Effect of on stereoisomers of lactic acids on microbially-mediated reductive dissolution of clay minerals: Insights from DFT and experimental studies

Abstract

Interactions between microorganisms and clay minerals occur ubiquitously in nature. It has been established that various factors can affect the interactions between microorganisms and clay minerals, such as temperature, pressure, pH, surface area and electron shuttle compounds. Lactic acid has two stereoisomer forms (L-(+)-lactic acid (L-(+)-lac) and D-(−)-lactic (D-(−)-lac)) and has widely been recognized as an important biomolecule that can be utilized as an electronic donor and/or carbon source by dissimilatory iron-reducing bacteria. Furthermore, lactic acid (endogenous or exogenous) has been shown to result in the dissolution of metal impurities from clay minerals. However, the synergistic effect of different lactic acid stereoisomers on abiotic processes (clay mineral dissolution) and biotic processes (microbial reduction) remains unclear, as well as the fundamental principles governing this process. To examine the differences between the effects of both stereoisomers of lactic acid on the interactions between microorganisms and clay minerals, experiments on microbially mobilized elements in clay mineral dissolution (MMED) were carried out in the presence of either L-(+)-lac or D-(−)-lac. Experimental results showed that the interdependence between coupled abiotic and biotic processes, created a synergy between these processes. Furthermore, to investigate the molecular or atomic level interactions, such as electrostatic charges and bond formation, that occur during the attachment of L-(+)-lac and D-(−)-lac onto mineral surfaces. Molecular dynamics (MD) simulations were used to distinguish between the adsorption pathways of L-(+)-lac and D-(−)-lac on clay minerals. The results of density functional theory (DFT) calculations indicated that the presence of lactic acid affected the electron distribution profile of clay mineral templates. The finding of this study provide insights into the role of lactic acid stereoisomers in promoting mineral-microbe interactrions.