Carbon dots internalization enhances electroactive biofilm formation and microbial acetate synthesis

This study investigated how carbon dots (CDs) and N-doped CDs modification of acetogenic bacteria enhanced the extracellular electron transfer (EET) and the formation of electroactive biofilm, which subsequently improved acetate production from CO2 in microbial electrosynthesis (MES). Characteristic fluorescence emission peaks of CDs and N-doped CDs were detected inside and outside of bacterial cells, respectively, suggesting that the CDs were successfully internalized by the cells. The (N-doped) CDs modified acetogens increased current output by 89-95%, and acetate production rate by 57-71%. The highest acetate production rate and electron recovery efficiency reached 2.1 mM d-1 and 91.3%, respectively. Acetoanaerobium and Sporomusa, which are considered to have different EET strategies, dominated the biocathodes modified with CDs and N-doped CDs, respectively. The CDs modification increased biofilm thickness by 86% and reached 41.0 +/- 1.3 mu m, which could be related to the increased secretion of flavins that enhanced the signal transduction interspecies. With the Ndoped CDs modified, the biofilm was occupied with active pyrrolic N/graphitic N (52%), and the expression of functional genes encoding cytochrome (cydB, cybH, CcdA) were enhanced significantly. This study offers insights into the EET mechanisms by acetogens and a viable approach to increase acetate production from CO2.