Atelocyanobacterium thalassa

Candidatus Atelocyanobacterium thalassa, also referred to as UCYN-A, is a nitrogen-fixing species of cyanobacteria that exists exclusively as an obligate symbiont. Despite being found in measurable quantities throughout the world's oceans, A. thalassa is not known to be free-living in any environment. Unlike typical cyanobacteria, its genome has undergone massive reduction, losing the genes for RuBisCO, photosystem II, and the TCA cycle. Consequently, it possesses no independent means of fixing carbon or generating energy through photosynthesis, rendering it entirely dependent on its host (so far only known to be Braarudosphaera bigelowii and a closely-related unnamed species).

This partnership is characterized by a strict metabolic exchange: A. thalassa fixes atmospheric nitrogen into ammonium for the host, while the host provides the essential carbon products the bacterium can no longer produce for itself. While various sublineages are distributed across diverse marine niches—from oligotrophic open waters to coastal regions—every known version of A. thalassa remains confined within a host cell.

In the more integrated form, specifically the UCYN-A2 sublineage within the alga Braarudosphaera bigelowii, the relationship has progressed so far that the bacterium is now considered a true organelle, termed a nitroplast. In these cases, the "bacterium" is imported with nuclear-encoded proteins and its division is synchronized with the host, mirroring the evolutionary history of mitochondria and chloroplasts. This discovery of the first nitrogen-fixing organelle in a eukaryote has major implications for agricultural science, as it demonstrates a biological pathway for potentially engineering crops that do not require nitrogen fertilizer.

Members of A. thalassa are spheroid in shape and are 1-2 μm in diameter, and provide nitrogen to ocean regions by fixing non biologically available atmospheric nitrogen into biologically available ammonium that other marine microorganisms can use. There are many sublineages of A. thalassa that are distributed across a wide range of marine environments and host organisms. It appears that some sublineages of A. thalassa have a preference for oligotrophic ocean waters while other sublineages prefer coastal waters. Much is still unknown about all of A. thalassa's hosts and host preferences.