Phytoplankton

Key Takeaways

Phytoplankton are mostly single-celled, photosynthetic organisms living near the surface in the sunlit euphotic zone — the top 200 to 300 meters — where they take up carbon dioxide, make carbohydrates, and release oxygen.
They generate roughly half of the atmosphere’s oxygen, as much each year as all land plants combined, and form the base of virtually every ocean food web.
Two groups dominate — larger algae like diatoms and far more abundant photosynthetic bacteria numbering hundreds of thousands per tablespoon of seawater — and together they help carry atmospheric carbon into the deep ocean when they die and sink.

Microscopic phytoplankton drifting in the sunlit ocean produce about half the oxygen on Earth and anchor nearly every marine food web. By turning carbon dioxide into living tissue, these tiny organisms also help drive the planet’s carbon cycle — which is why WHOI scientists study them so closely.

What are Phytoplankton?

Phytoplankton are mostly microscopic, single-celled photosynthetic organisms that live suspended in water. Like land plants, they take up carbon dioxide, make carbohydrates using light energy, and release oxygen. They are what is known as primary producers of the ocean—the organisms that form the base of the food chain.

Because they need light, phytoplankton live near the surface, where enough sunlight can penetrate to power photosynthesis. The thickness of this layer of the ocean—the euphotic zone—varies depending on water clarity, but is at most limited to the top 200 to 300 meters (600 to 900 feet), out of an average ocean depth of 4,000 meters (13,000 feet).

Phytoplankton comprise two very different kinds of organisms. The larger category include, single-celled algae known as protists—advanced eukaryotic cells, similar to protozoans. These forms include diatoms and are most abundant near coasts. Occasionally, these organisms form blooms—rapid population explosions—in response to changing seasons and the availability of nutrients such as nitrogen, iron, and phosphorus.

The other type of phytoplankton cells, more primitive but far more abundant than algae, is photosynthetic bacteria. These tiny cells, some only a micron across, are invisible but present in numbers of hundreds of thousands of cells per tablespoon of ocean water. Too small to be caught in any net, these organisms were unknown until the 1970s, when improved technology made them visible. Scientists now know these bacteria are responsible for half of the ocean's primary productivity and are the most abundant organisms in the sea. The group also includes cyanobacteria, which are believed to be among the oldest organisms on Earth and the origin of the photosynthetic organelles in plant cells known as chloroplasts.

Did you know?

Scientists usually divide plankton into three groups that align with major divisions of life.

Learn more

VIDEO [All videos about Phytoplankton]

What fuels New England’s fisheries?

New England's famous fisheries rely on tiny but mighty plankton. That's why scientists keep a close eye on the offshore food web that fuels the region's marine economy. Join WHOI biologist Heidi Sosik on a research cruise to the continental shelf off of Massachusetts.

Why are they important?

Phytoplankton are some of Earth's most critical organisms and so it is vital study and understand them. They generate about half the atmosphere's oxygen, as much per year as all land plants. Phytoplankton also form the base of virtually every ocean food web. In short, they make most other ocean life possible.

Through photosynthesis these organisms transform inorganic carbon in the atmosphere and in seawater into organic compounds, making them an essential part of Earth's carbon cycle. Because they take up carbon dioxide from the atmosphere, when they die they sink they carry this atmospheric carbon to the deep sea, making phytoplankton an important actor in the climate system. Phytoplankton growth is often limited by the scarcity of iron in the ocean. As a result, many people are discussing plans to fertilize large areas of the ocean with iron to promote phytoplankton blooms that would transfer more carbon from the atmosphere to the deep sea.

Phytoplankton are critical to other ocean biogeochemical cycles, as well. They take up, transform, and recycle elements needed by other organisms, and help cycle elements between species in the ocean. Photosynthetic bacteria are especially important in the nutrient-poor open ocean, where they scavenge and release scarce vitamins and other micronutrients that help sustain other marine life.

Some phytoplankton have a direct impact humans and other animals. Dense blooms of some organisms can deplete oxygen in coastal waters, causing fish and shellfish to suffocate. Other species produce toxins that cause can cause illness or death among humans and even whales that are either exposed to the toxins or eat shellfish that accumulate toxins. Such harmful algal blooms (HABs) cause significant economic loss every year in the seafood industry and in tourist communities, and scientists are working to understand the causes of these blooms and to devise ways to predict and prevent them.

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Arctic ecosystem

This abundant ecosystem supports large predators like walruses, polar bears, and whales.

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Isochrysis algae to biofuel and jet fuel

Researchers discovered that the algae Isochrysis can produce both biodiesel and jet fuel by utilizing its unique fats, despite its dark, sludgy oil at room temp.

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Jet Fuel from Algae?

Scientists have explored a way to make two types of fuel—biodiesel and jet fuel from different compounds in a single type of algae.

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Plankton blooms at ocean fronts

Plankton thrive where water masses meet—rising lighter water brings nutrients to the surface, fueling blooms in sunlit layers of the ocean.

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Ocean eddies transporting plankton throughout the water column

Plankton help reduce atmospheric carbon dioxide by using it to grow. Larger particles sink to store carbon, while scientists study how smaller particles might also descend.

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Different stages of photosynthesis

Phytoplankton photosynthesize in chloroplasts, using sunlight and CO? to make sugars and oxygen. Xanthophylls protect chlorophyll by adjusting light energy and burning excess as heat.

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How biological compounds from phytoplankton blooms enter the atmosphere

Biological compounds from phytoplankton blooms could end up trapped in centuries-old ice cores that scientists can use to measure ocean productivity over time.

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How Trichodesmium are essential cogs in the ecological machinery of the ocean

Trichodesmium are essential cogs in the ecological machinery that cycles chemicals through the oceanschemicals that sustain life and influence global climate.

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Carbon drawn into the ocean by plankton sinks and dissipates with depth

Only a small fraction of the carbon drawn into the ocean by plankton blooms makes it into the depths where it no longer can be exchanged with the atmosphere.

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Ciguatera Fish Poisoning cycle

Ciquatera fish poisoning, caused by toxins made by phytoplankton, can cause several gastrointestinal and neurological symptoms in people.

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Arctic ecosystem

This abundant ecosystem supports large predators like walruses, polar bears, and whales.

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Arctic ecosystem, including humans

Arctic animals, including humans, survive on the ocean's plentiful resources.

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Forecasting Where Ocean Life Thrives

Plankton thrive at ocean fronts, where lighter Atlantic water meets denser Mediterranean water, driving nutrient-rich mixing that fuels surface growth.

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Let the Sunshine In

Phytoplankton use chloroplasts to photosynthesize, adapting quickly to shifting light in the ocean to fuel growth with sunlight, carbon dioxide, and nutrients.

References

Imaging FlowCytobot

Real-time observation of phytoplankton.

Video Plankton Recorder
An underwater video microscope system that that takes images of plankton and particulate matter.

Ocean Iron Fertilization
October 19, 2007 “Give me half a tanker of iron, and I’ll give you an ice age” may rank as the catchiest line ever uttered by a biogeochemist. The man responsible was the late John Martin, former director of the Moss Landing Marine Laboratory, who discovered that sprinkling iron dust in the right ocean waters could trigger plankton blooms the size of a small city. In turn, the billions of cells produced might absorb enough heat-trapping carbon dioxide to cool the Earth’s warming atmosphere.

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