Algae

Algae are photosynthetic organisms that anchor to substrates in sunlit marine waters. They present diverse forms, ranging from microscopic single cells to multicellular seaweeds up to 50 cm (20 in) in length. Species commonly appear in various shades of green, red, or brown. Green algae often form filamentous tufts, broad sheets, or calcified structures that resemble stacked coins. Red algae typically grow as delicate branches, bushy growths, or hard, encrusting layers on rocks and dead coral. Brown algae are generally olive to dark brown, forming flattened blades or complex branched structures.

Algae convert sunlight into energy using chlorophyll and other pigments, functioning as primary producers and a key food source for marine herbivores such as parrotfish and surgeonfish. They occur abundantly on shallow reef flats, slopes, and rocky substrates, typically in depths down to 40 meters (130 ft). A distinctive form found in the 🌊 Red Sea is Valonia ventricosa, a single-celled green alga that grows into large, iridescent spheres several centimeters across.