Moreover,bacterially induced carbonate mineralization has been proposed as a novel and environmental friend strategy for the conservation of deteriorated ornamental stone.At present,many research results of microbial carbonate minerals are available,but still numerous key issues are unclear.For example,location of mineral nucleation sites,factors controlling the formation of minerals and so on.To better understand the formation mechanism of carbonate minerals by microbes,culture experiments with Lysinibacillus sp.strain GW-2 and Arthrobacter sp.strain MF-2 were performed using B4 and M2 medium without carbonate ions for 50 days,respectively.A series of sterile control experiments without bacteria were run simultaneously.During the incubation,cell density,the quantity of precipitate,the extracellular polysaccharide(EPS)content,the low-molecular-weight organic acid concentration,the pH,the CA,the electrical conductivity,and the Ca2+ and Mg2+ concentrations of the medium were determined.The morphologies of precipitated carbonates were observed using scanning electron microscopy,and their mineral species were determined by X-ray diffraction,and provide more evidence for studying the formation process and its mechanism of carbonate biomineralization.The main results are as follows:(1)Lysinibacillus sp.strain GW-2 and Arthrobacter sp.Strain MF-2 both have the ability to induce the formation of carbonate minerals.They can use the CO2 in air or bacterial respiration induce the formation of carbonate minerals.(2)Morphologies and species of the carbonate minerals were different between GW-2 and MF-2.The three carbonate minerals by GW-2 formed according to following trend:amorphous calcium carbonate ? Huntite ? High-Mg calcite.The morphologies include dumbbell-shaped,cauliflower-shaped,spherical,hemispherical,platy and irregular shape.The precipitate induced by strain MF-2 experienced from ACC to High-Mg calcite.The morphologies include platy,spherical and irregular morphology.(3)Huntite might be formed through ageing of amorphous calcium carbonate,whereas high-Mg calcite might be formed through demagnesium of the huntite by strain GW-2;High-Mg calcite may be directly converted from ACC by strain MF-2.(4)The quantity of precipitate significantly negatively correlated with the EC and the Ca2+and Mg2+ concentrations by strain GW-2,with r = 0.89,0.93,and 0.98,P<0.0019 respectively.The quantity of precipitate significantly negatively correlated with the EC and the Ca2+ and Mg2+concentrations by strain MF-2,with r = 0.86,0.89,and 0.92,P<0.001,respectively.These results suggested that the decrease in the EC and the Ca2+ and Mg2+ concentrations could indirectly indicate the precipitation of carbonate(5)A significant positive correlation between the quantity of precipitate and the pH by strain MF-2,with r=0.95,P<0.05 and by strain GW-2,with r=0.79,P<0.05.Increased of pH provide the necessary physicochemical conditions for carbonate precipitation.(6)The average precipitation rate significantly positively correlated with cell density and EPS by strain GW-2,with r = 0.67,and 0.62,P<0.05,respectively.The average precipitation rate significantly positively correlated with cell density and EPS by strain MF-2,with r = 0.64,and 0.61,P<0.05,respectively.These results could imply that bacterial cells and EPS acting as a nucleation template are a significant effecting mechanism for carbonate precipitation.These results can help to improve the understanding of the formation process of carbonate minerals by bacteria,and provide a scientific basis for the further clarify its mechanism. |
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