Long-Term Succession Shows Interspecies Competition ofGeobacterin Exoelectrogenic Biofilms

Geobacterspp. are well-known exoelectrogenic microorganisms that often predominate acetate-fed biofilms in microbial fuel cells (MFCs) and other bioelectrochemical systems (BESs). By using an amplicon sequence variance analysis (at one nucleotide resolution), we observed a succession between two closely related species (98% similarity in 16S RNA),Geobacter sulfurreducensandGeobacter anodireducens, in the long-term studies (20 months) of MFC biofilms.Geobacterspp. predominated in the near-electrode portion of the biofilm, while the outer layer contained an abundance of aerobes, which may have helped to consume oxygen but reduced the relative abundance ofGeobacter. Removal of the outer aerobes by norspermidine washing of biofilms revealed a transition fromG. sulfurreducenstoG. anodireducens. This succession was also found to occur rapidly in co-cultures in BES tests even in the absence of oxygen, suggesting that oxygen was not a critical factor.G. sulfurreducenslikely dominated in early biofilms by its relatively larger cell size and production of extracellular polymeric substances (individual advantages), whileG. anodireducenslater predominated due to greater cell numbers (quantitative advantage). Our findings revealed the interspecies competition in the long-term evolution ofGeobactergenus, providing microscopic insights intoGeobacter’s niche and competitiveness in complex electroactive microbial consortia.