blue-green alga

from the GNU version of the Collaborative International Dictionary of English.

• any of a group of photosynthetic microorganisms possessing characteristics of both bacteria and plants. When classed as bacteria, they are assigned to the Cyanobacteria; when classed as plants, they are assigned to the Cyanophyta. They are now known to be prokaryotic, and are usually called cyanobacteria in technical contexts. See cyanobacterium.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any member of the cyanobacteria; a primitive photosynthetic organism similar to bacteria which is thought to have generated the first oxygen in the Earth's atmosphere.