Cyanobacteria , also called Cyanobacteriota or Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name cyanobacteria refers to their color  'blue'), which similarly forms the basis of cyanobacteria's common name, blue-green algae, although they are not usually scientifically classified as algae.[note 1] They appear to have originated in a freshwater or terrestrial environment. Cyanobacteria produce a range of toxins known as cyanotoxins that can cause harmful health effects in humans and animals.