The green alga Caulerpa taxifolia with feather-like branches. The leaf is 5–65 cm in length. Photo: NOAA.
Green algae are organisms that belong to the phylum Chlorophycota of the kingdom Protista, a large group of marine and freshwater algae. Chlorophytes have several features that make them more like green plants than ordinary algae: they have cup-shaped chloroplasts that contain photosynthetic pigments similar to those in higher plants (chlorophylls a and b, as well as the secondary pigments carotenes, lutein, and zeaxanthin) and have a green color; they have cell walls made of cellulose; and they store food in the form of starch. Some produce cells with flagella at some stage in their lives. Green algae range in size from microscopic, single-cell types (some with flagella) to large complex seaweeds.