The green algae are a diverse group of photosynthetic organisms that thrive in aquatic and some terrestrial environments. They belong to the kingdom Plantae, and one branch of this group led to the land plants. They are primarily characterized by their green pigmentation, due to the presence of chlorophyll a and chlorophyll b. As with all algae, these pigments have an active role in photosynthesis, converting light energy into chemical energy and contributing significantly to the primary productivity of freshwater ecosystems. In addition, green algae also have complementary accessory pigments, the carotenoids, that expand the usable wavelengths for photosynthesis and pass that energy on to chlorophyll a. Carotenoids are also involved in photoprotection and can impart their coloration on the cells observed.