An order of red algae (Rhodophyceae), commonly called coralline algae. Only one family is recognized, the Corallinaceae, which formerly was assigned to the order Cryptonemiales. These algae are distinguished by the impregnation of cell walls with calcite, a form of calcium carbonate, which causes the thallus to be stony or brittle. See also Rhodophyceae.
Coralline algae, comprising about 40 genera and 500 species, are widespread, abundant, and ecologically important. They are divisible into two groups on the basis of the presence or absence of uncalcified, moderately flexible joints (genicula) between calcified segments (intergenicula). The most simple nonarticulated coralline algae are individual crusts of varying extent and thickness (to 8 in. or 20 cm thick). These often become confluent and cover large expanses of substrate. Many crusts bear rounded or pointed, branched or unbranched protuberances that can break off and continue to grow as free nodules known collectively as maerl.
Coralline algae are exclusively marine, although some species can tolerate a reduction in salinity to 13 parts per thousand. Some species thrive only where light is intense, as at the crest of a coral reef, while others grow only in shaded habitats or in deep water. Most species require constant immersion.
The ecosystem in which coralline algae are most important is the coral reef, where they are primary producers, adding carbon to the ecosystem, adding new material to the reefs, and cementing together other calcareous organisms. They have been engaged in similar activities through the millennia, with modern genera recognizable in limestones at least as old as 150 million years (Jurassic).
