Abstract
The morphology, crystal structure, and elemental composition of biominerals are commonly different from chemically synthesized minerals, but the reasons for these are not fully understood. A facultative anaerobic bacterium, Enterobacter ludwigii SYB1, is used in experiments to document the hydrochemistry, mineral crystallization, and cell surface characteristics of biomineralization. It was found that carbonate anhydrase and ammonia production were major factors influencing the alkalinity and saturation of the closed biosystem. X-ray diffraction (XRD) spectra showed that calcite, monohydrocalcite (MHC), and dypingite formed in samples with bacterial cells. It was also found that the (222) plane of MHC was the preferred orientation compared to standard data. Scanning transmission electron microscopy (STEM) analysis of cell slices provides direct evidence of concentrated calcium and magnesium ions on the surface of extracellular polymeric substances (EPS). In addition, high-resolution transmission electron microscopy (HRTEM) showed that crystallized nanoparticles were formed within the EPS. Thus, the mechanism of the biomineralization induced by E. ludwigii SYB1 can be divided into three stages: (i) the production of carbonate anhydrase and ammonia increases the alkalinity and saturation state of the milieu, (ii) free calcium and magnesium ions are adsorbed and chelated onto EPS, and (iii) nanominerals crystallize and grow within the EPS. Seventeen kinds of amino acids were identified within both biotic MHC and the EPS of SYB1, while the percentages of glutamic and aspartic acid in MHC increased significantly (p < 0.05). Furthermore, the adsorption energy was calculated for various amino acids on seven diffracted crystal faces, with preferential adsorption demonstrated on (111) and (222) faces. At the same time, the lowest adsorption energy was always that of glutamic and aspartic acid for the same crystal plane. These results suggest that aspartic and glutamic acid always mix preferentially in the crystal lattice of MHC and that differential adsorption of amino acids on crystal planes can lead to their preferred orientation. Moreover, the mixing of amino acids in the mineral structure may also have a certain influence on the mineral lattice dislocations, thus enhancing the thermodynamic characteristics.
Highlights
As one of the most extensive processes on the Earth’s surface, microbially induced carbonate precipitation (MICP) plays a key role in biogeochemical cycles of carbon, as well as influences global climate and seawater chemistry (Lowenstam and Weiner, 1989; Li et al, 2019)
The phylogenetic tree was constructed by the neighbor-joining (NJ) method using MEGA 6.0 software, and it clearly showed that the strain SYB1 clustered with members of E. ludwigii strains (Figure 1)
Given the unique surface texture, elemental composition, and involved organic matters of the minerals observed in the present experiments, it is reasonable to conclude that the facultative anaerobic strain E. ludwigii SYB1 strongly affects the formation of carbonate under aerobic culture conditions
Summary
As one of the most extensive processes on the Earth’s surface, microbially induced carbonate precipitation (MICP) plays a key role in biogeochemical cycles of carbon, as well as influences global climate and seawater chemistry (Lowenstam and Weiner, 1989; Li et al, 2019). Vaterite, and aragonite are the dominant precipitates in Mg-free solutions (Zhuang et al, 2018; Li et al, 2019), while Mgcalcite (low Mg-calcite, high Mg-calcite, very high Mg-calcite), dolomite (ordered and disordered) (Deng et al, 2010; Qiu et al, 2017; Liu et al, 2019), and various hydro-Ca–Mg carbonates are mineral phases in Mg-rich solutions (Sanz-Montero et al, 2019; Zhao et al, 2020b) These biominerals usually exhibit complex morphologies, including spherulite, dumbbell, and cauliflower shape (Sánchez-Román et al, 2011; Qiu et al, 2017; Zhang et al, 2020). Under the influence of a microorganism’s activities, the entire process of mineral formation, including nucleation, crystal growth, mineral phase transformation, orientation, and particle assembly, can all be significantly affected
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