Quanta Magazine

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 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 terrestrial rhizobia.

If Gamma A’s genome is a chess game, its nitrogenase gene is a checkers piece thrown into a box: it must come from somewhere else.

It was strange enough, but the researchers hadn’t yet laid eyes on the organism in question, only on its genome. Using genetic techniques, they followed the rhizobium’s DNA to a marine diatom—one of the sea’s ubiquitous photosynthetic microscopic algae—of the genus Haslea. Inside each diatom were four to eight bacterial cells. These cells turned out to be two bacterial species, which the researchers named Diatomic Tectiglobe And Deep Tectiglobe.

Haslea Diatoms perform photosynthesis to create energy; they then pass some of this energy to Tectiglobewhich provides nitrogen to the diatom.

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, that are essential for providing nitrogen to food webs.

To unpack these tortuous stories, the researchers reconstructed evolutionary trees for rhizobia and Tectiglobe bacteria. The results suggest that both groups acquired the ancient nitrogenase gene from other bacteria through horizontal gene transfer at different times in their evolutionary history. The authors also hypothesized that Tectiglobe developed its symbiotic relationship independently and earlier than its better-known terrestrial cousin.

Tectiglobe performs important biochemical work in the ocean. Researchers have estimated that Tectiglobe fixes nitrogen at a rate slightly less than half that of Trichodesmiumcyanobacteria previously thought to dominate oceanic nitrogen fixation. Tectiglobe-Diatom partners are found in oceans around the world. This relationship appears to be a significant part of nitrogen fixation on Earth.

The Symbiotic Spectrum

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

“This is how this little, isolated, solitary diatom can support itself,” says Angelicque White, an oceanographer at the University of Hawaii who was not involved in the work. “These unusual associations break our simplified description of how ecosystems work. They’re far from land and nutrient sources, so these organisms have to adapt somehow.”

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

It’s the same scenario that produced mitochondria and chloroplasts: both organelles were once free-living bacteria that became symbionts of larger cells and eventually settled there permanently. Tectiglobe Species, such as mitochondria and chloroplasts, have relatively small genomes, suggesting that they have given up genes they no longer need because the diatom host provides them. When Tschitschko observed the host and symbiont dividing to reproduce, their divisions occurred simultaneously.

These two qualities – a small genome and pairwise reproduction – indicate a lasting and stable symbiosis. Tectiglobe is certainly on the way to losing even more of its genome and becoming an organelle requires further research.

“There’s definitely a spectrum of symbioses, from loose symbioses to organelles, 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-supplying cyanobacterium that had become an organelle within an algal cell. This is clearly a recurring theme in the world of nitrogen fixation. After all, if you had the opportunity to make your own vital nutrient by adopting a pet, how could you resist?