Phytoplankton are microscopic one-celled plants that are at the base of the aquatic food chains. Phytoplankton usually thrive in the upper sunlit layer of the world’s oceans, lakes and rivers. This allows them to use the energy from the sun for the process of photosynthesis. Phytoplankton are primary producers, meaning that they provide the food for larger marine animals. Phytoplankton rely on photosynthesis to produce organic compounds from carbon dioxide (CO2) contained in the ocean water. To get enough energy from the sun, phytoplankton usually thrives in the euphotic zone, which is the ocean layer where sunlight is able to enter. Phytoplankton is responsible for approximately half of all the oxygen in the Earth's atmosphere.
Since phytoplankton are very small organisms, they’re hardly seen with the unaided eye; however, when phytoplankton is present in large numbers in a body of water, they usually give the body of water a green discoloration or other pigmentation which is determined by the species of phytoplankton present in the water. There are around 500 known species of phytoplankton. Most species of phytoplankton are photosynthetic autotrophs; however, there are some species that are mixotrophic (autotroph and heterotroph) and others are heterotrophic, such as zooplankton.
Phytoplankton are at the base of the marine food web, since many other marine organisms depend on them for survival. To produce organic carbon, needed by all living organisms, they must live in the sunlit region of the world’s bodies of water. Phytoplankton, like plants, use the energy from the sun to convert inorganic carbon dioxide (CO2) and water to organic carbon and oxygen. Phytoplankton are very diverse marine plants, including eubacteria and archaebacteria prokaryotes, as well as protistan eukaryotes. The most important groups of phytoplankton include the dinoflagellates, diatoms, coccolithophorids and cyanobacteria.
Dinoflagellates are unicellular and have two flagella; one extending out from the cell and the other located in a groove encircling the cell. Dinoflagellates are one of the main causes of harmful algal blooms know as red tides. Algal blooms can be very toxic to humans and fish that come into contact with it, while it can be fatal if consumed from predatory fish, such as the paralytic shellfish. Diatoms are photosynthetic unicellular eukaryotes. Diatoms can be of two shapes; elongated (pennate) or circular (centric). The cell wall of a diatom is made of silica and consists of two closely fitting halves. Diatoms occur as single cells or in colonies.
Coccolithophores are unicellular algae with two smooth flagella. They have a haptonema, which is used to gather food and deliver it to a food vacuole. Coccolithophores possess calcium carbonate scales known as coccoliths. Large populations of coccolithophores can produce algal blooms, turning sea water into a solid turquoise hue. Cyanobacteria, also known as blue-green algae, are unicellular and colonial microorganisms. They generally have a gelatinous cell wall. They are found in the ocean, freshwater and land ecosystems of the world. It is believed that cyanobacteria created the conditions that changed the composition of the early atmosphere, leading to the evolution of oxygen-tolerant life forms on Earth.
Under certain conditions, the populations of phytoplankton can grow significantly, producing algal blooms. Blooms can extend for hundreds of square kilometers in the ocean and are easily detected from satellite images. Marine plankton, including phytoplankton and zooplankton serve as food for larger marine animals, such as sharks, manta rays and whales, which can ingest millions of plankton in one mouthful. According to noaa.gov, the diversity of aquatic life is highly dependent on the amount of phytoplankton in the ocean and fresh water bodies of the world.