What pigment gives many red algae their characteristic red or purplish color?

Phycoerythrin is the pigment that gives many red algae their characteristic red or purplish color.

Phycoerythrin belongs to a group of water-soluble light-harvesting proteins called phycobiliproteins. Red algae contain it in structures called phycobilisomes, which are attached to their photosynthetic membranes. The pigment absorbs wavelengths of light that penetrate seawater relatively well and transfers captured energy toward chlorophyll a.

The amount and combination of pigments vary among species, so red algae are not always visibly red. Some appear brownish, greenish, or nearly black, particularly when viewed underwater or when other pigments dominate. Their color also depends on depth, lighting, and tissue thickness.

Phycoerythrin is not exclusive to red algae: some cyanobacteria and other organisms also contain it. It should not be confused with fucoxanthin, a brown-algal pigment, or with chlorophyll, the central photosynthetic pigment found across many photosynthetic lineages.