First Nitrogen-Fixing Organelle Discovered in a Marine Alga, Provides Insight on the Role of Symbiotic Interactions to Evolution of Chloroplast and Mitochondria

It has been widely accepted that only bacteria can take nitrogen from the atmosphere and convert it into a form that living organisms can use. Some plants, called legumes, can fix nitrogen by fostering symbiotic bacteria in root nodules. A recent discovery, however, upends that rule.

A Decades-Long Biological Mystery

In 1998, marine science professor Jonathan Zehr from the University of California Santa Cruz discovered a short DNA sequence of an unknown nitrogen-fixing cyanobacterium in the Pacific Ocean. Zehr and his fellow scientists spent years studying the mysterious organism, which they later called UCYN-A.

At the same time, paleontologist Kyoko Hagino from Kochi University in Japan was attempting to culture a marine alga that was the host organism for UCYN-A. After more than a decade of 300 sampling expeditions, Hagino succeeded in growing the alga in culture, allowing other scientists to study the organism and its host together in the laboratory.

For many years, experts considered UCYN-A an endosymbiont closely related to an alga. However, two recent studies suggest that it has co-evolved with its host during past symbiosis and currently fits the criteria for an organelle.

In a paper entitled "Metabolic trade-offs constrain the cell size ratio in a nitrogen-fixing symbiosis," Zehr and his international colleagues show that the size and ratio between UCYN-A and their algal hosts is the same across several species of the marine haptophyte algae Braarudosphaera Bigelow. The scientists used a model to demonstrate that the growth of host cells and UCYN-A is controlled by the exchange of nutrients with their associated metabolisms. This synchronization in growth rate led the scientists to call the organism "organelle-like."

However, the research team did not confidently consider UCYN-A an organelle until other lines of evidence were confirmed. In the paper "Nitrogen-fixing organelle in a marine alga," a group of experts shows that UCYN-A imports proteins from its host cells. According to Zehr, this is one of the hallmarks of something moving from an endosymbiont to an organelle. They begin throwing away pieces of DNA, with their genomes getting smaller and smaller until they start relying on the mother cell for the gene products to be transported into the cell. This discovery is the first instance of a nitrogen-fixing organelle, which the researchers called a nitroplast.

Changing Perspectives on Symbiosis

The collected evidence leaves little doubt that UCYN-A has surpassed the role of a symbiont. Although mitochondria and chloroplasts evolved billions of years ago, the nitroplast evolved about 100 million years ago. This provides experts with a new, more recent perspective on organellogenesis.

The newly discovered organelle also offers insight into ocean ecosystems. Since all organisms require nitrogen in biologically usable form, UCYN-A is globally essential due to its ability to fix nitrogen from the atmosphere. Additionally, it has been discovered everywhere from the tropics to the Arctic Ocean, and it fixes a significant amount of nitrogen.

This breakthrough also has the potential to change agriculture. For decades, experts have tried to figure out how to incorporate natural nitrogen fixation into agriculture. The system offers a new perspective on nitrogen fixation and can offer clues as to how such an organelle can be engineered into crop plants.

Synthesizing ammonia fertilizers from atmospheric nitrogen enables agriculture to take off in the early part of the 20th century. The technology-enhanced 50% of global food production is known as the Haber-Bosch process. It also creates a large amount of carbon dioxide.