Engineering Exopolysaccharide Biosynthesis of Shewanella oneidensis to Promote Electroactive Biofilm\nFormation for Liquor Wastewater Treatment

Microbial\nelectrochemical systems (MESs), as a green and sustainable\ntechnology, can decompose organics in wastewater to recover bioelectricity.\nElectroactive biofilms, a microbial community structure encased in\na self-produced matrix, play a decisive role in determining the efficiency\nof MESs. However, as an essential component of the biofilm matrix,\nthe role of exopolysaccharides in electroactive biofilm formation\nand their influence on extracellular electron transfer (EET) have\nbeen rarely studied. Herein, to explore the effects of exopolysaccharides\non biofilm formation and EET rate, we first inhibited the key genes\nresponsible for exopolysaccharide biosynthesis (namely, <i>so_3171</i>, <i>so_3172</i>, <i>so_3177</i>, and <i>so_3178</i>) by using antisense RNA in Shewanella\noneidensis MR-1. Then, to explore the underlying mechanisms\nwhy inhibition of exopolysaccharide synthesis could enhance biofilm\nformation and promote the EET rate, we characterized cell physiology\nand electrophysiology. The results showed inhibition of exopolysaccharide\nbiosynthesis not only altered cell surface hydrophobicity and promoted\nintercellular adhesion and aggregation, but also increased biosynthesis\nof <i>c</i>-type cytochromes and decreased interfacial resistance,\nthus promoting electroactive biofilm formation and improving the EET\nrate of S. oneidensis. Lastly, to evaluate\nand intensify the capability of exopolysaccharide-reduced strains\nin harvesting electrical energy from actual liquor wastewater, engineered\nstrain Δ3171-as3177 was further constructed to treat an actual\nthin stillage. The results showed that the output power density reached\n380.98 mW m<sup>–2</sup>, 11.1-fold higher than that of WT\nstrain, which exhibited excellent capability of harvesting electricity\nfrom actual liquor wastewater. This study sheds light on the underlying\nmechanism of how inhibition of exopolysaccharides impacts electroactive\nbiofilm formation and EET rate, which suggested that regulating exopolysaccharide\nbiosynthesis is a promising avenue for increasing the EET rate.