Enriching electroactive microorganisms from ferruginous lake waters - Mind the sulfate reducers!

Electroactive microorganisms are pivotal players in mineral transformation within redox interfaces characterized by pronounced oxygen and dissolved metal gradients. Yet, their systematic cultivation from such environments remains elusive. Here, we conducted an anodic enrichment using anoxic ferruginous waters from a post-mining lake as inoculum. Weak electrogenicity (j = ∼5 µA cm^-2) depended on electroactive planktonic cells rather than anodic biofilms, with a preference for formate as electron donor. Addition of yeast extract decreased the lag phase but did not increase current densities. The enriched bacterial community varied depending on the substrate composition but mainly comprised of sulfate- and nitrate-reducing bacteria (e.g., Desulfatomaculum spp. and Stenotrophomonas spp.). A secondary enrichment strategy resulted in different bacterial communities composed of iron-reducing (e.g., Klebsiella spp.) and fermentative bacteria (e.g., Paeniclostridium spp.). Secondary electron microscopy and energy-dispersive X-ray spectroscopy results indicate the precipitation of sulfur- and iron-rich organomineral aggregates at the anode surface, presumably impeding current production. Our findings indicate that (i) anoxic waters containing geogenically derived metals can be used to enrich weak electricigens, and (ii) it is necessary to specifically inhibit sulfate reducers. Otherwise, sulfate reducers tend to dominate over EAM during cultivation, which can lead to anode passivation due to biomineralization.