Cyanobacterium Candidatus Atelocyanobacterium thalassa "probably makes a significant contribution to global nitrogen fixation in the oceans". It's found only in association with a tiny single cell algae, possibly living in a groove on its surface. The cyanobacterium lacks "genes needed to carry out photosynthesis and other essential metabolic pathways." The symbiotic algae provides it with carbon and the cyanobacterium fixes nitrogen for the algae. It can't be grown in the lab, which is why they're just noticing it now.

Arrows point to nitrogen-fixing cyanobacterial cells associated with the larger photosynthetic host cells.

Unusual Symbiosis in Marine Microorganisms Fertilizes Ocean by Fixi...