About coralline algae

A variety of members of the red alga phylum Rhodophyta accumulate calcium carbonate (limestone) within their tissues as they grow. Their pink or purple growths become hard and rock-like. Many form encrusting growths on rocks or on other marine plants such as seagrasses or macroalgae, while others have branching, sometimes bushy growth forms. Certain of the encrusting coralline red algae play important roles in reef formation (along with stony corals) by building new rock and by cementing the reef structure together.

Click for closeup of coralline algae encrusting a turtlegrass leaf.