Only bacteria are able to extract nitrogen from the atmosphere and transform it into a form that is useful for life, according to modern biology textbooks. Legume plants, for example, fix nitrogen by storing symbiotic bacteria in nodules on their roots. However, a new finding disproves this principle.
An international group of researchers has reported the first nitrogen-fixing organelle found in a eukaryotic cell in two recent publications. One eukaryotic cell engulfing a prokaryotic one to undergo primary endosymbiosis, which progresses from symbiosis to organelle formation, is the fourth example of this process in history. This is known as the organelle.
“It’s very rare that organelles arise from these types of things,” stated Tyler Coale, a UC Santa Cruz postdoctoral scientist and the lead author of two recent studies. “We believe that the initial instance of it gave rise to all complex life. Referring to the mitochondria’s genesis, he stated, “The first time we think it happened, it gave rise to all complex life. Everything more complicated than a bacterial cell owes its existence to that event,” “A billion years ago or so, it happened again with the chloroplast, and that gave us plants,” added Coale.
Thirdly, a microorganism resembling a chloroplast is involved. The most recent finding is the first instance of an organelle that fixes nitrogen, dubbed a nitroplast by the researchers.
Long-Running Mystery
Decades of research and a little bit of chance led to the discovery of the organelle. In 1998, Jonathan Zehr, a prominent professor of marine sciences at UC Santa Cruz, discovered in Pacific Ocean saltwater a brief DNA sequence that seemed to be from an unidentified nitrogen-fixing cyanobacterium. Zehr and associates investigated the unidentified bacterium for years, naming it UCYN-A.
Meanwhile, paleontologist Kyoko Hagino of Kochi University in Japan was laboriously attempting to culture a marine alga. It turned out to be UCYN-A’s host organism. Hagino eventually succeeded in growing the alga in culture after more than 300 sample excursions and more than ten years, which allowed other researchers to start collaborating on the lab study of UCYN-A and its marine alga host.
For many years, scientists believed that UCYN-A was an endosymbiont that shared a close relationship with an algae. However, it appears from the two recent studies that UCYN-A met the requirements for an organelle by coevolving with its host after symbiosis.
Origins of Organelles
Zehr et al. (MIT, Institut de Ciències del Mar, Barcelona, and University of Rhode Island) demonstrate in a March 2024 paper published in Cell that the size ratio between UCYN-A and their algal hosts is consistent among the various species of the marine haptophyte algae Braarudosphaera bigelowii.
Using a model, the researchers show how the exchange of nutrients regulates the growth of both UCYN-A and the host cell. Their metabolic processes are connected. Because of this growth rate synchronization, the researchers dubbed UCYN-A “organelle-like.”
“That’s exactly what happens with organelles,” stated Zehr. “If you look at the mitochondria and the chloroplast, it’s the same thing: they scale with the cell.”
However, until additional lines of evidence were confirmed, the scientists did not confidently label UCYN-A as an organelle. Zehr, Coale, Kendra Turk-Kubo, Wing Kwan Esther Mak, and colleagues from the University of California, San Francisco, the Lawrence Berkeley National Laboratory, National Taiwan Ocean University, and Kochi University in Japan demonstrate that UCYN-A imports proteins from its host cells in the journal Science cover article that was published today.
“That’s one of the hallmarks of something moving from an endosymbiont to an organelle,” stated Zehr. “They start throwing away pieces of DNA, and their genomes get smaller and smaller, and they start depending on the mother cell for those gene products—or the protein itself—to be transported into the cell.”
Coale was involved in the study’s proteomics. He conducted a comparison between the proteins present in the isolated UCYN-A and the complete algal host cell. His research revealed that the host cell produces proteins and marks them with a certain amino acid sequence that instructs the cell to transfer the proteins to the nitroplast. After then, the proteins are imported and used by the nitroplast. Some of the proteins have roles that Coale has determined; they close gaps in specific UCYN-A pathways.
“It’s kind of like this magical jigsaw puzzle that actually fits together and works,” Zehr remarked.
Researchers from UCSF demonstrate in the same study that UCYN-A is inherited similarly to other organelles and multiplies in synchronization with the alga cell.
