Atelocyanobacterium thalassa

Candidatus Atelocyanobacterium thalassa, also referred to as UCYN-A, is a diazotrophic species of cyanobacteria commonly found in measurable quantities throughout the world's oceans and some seas. Members of A. thalassa are spheroid in shape and are 1-2µm 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.

Unlike many other cyanobacteria, the genome of A. thalassa does not contain genes for RuBisCO, photosystem II, or the TCA cycle. Consequently, A. thalassa lacks the ability to fix carbon via photosynthesis. Some genes specific to the cyanobacteria group are also absent from the A. thalassa genome despite being an evolutionary descendant of this group. With the inability to fix their own carbon, A. thalassa are obligate symbionts that have been found within photosynthetic picoeukaryote algae.

Most notably, the UCYN-A2 sublineage has been observed as an endosymbiont in the alga Braarudosphaera bigelowii with a minimum of 1–2 endosymbionts per host. A. thalassa fixes nitrogen for the algae, while the algae provide carbon for A. thalassa through photosynthesis. In 2024, it was announced that Atelocyanobacterium thalassa living inside the alga Braarudosphaera bigelowii behave more like true organelles rather than distinct endosymbionts, and so they have been proposed to be called nitroplasts. It is thought that A. thalassa could be used in future to genetically modify crops in order to improve their growth and yield.

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.