Engineering Exopolysaccharide Biosynthesis of Shewanella oneidensis to Promote Electroactive Biofilm Formation for Liquor Wastewater Treatment.

Microbial electrochemical systems (MESs), as a green and sustainable technology, can decompose organics in wastewater to recover bioelectricity. Electroactive biofilms, a microbial community structure encased in a self-produced matrix, play a decisive role in determining the efficiency of MESs. However, as an essential component of the biofilm matrix, the role of exopolysaccharides in electroactive biofilm formation and their influence on extracellular electron transfer (EET) have been rarely studied. Herein, to explore the effects of exopolysaccharides on biofilm formation and EET rate, we first inhibited the key genes responsible for exopolysaccharide biosynthesis (namely, so_3171, so_3172, so_3177, and so_3178) by using antisense RNA in Shewanella oneidensis MR-1. Then, to explore the underlying mechanisms why inhibition of exopolysaccharide synthesis could enhance biofilm formation and promote the EET rate, we characterized cell physiology and electrophysiology. The results showed inhibition of exopolysaccharide biosynthesis not only altered cell surface hydrophobicity and promoted intercellular adhesion and aggregation, but also increased biosynthesis of c-type cytochromes and decreased interfacial resistance, thus promoting electroactive biofilm formation and improving the EET rate of S. oneidensis. Lastly, to evaluate and intensify the capability of exopolysaccharide-reduced strains in harvesting electrical energy from actual liquor wastewater, engineered strain Δ3171-as3177 was further constructed to treat an actual thin stillage. The results showed that the output power density reached 380.98 mW m^-2, 11.1-fold higher than that of WT strain, which exhibited excellent capability of harvesting electricity from actual liquor wastewater. This study sheds light on the underlying mechanism of how inhibition of exopolysaccharides impacts electroactive biofilm formation and EET rate, which suggested that regulating exopolysaccharide biosynthesis is a promising avenue for increasing the EET rate.