Synthesis of microbial exopolysaccharides and their mechanisms and applications in heavy metal remediation

Abstract:

The non-biodegradable nature of heavy metals (HMs) results in their long-term presence in the environment, leading to severe environmental pollution and posing a threat to human health and ecosystems. Compared with physical and chemical remediation techniques, microbial remediation is praised for the low cost, environmental friendliness, and high efficiency. When facing heavy metal stress or nutrient imbalance, microorganisms are stimulated to produce and secrete extracellular polysaccharides (EPSs). Therefore, the production of EPSs is regarded as one of the important strategies employed by microorganisms to combat HM stress. EPSs not only protect microorganisms in extreme conditions such as low temperature, high temperature, high salinity, or exposure to toxic compounds but also facilitate the communication and transfer of information and substances both inside and outside the cells. EPSs serve as a protective barrier to restrict the entry of HM ions into the cells and as a medium for communication. EPSs contain multiple negatively charged functional groups capable of complexing with HM ions, undergoing ion exchange, and participating in redox reactions, thereby reducing the bioavailability and toxicity of HMs. Microbial EPSs play a significant role in the remediation of HM-contaminated environments. However, there is currently a lack of a systematic review on the synthesis process of microbial EPSs, the mechanisms of the interaction of EPSs with HMs, and the application status of EPSs in the environments with HM stress. This article provides an overview of microbial EPSs and their classification, elaborates on the intracellular and extracellular biosynthesis mechanisms of bacterial EPSs, explores the interactions between microbial EPSs and HMs, and discusses research advances in the use of microbial EPSs for the remediation of HM pollution in water and soil environments. Finally, it looks ahead to the synthesis of EPSs and the role of EPSs in HM remediation, offering support for the further application of microbial EPSs in the remediation of environmental HM pollution.