Phytoplankton and algae overlap heavily, but they’re not exactly the same thing. Most phytoplankton are algae, yet the two terms describe different qualities of an organism. “Algae” is a biological classification referring to photosynthetic organisms that live in water. “Phytoplankton” describes a lifestyle: any photosynthetic organism that drifts with ocean or freshwater currents rather than swimming or anchoring itself. The vast majority of phytoplankton species are microscopic algae, but the category also includes cyanobacteria, which are technically bacteria, not algae at all.
How the Two Terms Overlap
Algae is the broader category. It includes everything from single-celled organisms invisible to the naked eye to giant kelp that can grow up to 60 meters long. Phytoplankton refers specifically to the microscopic, free-floating subset. So a single-celled alga drifting in the open ocean counts as both algae and phytoplankton. A massive kelp forest anchored to the seafloor is algae but not phytoplankton.
The simplest way to think about it: nearly all phytoplankton are algae, but not all algae are phytoplankton. Seaweed, for instance, is a macroalga. It lacks true roots, stems, and leaves, instead growing as a flat, leaf-like body called a thallus, sometimes with gas-filled structures for buoyancy. It stays put. Phytoplankton, by contrast, are microalgae with a high surface-area-to-volume ratio that lets them absorb nutrients efficiently and reproduce quickly while drifting wherever currents carry them.
The Cyanobacteria Exception
One important group of phytoplankton isn’t algae at all. Cyanobacteria, sometimes called blue-green algae, were historically grouped with algae because they photosynthesize using pigments similar to those in plants and true algae. But closer analysis of their cell structure revealed they’re bacteria. Unlike algae, they lack a nucleus and other internal compartments that define more complex cells. Despite the reclassification, they still function like algae in ecosystems, producing oxygen and forming the base of aquatic food webs. A single genus of cyanobacteria, Prochlorococcus, produces up to 20% of all the oxygen in Earth’s biosphere.
The Major Types of Phytoplankton
The two dominant classes of phytoplankton are diatoms and dinoflagellates. Diatoms have rigid shells made of interlocking silica parts, almost like microscopic glass boxes. Dinoflagellates use whip-like tails called flagella to move through the water and are covered with complex shells of their own. Surveys of freshwater lakes have identified over 80 genera of phytoplankton spanning seven different phyla, which gives a sense of how diverse this group really is.
Other notable groups include coccolithophores, tiny organisms armored in calcium carbonate plates, and the green algae that dominate many freshwater systems. Each group occupies a slightly different ecological niche based on water temperature, nutrient availability, and light conditions.
Why Phytoplankton Matter More Than Their Size Suggests
Phytoplankton are invisible individually, but collectively they drive some of the planet’s most important cycles. Roughly half of all the oxygen produced on Earth comes from the ocean, and the majority of that production is from plankton, including drifting algae and photosynthetic bacteria. Carbon fixation by marine phytoplankton also accounts for about half of Earth’s total primary production. Between 5 and 10 gigatons of carbon are exported to the deep ocean each year through what’s known as the biological pump, essentially phytoplankton absorbing carbon dioxide at the surface, dying, and sinking.
In food webs, phytoplankton sit at the very bottom. Herbivorous zooplankton graze on them. Small fish eat the zooplankton. Larger predators eat the fish. Krill, herring, and scallops all depend on phytoplankton directly as primary consumers, making these microscopic organisms the foundation that supports whales, seabirds, and commercial fisheries alike.
When Phytoplankton Become Dangerous
Not all phytoplankton blooms are harmless. Under the right conditions, certain species multiply explosively into harmful algal blooms. These blooms can produce potent toxins that accumulate in shellfish and fish, then sicken or kill animals and humans who eat them.
- Diatom toxins: The diatom Pseudo-nitzschia produces domoic acid, which can cause seizures, permanent short-term memory loss, and death when consumed at high levels through contaminated seafood.
- Dinoflagellate toxins: Alexandrium species produce saxitoxins responsible for paralytic shellfish poisoning. Karenia brevis releases brevetoxins that cause respiratory problems, especially dangerous for people with asthma.
- Cyanobacterial toxins: Freshwater blooms dominated by Microcystis produce microcystin, a liver toxin linked to gastrointestinal illness, liver damage, and potentially liver cancer with prolonged exposure.
These blooms are sometimes visible as discolored water, red or brown in saltwater (the classic “red tide”) and blue-green in freshwater lakes and reservoirs.
Phytoplankton as a Nutritional Supplement
Some microalgae species are now cultivated for human consumption. The genus Nannochloropsis, already widely used in aquaculture, is rich in omega-3 fatty acids (particularly EPA), carotenoids, polyphenols, and vitamins. It’s marketed as a marine phytoplankton supplement and proposed as a plant-based source of the same omega-3s typically obtained from fish oil. Spirulina and chlorella, both microalgae, have been in the supplement market longer, though spirulina is technically a cyanobacterium rather than a true alga.