Rhodophytes

Rhodophytes, or red algae, are a diverse group of mostly marine, photosynthetic eukaryotes classified in the phylum Rhodophyta within Archaeplastida. They owe their name to phycoerythrin, a red pigment that masks chlorophyll a and gives many species their red hue. Most are multicellular; some are filamentous or crustose.

Key features include largely non-motile life stages, storage of floridean starch, and cellulose-rich cell walls. Many also produce agar or carrageenan in their extracellular matrices, substances widely used as gelling and thickening agents.

Red algae are primarily marine, inhabiting coastal and reef environments, often at greater depths than other seaweeds. Some species occur in fresh water. Coralline algae deposit calcium carbonate in their walls and contribute to reef structure.

Reproduction is frequently complex, with alternation of generations. Many species display a triphasic life cycle involving a female gametophyte, a dependent carposporophyte, and a separate tetrasporophyte. Most stages are non-motile; male and female gametes usually lack flagella.

Taxonomically, red algae include Bangiophyceae and Florideophyceae; Florideophyceae accounts for the majority of species. They are among the oldest eukaryotic lineages, with extensive morphological and chemical diversity.

Economically, red algae provide agar and carrageenan used in foods, microbiology, cosmetics, and pharmaceuticals. Edible seaweeds such as Porphyra/Pyropia (nori) and various Gracilaria or Gelidium species are harvested or cultivated for consumption.