A Bacterial Twist: When Ocean Life Feeds the Land
Imagen de Aristal Branson en Pixabay
Recent results published in Nature Microbiology show a surprising microbial connection between marine environments and terrestrial plants: a marine strain that not only survives on land but actively fixes nitrogen when interacting with legumes. This unexpected link could reshape how we think about these interactions and may reveal new tools for a more sustainable agriculture.
Why Nitrogen Matters, and Who Makes It Available
Nitrogen is an essential nutrient for living organisms. From the structure of DNA to the backbone of proteins, it’s part of the very foundation of life. But how do organisms that don’t feed on others get their nitrogen? That’s the case for plants and algae, which build what they need through photosynthesis.
Although there is plenty of nitrogen in our atmosphere, its chemical structure makes it unusable by plants or algae. However, some microorganisms can transform this nitrogen into a more assimilable form through a process called nitrogen fixation. It’s not easy, though, and these nitrogen-fixing microorganisms need high amounts of energy to transform atmospheric nitrogen (N₂) into ammonium.
This makes them crucial for our ecosystems, as plants and algae are at the base of food webs, meaning many other organisms depend on feeding on them. If they don’t have nitrogen… we would be in deep trouble!
Nitrogen-fixing microorganisms inhabit terrestrial and marine ecosystems but belong to different groups. In land, the best-known nitrogen-fixing microbes belong to Rhizobium and Bradyrhizobium and form a close relationship with legumes. Similarly, in marine environments, cyanobacteria are the main group responsible for nitrogen fixation, making it available to algae and supporting the ecosystem.
Until recently, scientists thought these two ecosystems were very different in fixing nitrogen. But recent findings in Nature Microbiology challenge experts and show us an unexpected interaction among them.
Chandola and colleagues have found a marine bacteria capable of colonising terrestrial plants and providing them with nitrogen. This question the idea about how this symbiosis works, its evolution and the connection between land and sea. Want to know more? Let’s dive in!
Fixers Under The Sea
Cyanobacteria are the best-known nitrogen fixers in the ocean, but they may not be the only ones. Genomic surveys have suggested that other bacteria, including Bradyrhizobium, might also play a role in this process. However, isolating and growing these marine strains has proven difficult.
In previous works, Chandola and colleagues noticed that the marine algae Phaeodactylum tricornutum could tolerate nitrogen-deprived conditions. This was a hint of the presence of nitrogen-fixing partners in its microbiome.
Based on this evidence, they isolated several bacteria associated with the algae, and one of them turned out to belong to a group typically linked to terrestrial ecosystems: Bradyrhizobium.
From Ocean to Roots
To determine what this marine Bradyrhizobium could do, researchers analysed its DNA. They used PacBio technology, allowing them to read the full genome of the bacteria, and compared it to both terrestrial Bradyrhizobium and marine nitrogen fixers.
The results were clear: this marine strain was closely related to photosynthetic terrestrial Bradyrhizobium. Similar to them, it lacked the common genes needed for nodule formation (nodA, nodB and nodC) but still carried nodD, a gene involved in detecting plant signals.
But genetic similarity wasn’t limited to its land-based cousins. When compared to marine nitrogen-fixers, the strain also shared genes involved in environmental adaptation.
Despite these resemblances, its genome showed enough differences to be likely classified as a new species.
All of this was very interesting, but real-life evidence was lacking. And here is where they found the most surprising results!
Going Out Of The Sea: The Resilient Nitrogen-fixing Capability
Given its genetic similarity to terrestrial Bradyrhizobium, one key question emerged: could this marine bacteria help land plants obtain nitrogen?
To find out, the researchers inoculated the roots of a terrestrial legume, Aeschynomene indica, with the newly isolated marine Bradyrhizobium. The plants were grown under sterile conditions without any added nitrogen source, ensuring that any nitrogen available to them would be the result of bacterial fixation.
After 14 days, when the scientists examined the plant roots, they had developed several root nodules. Further analyses proved that the nodule was successfully fixing nitrogen.
To further confirm its photosynthetic capabilities, the researchers exposed the marine strain to light. They observed the production of pigments and characteristic signals of photosynthetic strains, further reinforcing its similarity with terrestrial Bradyrhizobium.
New Questions for Evolution and Agriculture
This finding sets a precedent in the field, as it’s the first time a marine nitrogen-fixing bacteria has been proven to form an interaction with a terrestrial plant!
As with many groundbreaking findings, it raises many questions about the evolution of these interactions. Did this bacteria adapt to marine conditions after evolving on land? Or did nitrogen-fixing symbiosis emerge independently in both environments?
Beyond posing a challenge to scientific theories, the practical potential is huge. These marine bacteria can deliver nitrogen to plants, just like their terrestrial counterparts, but with an added advantage: they’re adapted to extreme conditions like high salinity and nutrient stress. As agricultural soils get drier and salinity increases, these microbes could become powerful tools in securing future food production worldwide.
References
Chandola, U., Manirakiza, E., Maillard, M., Lavier Aydat, L. J., Camuel, A., Trottier, C., ... & Tirichine, L. (2025). A Bradyrhizobium isolate from a marine diatom induces nitrogen-fixing nodules in a terrestrial legume. Nature Microbiology, 1-12. https://doi.org/10.1038/s41564-025-02105-5
