Green algae are divided into two main groups, chlorophytes and charophytes. More than 7,000 species of chlorophytes have been identified. Most live in fresh water, but there are also many marine and some terrestrial species. The simplest chlorophytes are unicellular organisms such as Chlamydomonas, which resemble the gametes and zoospores of more complex chlorophytes. Various species of unicellular chlorophytes exist as plankton or inhabit damp soil. Some live symbiotically within other eukaryotes, contributing part of their photosynthetic output to the food supply of their hosts. Some chlorophytes have even adapted to one of the last habitats you might expect to find them: snow. Chlorophytes carry out photosynthesis despite subfreezing temperatures and intense visible and ultraviolet radiation. Other chlorophytes contain similar protective compounds in their cell wall or in a durable coat that surrounds the zygote.