Host-microbe symbioses are an essential component of many ecological systems, playing critical roles in the physiology and evolution of all involved partners. In this context, the bacterial family that includes Coxiella burnetii, the causative agent of Q fever, is of particular interest. The Coxiellaceae family is a complex group with members that have adopted a variety of specializations. Closely related lineages to C. burnetii are tick mutualists (Coxiella-like endosymbionts) and aquatic bacteria that may include both free living and symbiotic species. Additionally, four related genera within this family include symbionts of insects and amoebae. Exactly how and when pathogenicity and mutualism evolved in this lineage is not clear, thus remaining a valuable line of enquiry that can help establish general principles on these lifestyle transitions.

A new study by Santos-Garcia and colleagues (2023) places the spotlight on this bacterial group, obtaining new insights through comparative genomics. The authors add two genomes, one of them a circular contig representing a highly reduced (0.9 Mb) chromosome, that increase the resolution of key branches in the Coxiella evolutionary tree. These include a sister group to C. burnetii and the group immediately subtending them, both entirely containing Coxiella-like endosymbionts. By analyzing genetic potential for metabolism, cell dimorphism, virulence and acidophily, the authors find evidence for the ancestrality of genes associated with a pathogenic lifestyle, and support a scenario by which mutualism arose multiple times in a parasitic lineage. In this context shines a pathogenicity island acquired in the common ancestor of this group and subsequently eroded in mutualistic lineages. This scenario highlights the importance of pre-adaptations that facilitate evolutionary specializations, such as the capabilities for B vitamin biosynthesis (key feature in the adaptation to a mutualistic relationship with organisms with B-vitamin-poor diets) and pH homeostasis (harnessed by C. burnetii for infection). 

Microbial groups at the crossroads of parasitism and mutualism help us understand the mechanisms underpinning these evolutionary strategies (see e.g. Drew et al, 2021). Transitions in endosymbiosis, including shifts in the parasitism-mutualism continuum, adaptation to new partners, or switches between free-living and host-associated lifestyles, affect the structure of ecological networks, and understanding them can yield crucial insights into how to manipulate microbial symbioses for health outcomes, sustainable agriculture or ecosystem conservation. The Coxiellaceae, by including a diverse set of mutualistic, parasitic and possibly free-living lineages, are a fantastic model group to tackle these questions. Together with other host-associated bacteria, such as Sodalis (Clayton et al, 2012) or Pantoea (Walterson and Stavrinides, 2015) species, these ecologically diverse microbes are valuable assets in the quest to decipher the molecular basis of lifestyle transitions in endosymbiosis.

REFERENCES

Clayton, A.L., et al (2012). A novel human-infection-derived bacterium provides insights into the evolutionary origins of mutualistic insect–bacterial symbioses. PLoS Genetics, 8: e1002990. https://doi.org/10.1371/journal.pgen.1002990

Drew, G.C., Stevens, E.J., King, K.C. (2021). Microbial evolution and transitions along the parasite-mutualist continuum. Nature Reviews Microbiology, 19: 623-638. https://doi.org/10.1038/s41579-021-00550-7

Santos-Garcia, D., et al. (2023) Genomic changes during the evolution of the Coxiella genus along the parasitism-mutualism continuum. bioRxiv, 2022.10.26.513839, ver. 4 peer-reviewed and recommended by Peer Community In Microbiology. https://doi.org/10.1101/2022.10.26.513839

Walterson, A.M., Stavrinides, J. (2015). Pantoea: insights into a highly versatile and diverse genus within the Enterobacteriaceae. FEMS Microbiology Reviews, 39: 968-984. https://doi.org/10.1093/femsre/fuv027

Nitrogen fixation and elimination are two key microbial processes that significantly impact the release (and removal) of reactive nitrogen into natural ecosystems. Unlike global change, caused by the emission of greenhouse gasses into our atmosphere, the release of reactive nitrogen into our biosphere only recently (in the last years) received the necessary public attention. Hence, novel techniques for (1) reactive nitrogen recovery, (2) energy-effective removal, and (3) sustainable nitrogen fixation are essential to prevent the nitrogen cycle from spinning out of control without also putting an additional burden on our precious natural resources or increasing the emission of greenhouse gasses.

In this research paper by Rous et al. (2023), the authors investigated the use of a biocathode in a bioelectrochemical system (BES) for sustainable fixation of N2 into NH3, using electricity as a sustainable energy source and CO2 as the only carbon source. A critical element in their study was the enrichment of N2-fixating bacteria, starting from soil samples, in an effort to achieve effective nitrogen fixation. A comparison between the enriched culture and a pure culture of diazotrophic hydrogenotrophic bacteria confirmed comparable results for N2 fixation, indicating that the enrichment process was a viable and successful approach. Although pure culture biotechnological processes have their merits, it is clear that the usage of an enriched microbial culture allows for a more simple, robust, and open microbial process, compared to pure culture systems.

This approach does enable a sustainable way of N2 (and by extension CO2) fixation, as it relies on electricity directly (or indirectly through H2) and CO2 only, but it does suffer from low coulombic efficiencies (<5%). This indicates that, even though the results are promising, there is room for optimization, especially concerning the production of (unwanted) side products, such as acetate and other microbial metabolites. This reflects a key challenge and potential disadvantage of mixed or enriched cultures compared to pure cultures.

It is in that framework that this study provides an interesting, highly relevant view on the potential of bioelectrochemical nitrogen fixation using enriched cultures, yet, it also implies the need to either find a purpose for the byproducts, such as acetate, and/or achieve a more effective enrichment strategy to achieve an increased coulombic efficiency towards sustainable nitrogen fixation.

Reference

Rous A., Santa-Catalina G., Desmond-Le Quéméner E., Eric Trably E. and Nicolas Bernet N. (2023). Comparison of enrichment methods for efficient nitrogen fixation on a biocathode. bioRxiv, 2023.03.02.530809, ver 5, peer-reviewed and recommended by Peer Community in Microbiology. https://doi.org/10.1101/2023.03.02.530809​