Dissolved organic matter (DOM) enhances the competitiveness of weak exoelectrogens in a soil electroactive biofilm

Dissolved organic matter (DOM) as critical redox active soil carbon plays a crucial role in shuttling electrons between bacteria and solid electron acceptors, such as iron oxides. However, research on DOM as an electron shuttle has traditionally focused on its impact on typical iron-reducing bacteria, namely strong exoelectrogens, like Geobacter. Besides these strong exoelectrogens, there is a significant presence of weak exoelectrogens in the soil, but studies examining how DOM affects their survival and competitiveness are lacking. This study focused on exploring the influence of DOM on weak exoelectrogens like Bacillus in the soil. By utilizing soil-bioelectrochemical systems (s-BESs) to enrich soil electroactive microorganisms, it investigated the relationship between the abundance of strong and weak exoelectrogens under conditions rich in DOM and conditions lacking DOM. The results showed that in the rich DOM treatment, the abundance of Geobacter was relatively lower (12 ± 0.5% vs. 41 ± 3%), and there was a significant negative correlation between the abundance changes of 18 weak exoelectrogens and Geobacter. This suggests that DOM caused a decrease in the population of strong exoelectrogens (e.g., Geobacter) while simultaneously promoting the growth of weak exoelectrogens (e.g., Bacillus and Sedimentibacter). Based on this, we propose that DOM, acting as an electron shuttle, creates favorable ecological niches for the thriving and propagation of weak exoelectrogens, enhancing their competitiveness within the microbial community. This new understanding provides deeper insights into the significance of DOM electron shuttling in soil microbial ecology, and raises the question: is the role of weak exoelectrogens in soil iron cycling underestimated due to the existence of DOM? Graphical Abstract: (Figure presented.) © The Author(s) 2024.