The red algae or Rhodophyta are one of the oldest groups of eukaryotic algae. The majority of species are found in the Florideophyceae and consist mostly of multicellular, marine algae, including many notable seaweeds. Approximately 5% of red algae occur in freshwater environments with greater concentrations found in the warmer area.
Unicellular members of the Cyanidiophyceae are thermoacidophiles and are found in hot springs and other acidic environments. The remaining taxa are found in marine environments. Most rhodophytes are marine with a worldwide distribution and are often found at deeper depths compared to other seaweeds because of dominance in certain pigments within their chloroplasts. Some marine species are found on sandy shores, while most others can be found attached to rocky substrata. Freshwater species account for 5% of red algal diversity, but they also have a worldwide distribution in various habitats; they generally prefer clean, high-flow streams with clear waters and rocky bottoms, but with some exceptions. A few freshwater species are found in black waters with sandy bottoms and even fewer are found in more lentic waters. Both marine and freshwater taxa are represented by free-living macroalgal forms and smaller endo/epiphytic/zoic forms, meaning they live in or on other algae, plants, and animals. In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts.
The coralline algae, which secrete calcium carbonate and play a major role in building coral reefs. Red algae such as dulse and laver are a traditional part of European and Asian cuisines and are used to make other products such as agar, carrageenans and other food additives.