Abstract
The present study is focused on the effect of biofilm medium chemistry on oxalate crystallization and contributes to the study of the patterns of microbial biomineralization and the development of nature-like technologies, using the metabolism of microscopic fungi. Calcium oxalates (weddellite and whewellite in different ratios) were synthesized by chemical precipitation in a weakly acidic environment (pH = 4–6), as is typical for the stationary phase of micromycetes growth, with a ratio of Ca2+/C2O42− = 4.0–5.5, at room temperature. Additives, which are common for biofilms on the surface of stone in an urban environment (citric, malic, succinic and fumaric acids; and K+, Mg2+, Fe3+, Sr2+, SO42+, PO43+ and CO32+ ions), were added to the solutions. The resulting precipitates were studied via X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). It was revealed that organic acids, excreted by micromicetes, and some environmental ions, as well as their combinations, significantly affect the weddellite/whewellite ratio and the morphology of their phases (including the appearance of tetragonal prism faces of weddellite). The strongest unique effect leading to intensive crystallization of weddellite was only caused by the presence of citric acid additive in the medium. Minor changes in the composition of the additive components can lead to significant changes in the weddellite/whewellite ratio. The effect of the combination of additives on this ratio does not obey the law of additivity. The content of weddellite in the systems containing a representative set of both organic acids and environmental ions is ~20 wt%, which is in good agreement with natural systems.
Highlights
In recent years, the scientific community from all over the world has shown a significant interest in the mechanisms of biomineralization with the participation of microorganisms, which is associated with the study of modern mineral formation at nano- and microlevels, as well as processes and phenomena occurring at the border of living and non-living [1,2,3]
The results of the experimental study (Appendix A Tables A1 and A2) made it possible to identify the chemical components of biofilms and their combinations, the presence of which led to the formation of weddellite in the area of whewellite crystallization in a narrow interval of pH value of the initial solutions (pHinit) = 4.5–6.5, which characterizes the acidity of the medium during the stationary phase of micromycete growth [19]
The experimental study showed that the changes in the chemical composition of natural biofilms, which are open systems, can significantly influence the crystallization of calcium oxalates occurring under the action of fungi and lichens
Summary
The scientific community from all over the world has shown a significant interest in the mechanisms of biomineralization with the participation of microorganisms, which is associated with the study of modern mineral formation at nano- and microlevels, as well as processes and phenomena occurring at the border of living and non-living [1,2,3]. Active microbe metabolism products which contain organic acids (including oxalic) dissolve the underlying substrate and initiate oxalate crystallization. Cations involved in their formation enter the biofilm from the underlying stone substrate, and the environment (atmospheric air and soil) [7]. Dews and fogs containing aerosols of various ions (including heavy metals) strongly contribute to oxalate crystallization
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