As photosynthetic organisms they emit oxygen and have chlorophyll and other pigments within two light-collecting photosystems. Algae can vary in size and their reproductive structures consist of cells that all have the potential to be fertile.
The algae are split into eight groups, differentiating between them by their cell structures and photosynthetic contents. Some may have other chlorophylls and related pigments beside just chlorophyll α, and the way the glucose units are linked in the polysaccharides can vary. Chloroplast structure, flagella and associated structures also differ.
By sequencing the nucleotides in ribosomal RNA and comparing them, it has been suggested that eukaryotic algae are less closely related to each other than nonphotosynthetic protists. This could be because the algae independently acquired their photosynthetic systems as either cyanobacterial or eukaryotic endosymbionts.
Algae are important producers in marine and freshwater environments and can be found both floating on the surface and on the lowest level of the body of water. A few grow in moist habitats exposed to the air.