In two recent papers, an international team of scientists describes the first known nitrogen-fixing organelle within a eukaryotic cell.
The organelle is the fourth example in history of primary endosymbiosis—the process by which a prokaryotic cell is engulfed by a eukaryotic cell and evolves beyond symbiosis into an organelle.
The newest discovery is the first example of a nitrogen-fixing organelle, which the researchers are calling a nitroplast.
But the two recent papers suggest that UCYN-A has co-evolved with its host past symbiosis and now fits criteria for an organelle.
But the scientists did not confidently call UCYN-A an organelle until confirming other lines of evidence.
“That’s one of the hallmarks of something moving from an endosymbiont to an organelle,” said Zehr.
He compared the proteins found within isolated UCYN-A with those found in the entire algal host cell.
He found that the host cell makes proteins and labels them with a specific amino acid sequence, which tells the cell to send them to the nitroplast.
According to modern biology textbooks, only bacteria are able to take atmospheric nitrogen and transform it into a form that can be used by living things. Legumes are among the plants that fix nitrogen through the symbiotic bacteria they contain in their root nodules. However, a new finding contradicts that principle.
An international group of researchers has reported the first nitrogen-fixing organelle found in a eukaryotic cell in two recent publications. The process by which a prokaryotic cell is swallowed by a eukaryotic cell and progresses beyond symbiosis to become an organelle is known as primary endosymbiosis, and the organelle is the fourth example of this process in history.
According to Tyler Coale, a postdoctoral scholar at UC Santa Cruz and first author of two recent papers, “it’s very rare that organelles arise from these types of things.”. “We believe that the first instance of it giving rise to all complex life. “That event, which pertains to the origins of the mitochondria, is responsible for everything more complex than a bacterial cell,” he stated. “It happened again with the chloroplast about a billion years ago, and that’s how we got plants,” Coale explained.
Thirdly, a microbe resembling a chloroplast is involved. The most recent finding is the first known instance of a nitroplast, an organelle that fixes nitrogen.
A long-standing mystery.
After decades of research and some good fortune, the organelle was discovered. In 1998, Jonathan Zehr, a distinguished 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. The unidentified organism, which Zehr and associates named UCYN-A, was the subject of years of study.
Paleontologist Kyoko Hagino of Kochi University in Japan was laboriously attempting to cultivate a marine alga at the same time. It turned out to be UCYN-A’s host organism. Hagino needed more than a decade and more than 300 sampling expeditions to successfully grow the alga in culture, which made it possible for other researchers to start collaborating on the lab study of UCYN-A and its marine alga host.
For a long time, scientists believed that UCYN-A was an endosymbiont that was intimately connected to an algae. However, it appears from the two recent papers that UCYN-A meets the requirements for an organelle because it co-evolved with its host beyond symbiosis.
Origin 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.
Utilizing 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. The researchers referred to UCYN-A as “organelle-like” because of this synchronization in growth rates. ****.
Zehr stated, “With organelles, that’s exactly what happens.”. The mitochondria and chloroplasts both scale with the cell, if you look at them. “.”.
However, until they could corroborate this with additional data, the scientists did not declare UCYN-A to be an organelle. Researchers from the University of California, San Francisco, the Lawrence Berkeley National Laboratory, National Taiwan Ocean University, Kochi University in Japan, and Zehr, Coale, Kendra Turk-Kubo, and Wing Kwan Esther Mak from UC Santa Cruz demonstrate that UCYN-A imports proteins from its host cells in the journal Science cover article that was published today.
According to Zehr, “that’s one of the hallmarks of something moving from an endosymbiont to an organelle.”. “They begin to discard DNA fragments, their genomes shrink, and they become dependent on the mother cell for the uptake of those gene products, or the protein itself, into the cell. “.”.
The proteomics for the study was handled by Coale. He made a comparison between the proteins present in the isolated UCYN-A and the entire algal host cell. His research revealed that the host cell produces proteins and marks them with a particular amino acid sequence that instructs the cell to transfer the proteins to the nitroplast. The proteins are subsequently used by the nitroplast after being imported. Some of the proteins have been identified by Coale to have specific roles in UCYN-A pathways that they bridge.
Zehr described it as “almost like this magical jigsaw puzzle that actually fits together and works.”.
Researchers from UCSF demonstrate in the same paper that UCYN-A is inherited similarly to other organelles and replicates in synchrony with the alga cell.
Switching viewpoints.
It is highly unlikely that UCYN-A still functions as a symbiont in light of these independent lines of evidence. Additionally, scientists now have a more recent understanding of organellogenesis because the nitroplast appears to have evolved approximately 100 million years ago, whereas mitochondria and chloroplasts evolved billions of years ago.
The organelle sheds light on ocean ecosystems as well. The ability of UCYN-A to fix atmospheric nitrogen is essential for all living things, as it provides the necessary element for all biological processes. It is widely distributed and fixes a sizable amount of nitrogen, according to research from the tropics to the Arctic Ocean.
According to Zehr, “It’s not just another player.”.
A change in agriculture could also result from this discovery. Early in the 20th century, agriculture and global population growth were facilitated by the ability to create ammonia fertilizers from atmospheric nitrogen. Approximately half of the world’s food production is made possible by the Haber-Bosch process. Moreover, it produces a great deal of carbon dioxide, accounting for 14% of global emissions. Researchers have been attempting to figure out how to use natural nitrogen fixation in agriculture for decades.
According to Coale, “this system offers a fresh viewpoint on nitrogen fixation and may offer insights into how such an organelle could be incorporated into crop plants.”.
However, there are still many unanswered questions concerning UCYN-A and its algal host. The scientists intend to investigate various strains and go deeper into the functioning of UCYN-A and the algae.
In her new lab, UC Santa Cruz assistant professor Kendra Turk-Kubo will carry out the research. Although Zehr anticipates that more organisms with evolutionary histories resembling those of UCYN-A will be discovered, this discovery belongs in textbooks because it is the first of its kind.
