Formation Mechanism Of Microbial Induced Carbonate Mineral-research On Microstructure Based On Focused Ion Beam Technology

Carbonate precipitation is one of the most important biogeochemistry processes on the Earth.The contribution of microorganisms to the precipitation of carbonate minerals cannot be underestimated.For example,the formation of soil calcareous nodules,sediments of rivers,lakes and marine are closely related to microorganisms.In addition,the process of microbial-induced carbonate mineral precipitation has potential application value in in the reconstruction of ancient geological environment,green construction materials,as well as the treatment of heavy metal pollution,and the storage of CO₂.Calcified bacteria have long been recognized as the evidence on that bacterial cell serve as the heterogenous nucleation site for carbonate during the mineralization process.Until now,research on calcified bacteria mainly focus on the morphological investigation,but the regulation mechanism of bacterial cells on the morphologies of carbonate minerals still remain unclear,especially the lack of evidence in micro-regional information.To further explore the nucleation and growth mechanism of microbial carbonate minerals,Arthrobacter sp.strain MF-2,Bacillus cereus strain LV-1,Curvibacter sp.strain HJ-1 are used to carry out biomineralization experiments in this research.Meanwhile,the mineralization system with（NH₄）₂CO₃ diffusion is used to carry out experiments induced by bacterial cell concentrate of strain HJ-1.This research focuses on the characterization and analysis of mineral phase,especially analysis of microscale structures,supplemented by solution chemistry and biological index analysis.We discussed the solution conditions of microbial induced carbonate precipitation,bacterial calcification and morphological transformation and evolution between major mineral morphologies.The main results and conclusions are as follows:1)The characteristics of carbonate minerals formed by the three bacterial mineralization experiments.ACC was formed preferentially and converted to calcite in the later period in the three bacterial mineralization experiments,indicating that ACC is the precursor of bacterial induced carbonate minerals.The minerals induced by the MF-2 strain are mainly spherical,while the minerals induced by the LV-1 and HJ-1 strains are mainly invaginated.Rod-shaped minerals and small pores in the form of bacteria can be found on the surface or inside of these minerals.2)Element composition and distribution of sections of rod-shaped mineral.C,O,P,S,and N come from the organic residues of bacterial cells,confirming that carbonate can nucleate on the surface of bacterial cells to form calcified bacteria.Gram-positive bacteria MF-2 and LV-1 strains tend to form a calcified layer on the cell surface after calcification,while Gram-negative bacteria HJ-1 strains are calcified throughout the bacteria,and there is no obvious layering of components.This shows that the formation of the calcified layer is related to the structure and chemical composition of the bacterial cell wall.3)The crystallization order of the calcified layer from the inside to the outside follows the Ostwald-Lussac rule of step.SAED patterns show that the mineral composition of the calcified bacteria of MF-2 strain from the inside out is ACC→vaterite;the mineral composition of the calcified bacteria of LV-1 strain from the inside out is ACC→vaterite→calcite;the mineral composition of HJ-1 strain calcified bacteria from the inside out is ACC→vaterite.As heterogeneous nucleation sites in the mineralization system,bacterial cells preferentially induce ACC to form on its surface,and then transform to vaterite and calcite and completely encapsulate bacterial cells to form calcified bacteria.4)Calcified bacteria serve as a kind of structural unit of the invaginated carbonate minerals.Strains LV-1 and HJ-1 can induce the formation of invaginated carbonate minerals with protruding surroundings,relatively flat in the middle and epitaxial growth trends at the edge.The invaginated carbonate minerals are formed by using bubbles at the bacterial biofilm interface as a template and calcified bacteria as structural units.The specific evolution pathway:calcified bacteria,nanoparticles,etc.→invaginated type（disc-shaped→sunflower-shaped→invaginated hemispherical shape）.5)Bacterial cells serve as templates for the formation of dumbbell-shaped carbonate.The cell concentrate of strain HJ-1 can induce the formation of dumbbell-shaped carbonate minerals in a supersaturated solution of Mg/Ca=5.The two ends of the dumbbell-shaped mineral have similar morphology and the same crystal orientation.There are traces of bacterial cells in the center,and the texture of the cut surface shows a trend of radial growth from the center to the two ends.This shows that dumbbell-shaped minerals are formed by bacterial cells as a template,through the orientated accumulation of nanoparticles at the two ends and growth,and the specific evolution pathway:bacterial cell→calcified bacteria→rod-shaped→dumbbell shaped.This study provides micro-regional evidence for that bacterial cell serve as nucleation site for carbonate minerals,and deeply explores the regulation and influence of bacterial cells on carbonate morphologies.Overall,this study provides new idea for the mechanism of microbial-induced carbonate precipitation.