Microbial communities in biological electrochemical systems

Doctoral thesis, 2024

Biological electrochemical systems (BES) can be used as biosensors and for recovery of resources from waste streams. BES utilizes microbial communities that grow on the surface of electrodes in the form of biofilms. Electrogenic bacteria residing in the anode biofilm initiate oxidation reactions, resulting in the release of electrons and subsequent electrical current generation. The electrons flow to the cathode where reduction reactions take place. Microbial biofilms may also be involved in the catalysis of cathode reactions. Many factors are involved in shaping the composition and performance of the microbial communities in BES, most of which remain poorly understood.

 

In this thesis, the impact of electrode material and biotic interactions on performance and microbial community assembly was investigated in microbial electrolysis cells (MECs) oxidizing volatile fatty acids at the anode. MECs are a type of BES that require an applied electric potential to generate products such as H2, CH4, and acetate at the cathode. MECs with mixed-culture biofilms on both the anode and the cathode were studied. Two experiments were conducted. The first was a comparison of MECs with three different cathode materials: carbon nanoparticles, titanium, and steel. The second was a comparison of MECs with three different anode materials: carbon cloth, graphene, and nickel. Furthermore, the effect of dispersal limitation as well as the presence of viruses and their associations with microorganisms was investigated. MECs with carbon cloth anodes had the highest current density and shortest lag time during startup. In contrast, no significant impact of cathode material on MEC performance was seen. The anode communities were dominated by electrogens from the Desulfobacterota phylum, while the cathodes were dominated by methanogens from the Methanobacteriaceae family. Stochastic initial attachment by competing electrogens on the anode explained variations in the startup time between replicate MECs. In each experiment at least two different Desulfobacterota species competed for dominance on the anode. MECs that enabled dispersal between the system tended to have the same dominating taxa.  Biotic interactions also affected the microbial communities in the system. Network analysis showed that the anode communities had a greater number of negative interactions between taxa compared to the cathode. Due to the need for direct contact by electrogens to transfer electrons to the anode, there is a higher competitive element to the colonization of the anode biofilm. Viral infection is another type of biotic interaction. Analysis of the prokaryotic and viral communities resulted in the identification of CRISPR-based and prophage virus-host associations, indicating previous infections and prophage inductions of electrochemically active microorganisms. These findings suggest that while there is selective pressure for electrogenic bacteria on the anode, stochastic factors, and biotic interactions play a larger role compared to electrode material in shaping the anode community.