Blue-green algae are true algae and belong with plants.

“Blue-green algae” was not an arbitrary name. These organisms form visible mats and blooms, contain photosynthetic pigments, and release oxygen, so they fit naturally into older botanical systems that grouped photosynthetic organisms with plants and algae. For generations they were studied largely by phycologists and classified as algae.

Their cells told a different story. Improved microscopy and biochemistry showed no membrane-bound nucleus and no chloroplasts. Instead, photosynthesis takes place on internal membranes inside a bacterial-type cell. As microbiology and molecular classification matured, those bacterial features became decisive. The organisms were increasingly treated as cyanobacteria—photosynthetic bacteria—rather than as a branch of eukaryotic algae.

The old name survives in everyday speech, environmental monitoring, and public-health warnings, which can make the reclassification easy to miss. Cyanobacteria really do perform oxygenic photosynthesis like algae and plants; they simply evolved on the bacterial side of life’s deepest cellular divide.