--- title: "Are Phytoplankton Algae? Plankton vs Algae Explained" url: https://thesea.org/phytoplankton-vs-algae/ date: 2026-11-13 modified: 2026-11-13 lang: en author: "Blane Perun" description: "TheSea.Org▸Bioluminescence & Plankton▸Plankton▸Phytoplankton vs Algae Are Phytoplankton Algae? — Plankton vs Algae Explained Bioluminescence & Plankton By Blane Perun November 13, 2026 9 min read Are phytoplankton algae? Mostly, yes...." categories: - "Bioluminescence & Plankton" word_count: 1909 --- # Are Phytoplankton Algae? Plankton vs Algae Explained [TheSea.Org](/)▸[Bioluminescence & Plankton](/bioluminescence/)▸[Plankton](/plankton/)▸Phytoplankton vs Algae # Are Phytoplankton Algae? — Plankton vs Algae Explained Bioluminescence & Plankton By [Blane Perun](/about-blane-perun/) November 13, 2026 9 min read Are phytoplankton algae? **Mostly, yes.** Phytoplankton are the drifting, photosynthetic organisms of the plankton, and most of them are **microscopic algae**: diatoms, dinoflagellates, coccolithophores and green algae. The big exception is **cyanobacteria**, which photosynthesise but are bacteria, not algae. And the reverse is not true: **seaweeds are algae but not plankton**, because they are anchored rather than drifting. Quick facts Phytoplankton means“Drifting plants” (Greek) Algae meansPhotosynthetic organisms that are not true plants Algal phytoplanktonDiatoms, dinoflagellates, coccolithophores, green algae Non-algal phytoplanktonCyanobacteria (e.g. Prochlorococcus, Trichodesmium) Algae that are not planktonKelp, sea lettuce, coralline algae, hair algae OxygenRoughly half of Earth’s photosynthesis SizeUnder 1 µm to about 1 mm “Plankton” and “algae” get used almost interchangeably, especially when the water turns green, brown or red. They are not the same thing. One word describes how an organism lives, the other describes what kind of organism it is. Once you see that difference, a lot of confusing questions answer themselves: why a diatom is both plankton and algae, why kelp is algae but not plankton, and why the “blue-green algae” that cause lake blooms are not really algae at all. As someone who has kept reef tanks since 1996 and dealt with nuisance algae over them, I find the distinction is more than just vocabulary. It changes how you think about what is growing in the water. Plankton is a lifestyle, algae is a type of organism Plankton is any organism that drifts with the currents and cannot swim against them. Algae is a loose group of photosynthetic organisms that are not land plants. The two categories overlap, but they answer different questions. The word plankton comes from the Greek for “wanderer” or “drifter”. It includes anything that drifts: bacteria, viruses, single-celled algae, tiny crustaceans, fish larvae and even large jellyfish. It is defined by behaviour and ecology, not by family tree. Within plankton, biologists usually separate the **phytoplankton**, which photosynthesise, from the **zooplankton**, which eat other things. Our main [plankton](/plankton/) guide and [zooplankton](/zooplankton/) page go deeper into that split. Algae, on the other hand, is a grouping by type. It covers a huge range of photosynthetic organisms, from single cells smaller than a red blood cell to giant kelp more than 30 metres long. What they share is photosynthesis with chlorophyll a, and no true roots, stems, leaves or vascular tissue. Algae is not a single natural lineage; green, red and brown algae and the many single-celled groups sit on very different branches of the tree of life. It is a useful everyday label rather than a strict scientific one. ![Coccolithophores, diatoms and flagellates under a microscope, the algae that make up phytoplankton](https://thesea.org/wp-content/uploads/phytoplankton-vs-algae-1-thesea-org-by-blane-perun.webp)Coccolithophores, diatoms and flagellates under a microscope, the algae that make up phytoplankton The algae that make up most phytoplankton When an oceanographer talks about phytoplankton, the organisms they mean are mostly microalgae. The main groups: **Diatoms** are single cells, sometimes in chains, encased in a glass shell of silica made of two halves that fit like a pillbox. They dominate cool, nutrient-rich water and spring blooms, and they are responsible for a large share of ocean photosynthesis. In aquariums they are the brown dusting on new glass and sand; see [diatoms in reef tanks](/diatoms-reef-tank/). **Dinoflagellates** have two whip-like flagella and many have armour plates of cellulose. Some photosynthesise, some hunt, and many do both. They include the species behind red tides and the glowing waves of bioluminescent bays. See [bioluminescent plankton](/bioluminescent-plankton/) and [red tide](/red-tide/). **Coccolithophores** cover themselves in tiny plates of calcium carbonate. Their blooms turn huge areas of ocean milky turquoise in satellite images, and their remains built chalk deposits such as the White Cliffs of Dover. **Green algae and other small flagellates**, including prasinophytes and haptophytes, are abundant in open water, many only a few micrometres across. | Organism | Algae? | Plankton? | Why | | -------- | ------ | --------- | --- | | Diatom | Yes | Yes (phytoplankton) | Single-celled alga that drifts | | Dinoflagellate | Yes (most) | Yes | Flagellated alga; some also eat prey | | Coccolithophore | Yes | Yes | Chalk-plated alga that drifts | | Cyanobacteria (Prochlorococcus) | No | Yes (phytoplankton) | Photosynthetic bacterium | | Giant kelp | Yes | No | Attached to the bottom | | Sargassum | Yes | Floats, but not usually called plankton | Large free-floating seaweed (pleuston) | | Hair algae in a reef tank | Yes | No | Grows attached to rock | | Copepod | No | Yes (zooplankton) | Tiny crustacean, an animal | Cyanobacteria: phytoplankton that are not algae The biggest exception is cyanobacteria. They were long called blue-green algae, and the name still appears on lake warning signs. But cyanobacteria are bacteria: their cells lack a nucleus and the other internal compartments that true algae have. In fact, the chloroplasts inside every alga and plant are descended from an ancient cyanobacterium that was engulfed by another cell, which is why the two look so alike in what they do. In the open ocean, cyanobacteria are some of the most important phytoplankton of all. *Prochlorococcus*, a cell well under a micrometre across, is thought to be the most abundant photosynthetic organism on Earth, and its cousin *Synechococcus* is almost as widespread. *Trichodesmium* forms floating rafts in warm seas and can fix nitrogen from the air, fertilising nutrient-poor tropical water. Sailors called its blooms “sea sawdust”. All of these are phytoplankton by lifestyle, but not algae by type. ~50%Rough share of Earth’s photosynthesis done by phytoplankton Under 1 µmSize of Prochlorococcus, a key non-algal phytoplankter 30 m+Giant kelp: algae, but not plankton Algae that are not plankton Going the other way, a great many algae are not plankton because they are fixed in place. Kelp, sea lettuce, red coralline algae that cement coral reefs together, turf algae and the green hair algae that plague aquariums all grow attached to rock, sand or other organisms. Biologists call these macroalgae or benthic algae. Some microalgae are not plankton either: the zooxanthellae living inside coral tissue are dinoflagellates, but they live in the coral, not in the water column. Benthic diatoms carpet sand flats and mud. See [how algae help the coral reef](/how-do-algae-help-the-coral-reef/) for the roles they play on reefs. Floating seaweed such as Sargassum sits awkwardly between the categories. It drifts, but it is large and lives at the surface, so it is usually described as pleuston or a floating habitat rather than plankton. ![Reef tank beside bottles of green phytoplankton culture, plankton vs algae at home](https://thesea.org/wp-content/uploads/phytoplankton-vs-algae-2-thesea-org-by-blane-perun.webp)Reef tank beside bottles of green phytoplankton culture, plankton vs algae at home What I learned about plankton and algae in my reef tanks In my reef systems the difference between algae in the water and algae on the rock was practical, not academic. When I set up the lighting over my multi-tier reef system, I deliberately chose lower-wattage metal halides over the shallow lagoon section, because brighter light at that depth would have meant a constant fight with nuisance algae. Later, when I increased intensity, I had to adjust temperature and other parameters to keep it in check. That nuisance algae was benthic, growing on rock and glass. It is a different problem from the water itself turning green, which is a phytoplankton bloom. That distinction matters for how you respond. Hair algae on rock is about nutrients, light and grazers. A green-water bloom is free-floating cells, and the tools are different. On the other side, many reefers deliberately add live or bottled phytoplankton to feed clams, sponges, feather dusters and the copepods that feed fish. That is the same group of organisms, used as food. Our guide to [phytoplankton for reef tanks](/phytoplankton-reef-tank/) covers which species are sold and who eats them, and [green hair algae in reef tanks](/green-hair-algae-reef-tank/) covers the attached kind. Why the difference matters in the ocean The same split shapes how scientists study the sea. Phytoplankton are counted with