# The Restorative Effects of Electron Mediators on the Formation of Electroactive Biofilms in Geobacter sulfurreducens

## TL;DR

Adding FMN helps restore biofilm formation and improves electron transfer in Geobacter mutants lacking key components.

## Contribution

FMN acts as both an electron shuttle and a cofactor in defective Geobacter biofilms, offering a new strategy for bioelectrochemical systems.

## Key findings

- FMN increased biofilm thickness by 250% in PCAΔ1496 and 33% in PCAΔ1501.
- FMN boosted current outputs by 175-fold in PCAΔ1496 and 317.7% in PCAΔ1501.
- FMN binds to outer membrane c-type cytochromes and enhances electron exchange capacity.

## Abstract

Electroactive biofilms (EABs) are essential for the performance of bioelectrochemical systems (BESs), but their formation in Geobacter, critically on conductive pili and exopolysaccharides, limits application under conditions where these components are deficient. Herein, we investigated the restorative effects of exogenous flavin mononucleotide (FMN) on EAB formation and extracellular electron transfer (EET) in two defective mutants of Geobacter sulfurreducens: the pili-deficient PCAΔ1496 and exopolysaccharides-deficient PCAΔ1501. Results show that FMN significantly promoted biofilm thickness in PCAΔ1496 (250%) and PCAΔ1501 (33%), while boosting maximum current outputs by 175-fold and 317.7%, respectively. Spectroscopic and electrochemical analyses revealed that FMN incorporates into biofilms, binds to outer membrane c-type cytochromes (c-Cyts), and enhances electron exchange capacity. Differential pulse voltammetry further confirmed that FMN did not exist independently in the biofilm but bound to outer membrane c-Cyts as a cofactor. Collectively, exogenous FMN plays dual roles (electron shuttle and cytochrome-bound cofactor) in defective Geobacter EABs, effectively restoring biofilm formation and enhancing EET efficiency. This study expands the understanding of the formation mechanism of Geobacter EABs and provides a novel strategy for optimizing BES performance.