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​

Plant pathogens can cause devastating damage to crop (Strange and Scott 2005) greatly affecting a food resource in growing need on our planet. A significant proportion of global crops require irrigation, and with this, bare the risk of being affected by irrigation-borne pathogens (Lamichhane and Bartoli, 2015). Detection of plant pathogens in irrigation water can effectively be used to minimize this risk. River water makes up a major irrigation water source. Morris et al., (2023), propose monitoring whole river catchments to understand plant pathogen population dynamics and generate models to prevent outbreaks, similar to practices regarding water-borne human pathogens.

Monitoring 270 km of the river Durance, Morris et al., (2023) reveal that two groups of bacteria known to host pathogenic strains, Pseudomonas syringae and the Soft Rot Pectobacteriaceae are present in relatively high numbers across the entire catchment or significant parts of it, respectively, with their abundance mostly correlated to water temperature. Nevertheless, despite their presence no major outbreaks have been reported in recent years. The authors suggest that the current environmental conditions in the lower, agriculture-dominated part of the catchment may not generate the necessary environment for an outbreak. Alternatively, as also suggested, though some potentially pathogenic variants were detected in the study, they may not match the crops currently grown in the area (Morris et al., 2023).

The authors thus bring up the need for large scale monitoring and call for observations on potential land-use changes in the area that may alter the sensitive and seemingly stable conditions in such a way that outbreaks will be triggered. Change of land use, specifically from rural to agricultural use, has been repeatedly recognized to influence biodiversity (e.g., Ionescu et al., 2022). Furthermore, agricultural environments, with a dense network of irrigation channels, natural and man-made ponds, and larger reservoirs, will accelerate the spread of organisms through multiple biotic and abiotic vectors (Karnatak and Wollrab, 2020), and with this likely plant- (and other) pathogens. Overall, the work by Morris et al., (2023) highlights that studying the presence and distribution of plant pathogens in water used for irrigation across large areas, is bound to identify which potential pathogens are omnipresent, awaiting for the right condition for an outbreak; and which are rather spread from, isolated, local sources and thus can be effectively mitigated.

References

Strange, R. N., and Scott, P. R. (2005). Plant disease: a threat to global food security. Annu. Rev. Phytopathol. 43, 83–116. https://doi.org/10.1146/annurev.phyto.43.113004.133839

Lamichhane, J.R. and Bartoli, C. (2015), Plant pathogenic bacteria in open irrigation systems: what risk for crop health? Plant Pathol, 64: 757-766. https://doi.org/10.1111/ppa.12371

C.E. Morris, C. Lacroix, C. Chandeysson, C. Guilbaud, C. Monteil, S. Piry, Rochelle Newall E., S. Fiorini, F. Van Gijsegem, M.A. Barny, O. Berge (2023) Comparative abundance and diversity of populations of the Pseudomonas syringae and Soft Rot Pectobacteriaceae species complexes throughout the Durance River catchment from its French Alps sources to its delta. bioRxiv, 2022.09.06.506731, ver. 3 peer-reviewed and recommended by Peer Community in Microbiology. https://doi.org/10.1101/2022.09.06.506731 

Ionescu, D., Bizic, M., Karnatak, R., Musseau, C. L., Onandia, G., Kasada, M., Berger, S. A., et al. (2022). From Microbes to Mammals: Pond Biodiversity Homogenization across Different Land-Use Types in an Agricultural Landscape. Ecological Monographs 92(3): e1523. https://doi.org/10.1002/ecm.1523