| The sedimentary mechanism of microbial carbonate rocks in nature is one of the research hot spots in the field of geological sedimentology.The carbonate formation has developed from the physical and chemical genesis in the past to the microbial genesis advocated by most researchers,and has achieved fruitful results,but there is little research on the effect of magnesium from different sources on the formation of microbial rocks.Based on this,a strain of Staphyloccus epidermidis Y2 was isolated and purified in this experiment,and the pH curve of the growth curve of the bacteria was determined.In order to explore the causes of the pH increase,qualitative experiments of ammonia and carbonic anhydrase activity of the bacteria were conducted.The concentrations of carbonate and bicarbonate ions and amino acids in bacterial extracellular polymers were also measured.After that,the strain was used to induce carbonate minerals using two different magnesium sources(magnesium sulfate and magnesium chloride)under different Mg/Ca ratios at 30?.The mineral characteristics were analyzed by polarization microscope,X-ray diffraction(XRD),scanning electron microscopy and energy spectrum(SEM-EDS),Fourier infrared testing(FTIR),X-ray photoelectron spectroscopy(XPS).High-resolution transmission electron microscopy and selected area electron diffraction(HRTEM-SAED),scanning transmission electron microscopy(STEM-EDS)were used to observe the distribution of mineral elements inside and outside the cells,as well as determine nucleation sites and whether intracellular mineralization occurs.The geological significance of microbial mineralization was analyzed by comparing the differences in morphology,elements and stable carbon isotopes between biogenic and non-biogenic calcite.The results confirmed that Slaphyloccus epidemidis Y2 could improve the pH of the environment by producing ammonia gas and carbonic anhydrase.The carbonic anhydrase catalyzed carbon dioxide hydration reaction released a large number of carbonate and bicarbonate ions,and increased the supersaturation of carbonate minerals in the culture system.The high content of glycine in the extracellular polymers of bacteria helps bacteria to survive in the highly saline environment.The mineral phase induced by the bacteria from different magnesium sources(MgSO4 and MgCl2)was the same.When magnesium was not added,the mineral phase was calcite.After adding magnesium,monohydrocalcite was produced in both magnesium sulfate and magnesium chloride groups.Different magnesium sources had great effects on the mineral density,crystal morphology and growth pattern.In magnesium chloride medium,the crystal cell density of calcite was lower than that of magnesium sulfate medium.In the medium of magnesium sulfate,the crystal grew in spiral shape,while the crystal in the medium of magnesium chloride grew in a concentric circle shape.Especially,at Mg/Ca ratio of 8,the minerals were composed of many regular microcrystals with geometric shapes in magnesium sulfate medium,while the minerals in magnesium chloride medium were composed of several granular mineral microcrystals.There is calcification inside the cell,but the minerals have no crystal structure.The study on the mineralization induced by halophiles under different Mg/Ca ratios and different magnesium sources may be expected to provide some theoretical bases for the the exploration of mineralization mechanism induced by microorganisms in the highly saline environment,and at the same time to provide some references for the sedimentary mechanism of marine carbonate rocks in the period of geological history.The study of microbiogenic calcite and monohydrocalcite has important reference value for the identification of microbial carbonate rocks and the restoration of sedimentary environment. |
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