Phytoplankton are microscopic organisms that drift in sunlit water and use photosynthesis to grow. They include diverse algae and cyanobacteria rather than one branch of the tree of life. Together they form the base of most marine food webs, produce a large share of Earth’s oxygen and influence the carbon cycle.
Phytoplankton versus zooplankton
“Plankton” describes organisms carried largely by currents, not a single species. Phytoplankton make organic matter using light. Zooplankton are animals or animal-like organisms that eat phytoplankton, other plankton or particles. Some organisms combine strategies or change roles during their life cycle.
Major phytoplankton groups
- Diatoms build intricate silica shells and often thrive where nutrients are plentiful.
- Dinoflagellates use flagella to move and include photosynthetic, predatory and toxin-producing species.
- Coccolithophores make plates of calcium carbonate that can brighten huge blooms in satellite images.
- Cyanobacteria, including tiny Prochlorococcus, dominate large nutrient-poor regions.
Why phytoplankton need light and nutrients
Photosynthesis confines most phytoplankton to the upper ocean where sufficient light penetrates. They also need nitrogen, phosphorus, iron and other nutrients. Wind and currents can mix nutrients upward, while river input and upwelling create productive regions.
Too few nutrients limit growth; too many from sewage or fertiliser can drive dense blooms and oxygen loss when organic matter decomposes.
Foundation of the marine food web
Copepods, krill, shellfish and many larvae graze phytoplankton. Small fish eat those consumers, and energy passes to tuna, seabirds, seals and whales. Food webs have many loops, but primary production sets much of the energy budget.
Phytoplankton, oxygen and carbon
Photosynthesis releases oxygen and takes up carbon dioxide. Most organic carbon is respired near the surface, but some particles and dead cells sink. This biological carbon pump transfers carbon to deeper water and sediment for periods ranging from days to millennia.
Are algal blooms good or bad?
Seasonal blooms are normal and support productive ecosystems. A harmful algal bloom is defined by damage: some species make toxins, clog fish gills or create low oxygen. Water colour alone cannot reveal whether a bloom is dangerous, so official testing matters.
How scientists observe phytoplankton
Satellites measure ocean colour and estimate chlorophyll over large areas. Researchers also use microscopes, genetic methods, pigments, imaging sensors and water samples to identify species and understand productivity.
For the planetary context, read how much oxygen the ocean produces.
Frequently asked questions
Are phytoplankton plants?
Some are algae traditionally described as plant-like, while cyanobacteria are bacteria. “Photosynthetic plankton” is more accurate than calling all of them plants.
Can you see phytoplankton?
Most individuals are microscopic, but dense blooms can change ocean colour and some colonies are visible.