Candidatus Atelocyanobacterium thalassa, also referred to as UCYN-A, is a nitrogen-fixing species of cyanobacteria commonly found in measurable quantities throughout the world's oceans and some seas.

[4] With the inability to fix their own carbon, A. thalassa are obligate symbionts that have been found within photosynthetic picoeukaryote algae.

[6] 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.

[7][8] It is thought that A. thalassa could be used in future to genetically modify crops in order to improve their growth and yield.

The assumption that N2 fixation only occurred via Trichodesmium and Richelia led to the conclusion that in the oceans, nitrogen output exceeded the input[citation needed].

The use of the polymerase chain reaction (PCR), removed the requirement of cultivation or microscopy to identify N2 fixing microorganisms.

[10] In 1989, a short nifH gene sequence was discovered[citation needed], and 15 years later it was revealed to be an unusual cyanobacterium that is widely distributed.

[16] Genes are lacking for photosystem II of the photosynthetic apparatus, RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), and enzymes of the Calvin and tricarboxylic acid (TCA) cycle.

[17][18] Due to the lack of metabolically essential genes, A. thalassa requires external sources of carbon and other biosynthetic compounds.

[16] The complete or partial lack of biosynthetic enzymes required for valine, leucine, isoleucine, phenylalanine, tyrosine and tryptophan biosynthesis further suggests the need for external sources of amino acids.

[4] Atelocyanobacterium thalassa may be a true endosymbiont and fully enclosed within the host's cell membrane or has molecular mechanisms to allow for secure attachment and transfer of metabolites.

[19] Atelocyanobacterium thalassa is unicellular, hence it does not have specialized cellular compartments (heterocysts) to protect the nitrogenase (nifH) from oxygen exposure.

Other nitrogen-fixing organisms employ temporal separation by fixing nitrogen only at night-time, however, A. thalassa has been found to express the nifH gene during the daylight.

The oligotypes of A. thalassa are based on its nitrogenase (nifH) sequences, and reveal thirteen positions of variance (entropy).