Researchers at the University of California, Santa Cruz, and Kochi University in Japan have discovered a new organelle, the nitroplast, in the marine algae species *Braarudosphaera bigelowii*. The nitroplast is a structure that can convert nitrogen gas into a form that is useful for cell growth, a process known as nitrogen fixation. This discovery challenges the long-held belief that only bacteria and archaea can perform nitrogen fixation.
The nitroplast is a product of a symbiotic relationship between the algae and a bacterium called UCYN-A, which was previously thought to be a separate organism. Genetic analysis has revealed that the bacteria and algae entered into a symbiotic relationship around 100 million years ago, leading to the development of the nitroplast organelle.
Researchers use two key criteria to determine whether a bacterial cell has become an organelle in a host cell. First, the cell structure in question must be passed down through generations of the host cell. Second, the structure must be reliant on proteins provided by the host cell. The nitroplast meets both of these criteria, as it splits in two just before the algae cell divides, ensuring that one nitroplast is passed down to each offspring. Additionally, the nitroplast lacks key proteins required for growth and relies on proteins provided by the algae to fill these gaps.