Enriching electroactive microorganisms from ferruginous lake waters â€“ Mind the sulfate reducers!
Electroactive microorganisms (EAM) are pivotal agents in mineral transformation, particularly within dynamic redox interfaces. Yet, the systematic cultivation of these EAM from such environments, characterized by pronounced oxygen and dissolved metal gradients, remains elusive. Current knowledge is primarily based on anecdotal reports rather than comprehensive studies, underscoring the need for a more rigorous exploration of this fascinating microbial realm. Here we conducted an anodic enrichment (0.2 V vs. SHE) using anoxic ferruginous waters from a post-mining lake as inoculum. The observed weak electrogenicity was dependent on the activity of electroactive planktonic cells rather than being driven by anodic biofilms. The cells preferred formate as electron donor; potentially linked to microbial acetogenesis. Addition of yeast extract to the medium decreased the lag phase but did not increase current densities. The enriched bacterial community varied depending on the substrate composition, mainly comprising of non-conventional EAM like sulfate- and nitrate-reducing bacteria (e.g., Desulfatomaculum ruminis and an amplicon sequence variant (ASV-4) of Stenotrophomonas, respectively). These bacteria could be regarded as weak electricigens considering their low current yields. Surprisingly, a secondary enrichment strategy resulted in considerably different bacterial community composion consisting of iron-reducing (e.g., Klebsiella spp.) and fermentative bacteria (e.g., Paeniclostridium ASV-3). Cyclic voltammetry, secondary electron microscopy, and energy-dispersive X-ray spectroscopy results indicate the precipitation of sulfur- and iron-rich organomineral aggregates at the anode surface. Precipitation resulted in anode passivation and presumably prevented current production by EAM. Our findings suggest that (i) non-conventional EAM with the potential to drive biomining in bioelectrochemical systems can be enriched by using geogenically metal-enriched anoxic waters as inoculum, and (ii) there is a need for specific inhibition of sulfate reducers indigenous to these waters since they seemingly outcompete EAM during cultivation and as their activity leads to anode passivation