Groundbreaking Discovery: First Nitrogen-Fixing Organelle Identified in Eukaryotic Cells

An international team of researchers has made a monumental breakthrough in biology by discovering the first known nitrogen-fixing organelle within a eukaryotic cell, challenging long-held beliefs that only bacteria could perform this essential function.

This organelle, termed a “nitroplast,” represents only the fourth instance of primary endosymbiosis documented—a pivotal process where a prokaryotic cell becomes part of a eukaryotic cell, evolving into an organelle.

The discovery was detailed in two recent papers, with significant contributions from Tyler Coale, a postdoctoral scholar at UC Santa Cruz. “This finding rewrites the textbooks, as it’s an event that has shaped complex life throughout Earth’s history, similar to the development of mitochondria and chloroplasts,” Coale explained.

The nitroplast was identified within marine algae where the UCYN-A, a type of cyanobacterium previously thought to be just a closely associated symbiont, has co-evolved with its algal host beyond mere symbiosis.

This relationship was first hinted at in 1998 when Jonathan Zehr, a distinguished professor of marine sciences at UC Santa Cruz, discovered a mysterious DNA sequence in Pacific Ocean seawater. It took decades of subsequent research and collaboration to understand the true nature of UCYN-A.

Recent studies published in Cell and Science journals confirm the integration of UCYN-A into the algal host, exhibiting classic organelle characteristics. These include synchronized growth with the host cell, the importation of proteins from the host, and inheritance patterns akin to other organelles. “The nitroplast scales with its host cell similar to how mitochondria and chloroplasts function within their eukaryotic cells,” Zehr added.

This evolution seems to have occurred around 100 million years ago, providing a relatively recent example of organellogenesis compared to the ancient origins of mitochondria and chloroplasts. The nitroplast’s ability to fix atmospheric nitrogen is crucial, especially considering its widespread presence in tropical to Arctic oceans, highlighting its significant ecological role.

Moreover, this discovery holds promising implications for agriculture by potentially offering new methods to naturally integrate nitrogen fixation into crops, thus reducing reliance on synthetic fertilizers which are a major source of global CO2 emissions.

As research continues, the scientific community anticipates further insights into UCYN-A and its integration as a nitroplast, hoping to uncover more such evolutionary phenomena in other organisms. Kendra Turk-Kubo, assistant professor at UC Santa Cruz, will further this research in her new laboratory, aiming to deepen understanding of natural nitrogen fixation and its applications.