A large specimen of the brown alga Saccharina latissima
‘Algae’ is an artificial concept for a diverse assemblage that spans 4 of the 5 supergroups of eukaryotes and 1 group of prokaryotes in the tree of life.
Although the precise evolutionary relationships of these groups are still open to debate, the name 'algae' is still used as a term of convenience.
Algae range from single cells through to complex multicellular forms several metres in length.
Eukaryotes
Organisms whose cells have a nucleus.
Prokaryotes
Organisms whose cells don't have a nucleus.