Green algae are a large and ecologically important group of oxygenic photosynthetic eukaryotes. They are diverse in terms of species number, morphology, biochemistry and ecology. Together with the embryophytic land plants, they form the Viridiplantae or green plants (also known as Chlorobionta or Chloroplastida). The Viridiplantae comprise two clades: the Chlorophyta and the Streptophyta. The Chlorophyta include a wide variety of marine, freshwater and terrestrial green algae. The Streptophyta include freshwater green algae (also known as charophytes) and the embryophytic land plants. As such, the green algae with the exclusion of the land plants form a paraphyletic group. Monophyly of the green plants, on the other hand, is well established based on ultrastructural, biochemical and molecular data. The green algae possess the following unifying traits, most of which are also shared with the land plants: Chloroplasts are enclosed by a double membrane, and include stacked thylakoids. Many species have pyrenoids, which are embedded in the chloroplast, penetrated by thylakoids, and surrounded by starch. Photosynthetic pigments include chlorophyll a and b, along with accessory pigments such as carotenes and xanthophylls.