Marine life appeared in the form of fish, which live exclusively in water. Some of these evolved into amphibians which spend portions of their lives in water and portions on land. Other fish evolved into land mammals and subsequently returned to the ocean as seals, dolphins or whales. Plant forms such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton, and particularly phytoplankton, are key primary producers forming the general foundation of the ocean food chain. Marine invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters including breathing tubes (see
Marine microorganism Earth's life and may play roles in both the carbon cycle and the nitrogen cycle. Algae can grow as single cells, or in long chains of cells. Green algae are a large group of photosynthetic eukaryotes that include many microscopic organisms. Green algae include unicellular and colonial flagellates as well as various colonial, coccoid, and filamentous forms. There are about 6000 species. Over 1500 species of fungi are known from marine environments. These are parasitic on marine algae or animals, or are saprobes on algae, corals, protozoan cysts, sea grasses, wood and other substrata, and can also be found in