Phytoplankton have some of the same features as plants that grow on the land (terrestrial plants) but they have many other differences. The term phytoplankton is a broad one, it refers to all photoautotrophic microorganisms in water-based food webs. They are very diverse, including protistan eukaryotes and both eubacterial and archaebacterial prokaryotes. The most important groups are diatoms, cyanobacteria, and dinoflagellates. 

Phytoplankton Appearance 

Most phytoplankton is too small to be seen by the human eye, without the help of a microscope. But, when there are large numbers, the surface of the water can change colors due to the presence of chlorophyll. 

There are two different classes of phytoplankton—dinoflagellates and diatoms. The first of these use a tail that resembles a whip to move through the water. They are single-celled eukaryotes that are usually are usually found in marine habitats but have also been located in freshwater. They are one of the largest groups of marine eukaryotes but smaller than the populations of diatoms. There are around 1,555 known marine species of dinoflagellates. Their bodies are covered with complex shells similar to those found around diatoms. 

Credit: ja:User:NEON / commons:User:NEON_ja, CC BY-SA 2.5, via Wikimedia Commons

Diatoms have shells that are made from a different substance than that which is found in dinoflagellates. They are made of interlocking parts with a more rigid structure. Diatoms do not use flagella to move through the water. Instead, they move as the ocean currents do. Diatoms make up a very large portion of Earth’s biomass and produce a significant portion of oxygen each year, around 20 to 50 percent. They are also around half of all organic material found in the oceans. 

Diatoms are unicellular, this means that they occur in colonies or as solitary shells and take various shapes. These include stars, zigzags, fans, ribbons, and more. They range in size from 2 to 200 micrometers. Their cell wall is made of silica, called a frustule that has structural coloration. This feature gave diatoms their nickname— “jewels of the sea” or “living opals.” 

Phytoplankton Habitat

Phytoplankton is an important part of a balanced ecosystem. They’re a crucial food source for many sea creatures, such as shrimp, snails, and jellyfish according to Ocean Service. They are the base of many marine food webs and inhabit the upper sunlit layer of almost all bodies of water, fresh and salt. They live in the photic zone of the ocean where it’s possible to engage in photosynthesis. There, they serve as a food source for many small creatures. They can be degraded by bacteria or viral lysis, a process in which the membrane of a cell is broken down through contact with a fluid containing lysed cells. 

Phytoplankton are also important in biogeochemical cycles. They take up and transform elements that are needed by other organisms. This means that the cycle of elements between species is continued uninterrupted. This is especially helpful, according to WHOI, in areas that are nutrient-poor. Phytoplankton scavenge and release vitamins and micronutrients that help to support the broader marine ecosystem. 

Phytoplankton bloom in the Barents Sea
Phytoplankton bloom in the Barents Sea, Credit: NASA Goddard Space Flight Center from Greenbelt, MD, USA, CC BY 2.0, via Wikimedia Commons

Phytoplankton can also vary depending on their location around the world. For example, oligotrophic regions like the South Pacific Gyre primarily play host to small-sized cells known as picoplankton that are composed of cyanobacteria. In other systems, those that are more productive, larger dinoflagellates are dominant. 

Phytoplankton Diet 

Phytoplankton contains chlorophyll, or the pigment that gives plants their green color. In most plants, it helps to absorb energy and ensure that they get their nutrients from sunlight during photosynthesis. It is also found in green vegetables and is used by some people as a health supplement. But, phytoplankton can’t survive on sunlight alone. They also need inorganic nutrients like nitrates, phosphates, and sulfur, according to Ocean Service. These nutrients are converted into proteins, fats, and carbohydrates. 

Dinoflagellates are known to be photosynthetic and mixotrophic. The first of these refers to the process of taking nutrients from the sun while the latter refers to using a combination of photosynthesis and ingestion of prey. 

Since they live in the photic zone of the ocean, they engage in photosynthesis. This means they take in carbon dioxide and release oxygen. It’s possible, if the sun’s radiation is too high, that phytoplankton starts to degrade, a process known as photodegradation. 

Phytoplankton and Harmful Algae Blooms 

It is possible for phytoplankton to grow out of control if an ecosystem contains too many nutrients, this creates something known as a harmful algae bloom or a HAB. Harmful algae bloom produce harmful effects on their environments and can impact fish, shellfish, marine mammals, birds, and people. There are several human illnesses that can be debilitating or fatal. Every coastal area experiences HABs. They are a concern to governments around the world due to the impact they can have on marine life, industry, and tourism. Climate change appears to be exasperating the problem. 

Harmful algae bloom in Lake Erie
Harmful algae bloom in Lake Erie, Credit: NOAA Great Lakes Environmental Research Laboratory, CC BY-SA 2.0, via Wikimedia Commons

Phytoplankton Reproduction 

When diatoms have the correct amount of sunlight and nutrients, they’re capable of doubling in size every twenty-four hours. They use a process known as asexual multiple fission. This means that the nucleus of the parent cell divides several times by amitosis, creating more nuclei. Then, daughter cells are created when the cytoplasms separate. In ideal conditions, diatoms live for around six days. When they die, their shells sink, carrying atmospheric carbon into the deep sea. 

Phytoplankton Facts

  • There are two different classes of phytoplankton—dinoflagellates and diatoms.
  • Phytoplankton can grow out of control and cause HAB’s or harmful algae blooms. 
  • They can be photosynthetic and mixotrophic.
  • Phytoplankton contains chlorophyll or the pigment that gives plants their green color. 
  • Most phytoplankton is too small to be seen by the unaided eye. 
  • Diatoms are sometimes called “jewels of the sea” or “living opals.” 
  • Phytoplankton is an important food source for many sea creatures, such as shrimp, snails, and jellyfish. 
  • Phytoplankton is used in aquaculture and mariculture. They are used as food for farmed animals in aquaculture and introduced into enclosures with the normal circulation of seawater in mariculture.