| Carbonate rocks,as an important part of Earth's lithosphere,are widely distributed and exist in various geological periods.Carbonate precipitation caused by different types of microorganisms is a very common phenomenon in the natural environment.Magnesium ions have an important effect on the formation of carbonate minerals induced by microorganisms,but the role of magnesium ions in mineralization process is seldom studied.Microorganisms and magnesium ions are two key factors affecting the mineralization mechanism.The study on the mechanism of magnesium ions' influence on mineralization under microbial action is helpful to understand the process and mechanism of microbial mineralization in laboratory and natural environment.In this paper,halophilic bacteria(Staphylococcus warneri YXY2)and sulfate reducing bacteria(Mgossypii gossypii HFF1 and Citrobacter freundii ZW123)were isolated and purified from the seabed sediments of Tangdao bay,Qingdao.A series of experiments were carried out to induce carbonate minerals by moderate halophiles and sulfate reducers at different Mg/Ca ratios,and the hydro-chemical parameters related to mineralization in the culture system were determined,and the mineralogical characterizations of bacterial induced minerals were also analyzed.The main research conclusions of this paper are as follows:(1)Microorganisms play an important regulatory role in the mineral formation process.Carbonate or phosphate minerals formed in the experimental groups with incabation of halophiles and sulfate reducing bacteria,while no minerals appeared in the non-inoculated control group,indicating that microorganisms played an important regulatory role in the formation process of minerals.The combined effect of ammonia,carbonic anhydrous,alkaline phosphatase secreted by bacteria increased alkalinity and mineral saturation of culture medium and provided favorable conditions for bacteria to induce the formation of carbonate and phosphate minerals.The organic functional groups,organic macromolecules and amino acids composistion within minerals induced by microorganisms are similar to that of bacterial extracellular polymeric substance(EPS),indicating the close relationship between mineral formation and EPS.The amino acids in the EPS are negatively charged by deprotonation and absorb magnesium and calcium ions.The particles with crystal structure and trace magnesium and calcium elements are present in the EPS,which may be the initial stage of mineral formation.Therefore,EPS serve as nucleation sites during the formation of biogenic minerals.In addition,there are also nanoscale amorphous particles within the cell,containing trace magnesium and calcium elements,which are generated by the free calcium in the medium entering the cell by diffusion and reacting with the carbonate inside the cell.(2)Microorganisms have important effects on mineralogical characterizations of minerals.The carbonate and phosphate minerals induced by halophiles and sulfate reducers have the characteristics of diverse morphologies and special element composition,indicating the close relationship between microorganisms and the formation of minerals.In addition,the participation of bacteria can improve the thermal stability of biogenic minerals.In addition,the crystals of biogenic aragonite and struvite have obvious preferential orientation,which is caused by the selective adsorption of amino acid molecules onto different crystal surfaces.The influence of microorganisms on mineralogical characterization provides a basis for the identification of biogenic and non-biogenic minerals.(3)The influence of magnesium ions on the mineralization products under the participation of microorganisms.Magnesium ions cannot enter the chemically induced aragonite crystals,but in the presence of halophore YXY2,the carboxyl group on the amino acid of the EPS of the bacteria can promote the dehydration of magnesium ions,so free magnesium ions enter the biogenic aragonite crystals.With the participation of sulfate reducing bacteria HFF1,dehydrated magnesium ions can participate in mineralization to form nesquehonite,indicating that microorganisms can break the nucleation energy barrier of magnesium ions.At the same time,magnesium ions can participate in the formation of monohydrocalcite and play an important role in maintaining the stability of monohydrocalcite.Aspartic acid and glutamate in the EPS of sulfate-reducing bacteria ZW123 can weaken the hydration of magnesium ions,leading free magnesium ions into calcite,and improve the magnesium content of Mg-rich calcite.The magnesium-rich calcite induced by ZW123 may be the precursor of proto-dolomite.The mechanism of magnesium ions entering Mg-rich calcite proposed in this paper may provide a reference for the "microbial dolomite model".(4)The Mg/Ca ratio has an effect on mineral species and crystal structure.When Mg/Ca ratio was 0,the minerals induced by moderate halophile YXY2 were mainly calcite and vaterite.When the Mg/Ca ratio were 2,4 and 6,the minerals formed were Mg-rich calcite and aragonite.At Mg/Ca ratio of 8,only aragonite formed.The half-peak width of aragonite increased with the increase of Mg/Ca ratio,indicating that the crystal development became worse.The mineral phases under different Mg/Ca ratios are nearly the same as those precipitated in the Period of "Aragonite Sea" and "Calcite Sea",which is of certain reference significance for the study of carbonate mineral precipitation in "Aragonite Sea" and "Calcite Sea".When Mg/Ca ratio was 0,the minerals induced by sulfate reducing bacteria HFF1 were mainly calcite and vaterite.When the Mg/Ca ratio were 3,6,9 and 12,the minerals formed were monohydrocalcite and nesquehonite;the percentage of nesquehonite gradually increased with the increase of M./Ca ratios.When Mg/Ca ratio was 0,the minerals induced by sulfate reducing bacteria ZW123 were mainly calcite and vaterite.When Mg/Ca ratio was 3,the mineral formed was Mg-rich calcite.When the Mg/Ca ratios were 6 and 9,the mineral formed was monohydrocalcite.At an Mg/Ca ratio of 12,only struvite formed.When the Mg/Ca ratios were 6 and 9,the morphology of monohydrocalcite were different.When the Mg/Ca ratio was 6,the monohydrocalcite had a staggered growth pattern,and when the Mg/Ca ratio was 9,the monohydrocalcite had a concentric growth pattern.Crystallinity and thermal stability of monohydrocalcite decreased with the increase of Mg/Ca ratio.In this paper,the formation process and mineral characterization of carbonate minerals induced by microorganisms under different Mg/Ca ratios were studied.The results showed that microorganisms had important effects on the formation and mineralogy characterization of carbonate minerals,and magnesium ions could affect mineralized products with the participation of microorganisms.This study has certain reference value to solve the problems of "dolomite genesis" and "aragonite sea" in geological history. |
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