|Similar Trichodesmium, Cyanothece, Gloeobacter, Prochlorococcus, Acaryochloris marina|
Candidatus Atelocyanobacterium thalassa (previously cyanobacterium UCYN-A,) is a species of cyanobacteria commonly found throughout the world's oceans. Unlike many other cyanobacteria, A. thalassa lacks the ability to perform photosynthesis. Instead, it is found in a symbiotic relationship with photosynthetic algae. A. thalassa fixes nitrogen for the algae, while the algae provides carbon for A. thalassa through photosynthesis.
A. thalassa was first described in 1998 as a nitrogen-fixing bacteria with a much-reduced genome. The microbe was originally given the name UCYN-A for "unicellular cyanobacteria group A". In 2012, A. thalassa was described to be in a symbiotic association with the unicellular algae Braarudosphaera bigelowii and Chrysochromulina parkeae.
A. thalassa lacks a variety of metabolic components common to cyanobacteria, including the oxygen-producing photosystem II complex of the photosynthetic apparatus, the carboxysome, enzymes of the Calvin and tricarboxylic acid cycles, as well as several enzymes involved in amino acid synthesis.