Attachment of Extracellular Metabolic Products of Lysinibacillus sp. DRG3 on Sand Surface under Variable Flow Velocities and Bioprocesses

The efficacy of microbially mediated stabilization of soil mass depends on soil aggregation and further depends on the complex interplay of environmental parameters, microbial extracellular metabolic products, and surface characteristics of soil particles. Failures of flood control dikes or similar structures often culminate from minor erosion of soil particles initiated by groundwater seepage. Although the introduction of microbial metabolic products in controlling soil erosion has been studied by researchers, the influence of slow fluvial activities on the composition, characteristics, and their impacts on attachment mechanisms of extracellular polymeric substances (EPS) on soil surfaces have remained unexplored. Impacts of slow fluvial activities on the amount and chemical composition of EPS produced by Lysinibacillus sp. DRG3, a nonpathogenic soil bacterium, and the attachment mechanisms of the EPS produced under noncalcifying, nonureolytic, and ureolytic calcifying bioprocesses on the sand surfaces were investigated. DRG3-inoculated specimens were incubated in the presence of steady circulation of aqueous media containing minimal concentrations of minerals and carbon and nitrogen sources to simulate groundwater movements through soil. Quantity, compactness, continuity, and viscosity of EPS and the amounts of carbohydrate, protein, lipid, DNA, and RNA found in EPS increased with circulation velocity and incubation duration. EPS were found to attach to sand through electrostatic interaction and hydrogen bonding. Internally, EPS components interacted with each other through electrostatic interaction, hydrogen bonding, and hydrophobic interaction. Electrostatic interaction appeared to weaken with increasing media circulation intensity and alkalinity. In contrast, EPS production and hydrogen bonding intensified under increased media circulation. Results of this investigation suggest microbe-mediated soil aggregation becomes stronger and more efficient under slow media circulation and are expected to have implications on microbially mediated soil stabilization, particularly in addressing soil erosion. This study provides useful insights for successful field implementation of biomediated soil stabilization. Work presented herein also demonstrates a role for microbial activities found in subterranean environments in strengthening an existing sand deposit.

In recent years, civil engineers have shown an interest in biomediated soil improvement for a variety of applications mainly to address the limitations of traditional ground improvement techniques such as higher cost and non environment friendliness. The microbes used in this study are soil residing, nonpathogenic, and respond positively in all three metabolic pathways. They can grow and produce EPS and calcite consuming nutrients from the minerals available in the groundwater. Therefore, this process is relatively less expensive. In the present study, neither high doses of nutrients were introduced that might have harmful effects on the surrounding ecosystem nor any harmful chemical generated as a byproduct of the precipitation reaction. Additionally, the process for imparting interparticle aggregation used in this study did not require the provision of nutritional supplements to implement the process in the field and was therefore self-sustaining. However, the efficacy of microbe-mediated soil improvement depends on microbially mediated soil particle aggregation. The understanding of the nature and mechanism of the EPS-sand interaction developed from this study will further contribute to the successful implementation of this technique in the field.