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.