water samples, filters, microscopes and satellites that measure ocean colour, because their chlorophyll changes how the sea reflects light. Benthic algae are surveyed by divers with quadrats and photos. Phytoplankton turn over in days, so a bloom can appear and vanish within weeks, while kelp forests and coralline crusts change over seasons and years. Both are at the base of the food web. Phytoplankton feed zooplankton, which feed fish, seabirds and whales; see [what eats plankton](/what-eats-plankton/) and the [ocean food web](/oceans-food-web/). Attached algae feed grazers like urchins, parrotfish and surgeonfish. When either gets out of balance, through nutrient pollution or loss of grazers, the result can be a harmful bloom in open water or seaweed overgrowing a reef. Mixotrophs: where the lines blur further Even the split between phytoplankton and zooplankton is not as clean as textbooks suggest. Many dinoflagellates, and a good number of other small flagellates, are mixotrophs: they photosynthesise when light and nutrients allow, and they also swallow bacteria or other cells when food is available. *Noctiluca scintillans*, the sea sparkle behind many glowing beaches, comes in a green form that carries algal partners inside it and a red form that simply eats. Some ciliates steal working chloroplasts from the algae they eat and use them for a while. So a single drifting cell can be part alga, part predator, depending on the day. For most purposes we still sort them by what they mainly do, but it is worth knowing that nature did not draw these lines for our convenience. See [sea sparkle](/sea-sparkle/) for one of the best-known examples. The same blurring shows up in coral. The zooxanthellae inside reef-building corals are dinoflagellate algae, close relatives of free-living phytoplankton, but they spend their working lives inside animal tissue. When corals bleach, they expel those algae into the water, where for a short time they are drifting cells again. The short version If you remember one thing, make it this: plankton is about where and how something lives, and algae is about what it is. Most phytoplankton are algae, so in casual conversation “plankton algae” is not wrong. But cyanobacteria are phytoplankton that are not algae, and seaweeds are algae that are not plankton. I would add one practical rule from the reef hobby: if the water itself is coloured, you are looking at plankton; if the colour is growing on a surface, you are looking at attached algae. ![Seaweed drifting in green phytoplankton-tinted water, comparing phytoplankton vs algae](https://thesea.org/wp-content/uploads/phytoplankton-vs-algae-3-thesea-org-by-blane-perun.webp)Seaweed drifting in green phytoplankton-tinted water, comparing phytoplankton vs algae Frequently asked questions Are phytoplankton algae? Most are microscopic algae such as diatoms and dinoflagellates. Cyanobacteria are an important exception: they are phytoplankton but bacteria, not algae. Is all algae plankton? No. Seaweeds like kelp and attached algae on rocks are algae but not plankton, because they do not drift. What is the difference between plankton and algae? Plankton describes organisms that drift with currents; algae describes a type of photosynthetic organism. Many organisms are both. Is blue-green algae a phytoplankton? Yes, when it drifts in the water. But blue-green algae are cyanobacteria, which are bacteria rather than true algae. Are diatoms algae or plankton? Both. Diatoms are single-celled algae, and most species live as phytoplankton. Is zooplankton algae? No. Zooplankton are animals and animal-like protists that eat other organisms, including phytoplankton. Sources & further reading - NOAA Ocean Service: What are phytoplankton? - NASA Earth Observatory: What are Phytoplankton? - Falkowski, P.G. (2012). Ocean science: the power of plankton. *Nature* 483: S17–S20. - Partensky, F., Hess, W.R. & Vaulot, D. (1999). *Prochlorococcus*, a marine photosynthetic prokaryote of global significance. *Microbiology and Molecular Biology Reviews* 63(1): 106–127. Continue reading More about plankton [Bioluminescence & PlanktonPlanktonPhytoplankton vs zooplankton. ](/plankton/) [Reef TanksPhytoplankton for Reef TanksWho eats it and how much. ](/phytoplankton-reef-tank/) [Section hubBioluminescence & PlanktonAll glowing and drifting life. ](/bioluminescence/) BP Last updated November 13, 2026 Written by Blane Perun Blane Perun has been exploring Earth’s oceans and marine life for over 25 years. Founder of TheSea.Org in 1999, underwater photographer, coral aquaculture pioneer, and explorer of 80+ countries. [About Blane Perun →](/about-blane-perun/) [More on bioluminescence & plankton →](/bioluminescence/)