Algae are at the base of many aquatic food chains in marine and freshwater habitats. They are autotrophic organisms, varying from single-cell to multicellular species. Species of algae are commonly found in marine and fresh water habitats, although, algae can also be found thriving on land in moist surfaces and as endosymbionts of other organisms. Most algae are photosynthetic, meaning they use sunlight to produce their own food. Unlike true plants, algae lack the distinct organs found in plants. Algae play an important role in the carbon cycle and the existence of nearly all marine life, which depend on algae for their survival.
Photosynthesis
Algae utilize the energy from the Sun to carry on photosynthesis. Photosynthesis occurs in organelles called chloroplasts. All algae contain chlorophyll a; however, algae may also contain other accessory pigments, which is what give species of algae their characteristic color. Most algae obtain their energy through photosynthesis, although some species obtain their energy from chemical reactions (chemotrophic), and some others are heterotrophs. A few species of algae form symbiotic associations with sea sponges, corals and lichens. During photosynthesis, algae remove carbon dioxide (CO2) from the ocean and releases oxygen (O) as a by-product. Thus, maintaining a balance of CO2 concentrations in the atmosphere and the ocean.
Unicellular algae
Single-celled algae inhabit the aquatic ecosystem in the form of phytoplankton. They have flagella which allow them to drift freely through the water. Single-celled algae usually drift in the upper layer of the ocean and freshwater bodies. This provides them with sufficient sunlight for photosynthesis. Algae may form unicellular, colonial or multicellular forms. Algae, such as diatoms are very small, while kelp (brown algae) can grow to about 40 meters (131 ft.) tall. Most single-celled algae are at the very base of many marine and freshwater food chains and many animal species, small and big, rely on them for food.
Multicellular algae
Motile unicellular species of algae often form complex aggregations of filaments, colonies or single multinucleated cells. The groups that may form multicellular thallus (body) are red, brown and some species of green algae. Seaweeds most often inhabit the littoral zone, most frequently the rocky shores. Seaweeds may extend several meters offshore, in some areas of the littoral zone. Some species, such as Sargassum are capable of floating through shallow waters of a coral reef, and other species have totally adapted to withstand the environmental stresses caused by changing temperatures and salinity in tidal pools.
Endosymbiotic relationships
Some species of algae may form symbiotic association with other organisms, in which the algae provides organic nutrients to the host organism, while at the same time obtaining protection from the host. Lichens form symbiotic associations with blue-green algae. In this relationship, the fungus protects the algae from the environment, while obtaining nutrients from the algae. Another type of symbiotic relationship occurs in the coral reef, where zooxanthellae live in a symbiotic association with a coral polyp, providing oxygen and energy. In return the polyp supplies the algae with carbon dioxide and other nutrients that the algae needs. Zooxanthellae may also form associations with jellyfish, clams, anemones, mollusks, among others.
Algae are most commonly found on water ecosystems in most marine and terrestrial habitats, including the ocean, lakes, rivers, ponds and moist surfaces. Temperature and salt concentrations may determine the type of algae found in a given habitat. Species of algae may show distinct colors, depending on the pigments present in them. While most algae species can only thrive in the aquatic environment, some species can live outside of the water; however, they still need a moist surface (mud, rocks, and humid substrates) on where to adhere to survive. Most marine animals, including whales, turtles, seals, shrimp, lobsters, octopuses, and starfish, among many others depend upon algae for feeding.
