All previously discovered cyanobacteria can use water as an oxygen donor in photosynthesis, producing molecular oxygen as a by-product. But Jonathan Zehr at the University of California, Santa Cruz, and his colleagues have found one that can't.
The new cyanobacterium, known as UCYN-A, has not been cultured, although significant parts of its DNA have been sequenced. It lacks photosystem II, which is necessary for water splitting, but retains photosystem I, in common with light-fuelled but non-oxygen-producing green sulphur bacteria. UCYN-A does not fix carbon dioxide into carbohydrates, but it does fix nitrogen.