Rhodophyta

Rhodophyta, commonly called red algae, is a diverse group of mostly marine, multicellular photosynthetic organisms. They inhabit coastal and open-ocean habitats from temperate to tropical seas. Their characteristic red color comes from phycoerythrin, a phycobiliprotein that masks chlorophyll a and enables photosynthesis at greater depths than many other algae.

Cell walls typically contain cellulose and a carbon-rich matrix, and many rhodophytes store floridean starch. Coralline reds deposit calcium carbonate in their walls, contributing to reef building in some environments. Most species lack motile cells in their life cycles.

Reproduction is often complex, with alternation of generations. Many species have three generations: a gametophyte, a diploid carposporophyte dependent on the female, and a tetrasporophyte producing tetraspores. Some show isomorphic generations, others heteromorphic. Gametes are usually non-flagellated.

They are major marine producers, contributing to primary production and providing habitats for invertebrates. Phycoerythrin enables growth at greater depths, giving them broad depth distributions. Freshwater rhodophytes exist but are fewer.

Economic importance: Red algae yield agar and carrageenan, extracted from genera such as Gelidium, Gracilaria, and Chondrus. Edible species including nori (Porphyra/Pyropia), dulse (Palmaria), and wakame (Undaria) are cultivated and consumed. Some species are studied for bioactive compounds.