Which substance is responsible for the red color in algae?

The red color in algae is mostly attributed to a group of pigments called phycoerythrin. Phycoerythrin belongs to the class of phycobiliproteins, protein complexes that act as accessory light-harvesting pigments in certain algae and cyanobacteria.

Phycoerythrin appears red because it absorbs light mostly in the blue-green region of the spectrum (peaking around 495 nm wavelength) and reflects red light. This pigment plays a role in photosynthesis by efficiently capturing light energy from the blue-green part of the spectrum and transferring it to chlorophyll a, the primary photosynthetic pigment. By maximizing light absorption, phycoerythrin allows algae to photosynthesize efficiently in deep water or shady environments where blue-green light penetrates reasonably well.

Along with phycoerythrin, other phycobiliproteins such as phycocyanin (appearing blue) and allophycocyanin (appearing blue-green) also contribute to the vibrant colors observed in diverse algal species.