Red seaweed: why is it red?

Everything has a colour, based on the wavelength of light reflected rather than absorbed. Red seaweeds contain pigments called phycoerythrins and phycocyanins that reflect red light and absorb blue-green light.

These are not the plants’ only pigments, however. Red seaweeds also contain chlorophyll – the pigment responsible for photosynthesis – as well as a number of others. Energy from the sun is captured and passed by the various red pigments into the chloroplasts.

To complicate matters further, not all red seaweeds are red. Species with lower concentrations of phycoerythrins can appear greenish-blue or brown to the naked eye, which can often cause problems with identification.

The pigments in red algae have proved extremely useful to science, particularly cancer research. Due to their fluorescent properties, the pigments can be used to chemically tag antibodies and their subsequent effects on tumour cells.

Main image © Getty Images