A cyanobiont is a cyanobacterium that lives in symbiosis with an eukaryote, sometimes inside the cells of the eukaryote. The cyanobiont fixes nitrogen, and sometimes also performs photosynthesis for the host organism. The cyanobiont is often vertically transmitted, this is the case for cyanobionts of diatoms, lichens, didemnid ascidians, Azolla ferns and some sponges. Some of these cyanobionts cannot live without the host organism.
References
Cyanobiont Wikipedia(Text) CC BY-SA