Long-Term Succession Shows Interspecies Competition of <i>Geobacter</i> in Exoelectrogenic Biofilms

Abstract

<i>Geobacter</i> spp. 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), <i>Geobacter sulfurreducens</i> and <i>Geobacter anodireducens</i>, in the long-term studies (20 months) of MFC biofilms. <i>Geobacter</i> spp. 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 of <i>Geobacter</i>. Removal of the outer aerobes by norspermidine washing of biofilms revealed a transition from <i>G. sulfurreducens</i> to <i>G. anodireducens</i>. 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. <i>G. sulfurreducens</i> likely dominated in early biofilms by its relatively larger cell size and production of extracellular polymeric substances (individual advantages), while <i>G. anodireducens</i> later predominated due to greater cell numbers (quantitative advantage). Our findings revealed the interspecies competition in the long-term evolution of <i>Geobacter</i> genus, providing microscopic insights into <i>Geobacter</i>'s niche and competitiveness in complex electroactive microbial consortia.