In the radioactive bacteria, Tschitschko and his colleagues detected the Gamma A version of the nitrogenase gene. They were on its trail. However, the gene was located in an exotic genomic environment. When they sequenced the DNA of the Gamma A bacterium, most of its genome was typical of a globally distributed class of bacteria called Alphaproteobacteria. Its nitrogenase gene, however, was taxonomically related to the land-based rhizobia.

If Gamma A’s genome is a chess set, its nitrogenase gene is a checkers piece thrown in the box: It has to have come from somewhere else.

This was odd enough, but the researchers still had not laid eyes on the organism in question, only its genome. Using genetic techniques, they tracked the rhizobia DNA to a marine diatom — one of the ubiquitous, photosynthetic microscopic algae of the sea — of the genus Haslea. Inside each diatom were four to eight bacterial cells. The cells turned out to be two bacterial species, which the researchers named Tectiglobus diatomicola and Tectiglobus profundi.

Haslea diatoms photosynthesize to create energy; then they hand over some of this energy to Tectiglobus, which supplies the diatom with nitrogen.

This mirrors the relationship between rhizobia and legumes on land, in which bacteria offer nitrogen to the plant in exchange for carbohydrates. Somehow, this nitrogenase gene found its way into two bacterial groups — and both went on to form symbiotic relationships, with very different host organisms, crucial for providing nitrogen to food webs.

To unpack these twisted histories, the researchers reconstructed evolutionary trees for the rhizobia and Tectiglobus bacteria. The results suggested that both groups acquired the ancient nitrogenase gene from other bacteria through horizontal gene transfer at different points in their evolutionary histories. The authors also speculated that Tectiglobus evolved its symbiotic relationship independently and earlier than its more widely known cousin onshore.

Tectiglobus is doing important biochemical work in the ocean. The researchers estimated that Tectiglobus is fixing nitrogen at slightly less than half the rate of Trichodesmium, the cyanobacteria previously thought to dominate oceanic nitrogen fixation. And the Tectiglobus-diatom partners are found in oceans throughout the world. The relationship appears to represent a significant chunk of nitrogen fixation on Earth.

The Symbiotic Spectrum

It makes sense that a diatom would want to carry an in-house nitrogen source: The ocean is a desert. Nutrients are scarce, and most microbes are in a perpetual state of near-starvation. A photosynthesizing diatom with its own unlimited source of energy, but with a need for nitrogen, offered Tectiglobus a safe and beneficial arrangement.

“This is the way this one isolated, lonely little diatom can meet its own needs,” said Angelicque White, an oceanographer at the University of Hawaiʻi who wasn’t involved in the work. “These unusual associations break down our simplified description of how ecosystems work. They’re far from land. They’re far from the sources of nutrients. And so these organisms have to adapt in some way.”

But what is the arrangement exactly? It has a whiff of an enduring symbiotic relationship, but it’s also possible that Tschitschko caught the bacteria in the middle of a transition to full-fledged organelle, in which case they would cease to be an independent organism.

This is the same scenario that produced mitochondria and chloroplasts: Both organelles were formerly free-living bacteria that became symbionts of larger cells and eventually moved in permanently. The two Tectiglobus species, like mitochondria and chloroplasts, have a rather small genome, suggesting that they have been jettisoning genes they no longer need because the diatom host provides for them. When Tschitschko observed the host and symbiont dividing to reproduce, their divisions occurred together.

Both of these qualities — a diminished genome and paired reproduction — point to a long-lasting and stable symbiosis. Whether Tectiglobus is definitely on its way to losing even more of its genome and becoming an organelle requires more research.

“Undoubtedly there’s a spectrum of symbioses, from loose symbioses to an organelle, and these organisms can be placed along that spectrum,” said Zehr, who was not involved in the new research. In 2024, his lab reported a nitrogen-providing cyanobacterium that had become an organelle within an algal cell. Clearly, this is a recurring theme in the world of nitrogen fixation. After all, if you had the chance to manufacture your own vital nutrient by taking on a pet, how could you resist?