Within Protista, algae are separated into the following taxonomic divisions: golden algae (Chrysophyta), yellow-green algae (Xanthophyta), diatoms (Bacillariophyta), brown algae (Phaeophyta), dinoflagellate algae (Pyrrophycophyta), cryptomonad algae (Cryptophyta), euglenoid algae (Euglenophyta), stoneworts (Charophyta), green algae (Chlorophyta), and red algae (Rhodophyta).
As some of these names indicate, pigmentation plays an important role in algal classification, and its correlation with distinctive structural and reproductive features shows that it is a fundamental trait. In most cases the pigmentation referred to is an accessory one that occurs in addition to the green photosynthetic pigment, chlorophyll, which the accessory pigment frequently masks; thus the externally visible color in some of the groups is not green. In at least two of the divisions (brown algae and red algae), there is strong evidence that the accessory pigment is functional in photosynthetic metabolism, absorbing light energy and transferring it to the chlorophyll.
All algae contain the form of chlorophyll known as chlorophyll a. However, other varieties of the compound may be present as well, depending on the species.
The following article does not discuss blue-green algae. Despite their name, members of this group, which make up the phylum Cyanophycota, are not true algae but are instead a form of bacteria.
