Electroactive bacteria: Effect of conjugative plasmids, role of interspecies communication, and discovery of new exoelectrogens

Mathias Fessler

Research output: Book/ReportPh.D. thesis


At the moment the world is facing major challenges that are critical to act upon immediately, in order to prevent irreversible climate changes. Researchers across all fields are addressing this with a variety of different solutions, all contributing to a reduction in greenhouse gas emissions. These include making public transport more accessible, increased recycling, development of plantbased food alternatives, power-to-X, and resource recovery from wastewater. Even just within the field of resource recovery, there are a multitude of promising technologies under development, which can generate electricity, remove toxic compounds, and synthesize valuable chemicals. Collectively these technologies are referred to as microbial electrochemical systems (MESs), and common for them are that they use wastewater as energy input and rely on the metabolism of electroactive bacteria (EAB).
In the absence of soluble electron acceptors, EAB have evolved to respire on insoluble extracellular electron acceptors. In nature, iron oxides are often used as terminal electron acceptors. However, electrodes may replace the iron oxides, which is exactly what is taken advantage of in MESs. The electron flow from the EAB to the electrode is intrinsic for the function of the MES and, therefore, the EAB are essential for the performance of the system. Needless to say, stronger EAB result in better reactor output. Despite their importance in MESs, the knowledge on EAB is still rather limited. The overall purpose of the PhD project presented here was to improve the fundamental understanding of EAB, which will eventually lead to the construction of better performing MESs. More specifically this was addressed by (a) reviewing the field and suggesting where it should move towards in the future for better performing MESs, (b) showing that natural conjugative plasmids can inhibit extracellular electron transfer (EET), and (c) identifying new electroactive species to broaden our understanding of the phenomenon.
Firstly, this PhD project features a thorough review and perspective on how the field should move forward from here, in order to improve MESs. The study, construction and application of MESs for sustainable resource recovery and wastewater treatment is still in its infancy, why the thesis presented here suggests to look at similar fields, such as microbial ecology, for inspiration. In many ways, microbial biofilm communities growing on electrodes in reactors resemble biofilms studied in other settings. Therefore, with a basis in the already existing knowledge on microbial interactions, it is proposed to focus on interactions in electroactive biofilms with special attention to the contributions from non-electroactive species and conjugative plasmids. It is important to establish the role of non-electroactive bacteria in these biofilms in the future, as they are often highly represented in electrode biofilms. Elucidating their contribution may present new and innovative means for optimization of MESs.
Secondly, the impact of conjugative plasmids on EET was investigated. Conjugative plasmids are commonly found in natural biofilms, where they facilitate physical stabilization, amongst other things. In this project, conjugative plasmids were originally designed to be efficient and easy-to-spread vectors of EET genes, to achieve better performing MESs, however, the conjugative plasmids actually had an inhibitory effect on electron transfer. Due to their high abundance in wastewater, addressing this negative effect was important in order to understand if and how these plasmids can limit MESs performance. By testing different electroactive species, numerous terminal electron acceptors, and using various gene knockouts it was shown that several conjugative plasmids specifically interfere with electron transfer mediated by electrically conductive cell surface nanowires. This was due to downregulated transcription of several essential nanowire genes. This is of significance, as some of the strongest electroactive bacteria use this electron export mechanism, and these species are often abundant in microbial reactors.
Finally, two species of magnetotactic bacteria were shown to be electroactive, which is the first report of electroactivity in this group of bacteria. Electroactive microbes with unique traits, such as magnetic organelles, have the potential to enable design of novel reactors, which is one of the reasons why it is important to continue to identify new EAB. Both of the magnetotactic species were able to generate current in a microbial fuel cell, and to reduce different iron oxides to a varying degree. This implicates magnetotactic bacteria in the biogeochemical iron cycle, and also suggests that they have a potential use in MESs.
In conclusion, the project presented here has added two new species to the list of known EAB, shown that conjugative plasmids substantially reduce electron export ability in nanowire-dependent EAB, and, with grounds in a thorough review of the field, proposed to look into the role of non-electroactive species in electroactive biofilms in the future. The findings reported here cannot be used in this instant to improve MESs directly. Instead, they shed light on a previously unknown inhibitor of EET and provide a deeper understanding of EET in general, which forms the basis for MES improvement in the future.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages113
Publication statusPublished - 2023


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