Researchers in the United States have identified a cellular organelle in algae capable of converting nitrogen gas into a form usable for cell growth. This discovery of a structure known as a nitroplast could enhance efforts in genetic engineering to boost nitrogen fixation in plants, thus increasing crop yields and reducing the need for fertilizers. The findings were published on April 11th in the journal Science.
According to Nature, "Nitrogen fixation is thought to occur only in bacteria and archaea." Jonathan Zehr, a marine ecologist at the University of California, Santa Cruz, and the lead author of the study, stated that the algae they discovered are "the first eukaryote known to fix nitrogen," referring to organisms like plants and animals.
The discovery stemmed from a 2012 finding by Zehr and colleagues, who observed a close interaction between the marine alga, Beijerinckia indica, and the nitrogen-fixing cyanobacterium UCYN-A. UCYN-A appeared to reside inside or on the algal cells of B. indica. As a hypothesis, Zehr and his team suggested that UCYN-A converts nitrogen gas into compounds usable for algal growth, such as ammonia, and in return, UCYN-A gains carbon-based energy from the algae.
Previous research considered UCYN-A and similar cyanobacteria as endosymbionts within marine single-celled algae. However, in the latest study, Zehr and colleagues argue that UCYN-A should be classified as an organelle within the algal cells rather than as a separate organism.
The researchers assessed whether UCYN-A had become an organelle within the host cell through two key criteria: first, the discussed cellular structure must be inherited through generations of the host cell, and second, the structure must depend on proteins provided by the host cell.
By imaging dozens of algal cells at different stages of cell division, the researchers found that UCYN-A divided alongside the algal cell before cell division, thus passing from parent to offspring. Additionally, UCYN-A, comprising over 8% of the volume of each host cell, lacked the crucial proteins needed for photosynthesis and the synthesis of genetic material, acquiring these proteins for growth from the algal cells.
These findings indicate that UCYN-A has tightly integrated into the structure of algal cells and division, importing proteins encoded by algal genomes, which are characteristics of organelles. This suggests that UCYN-A has evolved beyond an endosymbiotic relationship to function as a nitrogen-fixing organelle or nitroplast in the early stages of evolution.
Understanding how this nitroplast interacts with its host cell could aid in designing crops capable of nitrogen fixation. This would reduce the need for nitrogen-based fertilizers, thus mitigating environmental pollution.
"It's a great idea to have crops with their own nitrogen-fixing organelles," said Eva Nowack, a symbiosis expert at the University of Düsseldorf in Germany, though she noted that conferring this ability to plants is challenging, particularly ensuring stable inheritance of nitroplasts across generations.
For more information, see the related paper: Link.
