What Are Phytoplankton?

Phytoplankton are arguably the most critical species on Planet Earth. A microscopic type of marine algae, they are similar to land plants in that they contain chlorophyll and need sunlight in order to live and grow. Hence they live in the sunlit upper ocean. They were instrumental in helping life to take hold on the planet around 3.8 billion years ago, and they continue to create the oxygen and energy that all species need to survive. We explain all you need to know.
Diatoms phytoplankton
Assorted diatoms, a type of phytoplankton, viewed through a microscope. Photo: © Prof. Gordon T. Taylor. NOAA.

Phytoplankton: The World’s Great ‘Primary Producers’

Phytoplankton are the foundation of the marine food web in every ocean. They are the tiny primary producers (autotrophs) who turn sunlight into energy through photosynthesis and feed the entire marine environment. In the process they help to mitigate climate change and also produce most of the world’s oxygen.

Phytoplankton (also known as micro-algae) are microscopic, single-celled marine organisms that live suspended in water. Like terrestrial plants, they play a vital role in the carbon cycle, through the process of photosynthesis. They create their own energy by taking in carbon dioxide and sunlight, and they release oxygen as a by-product. Indeed, scientists believe that they may contribute 50-80 percent of all the oxygen in Earth’s atmosphere. 1 By absorbing carbon dioxide (some of which they return), phytoplankton prevent it from amplifying the greenhouse effect and therefore play an important role in lessening the effects of global warming around the world.

However, researchers have found that phytoplankton have declined substantially in the world’s oceans over the past century. From 1950 alone, phytoplankton numbers decreased by around 40 percent, probably in response to ocean warming around the world. 2

Phytoplankton originated in the oceans about 2.8 billion years ago, when the planet’s crust had cooled sufficiently to allow life to form, but they’re still incredibly important in the regulation of aquatic food webs, biogeochemical cycles and Earth’s climate system. 3

Phytoplankton bloom in the Barents Sea
Phytoplankton bloom in the Barents Sea, July 2020. The phytoplankton appear as a light blue swirling pattern against the dark blue ocean. The northern part of the Finnoscandian peninsula is in the lower right corner.Photo: © NASA Worldview

Phytoplankton Are Primary Producers

Phytoplankton are the main primary producers of the ocean. A primary producer converts an abiotic (non-living) source of energy (like sunlight) into energy stored in organic compounds (such as carbohydrates), which can then be used by other organisms who feed on biotic material.

Phytoplankton photosynthesize sunlight, carbon dioxide and water into energy. In effect, they turn themselves into a piece of food, and form the base of several food chains in the world’s seas and oceans, providing food for a wide variety of other sea creatures including zooplankton, krill, jellyfish and whales.

In total, phytoplankton account for roughly one percent of the planet’s biomass. 4 5

What’s the difference between Phytoplankton and Zooplankton?
Phytoplankton are drifting plants. They are autotrophs who use photosynthesis to create food energy from sunlight. Zooplankton are drifting animals who eat phytoplankton (and other zooplankton).

What’s the difference between autotrophs, heterotrophs and photoautotrophs?
Autotrophs do not need a living source of food energy. They are able to create food energy in the form of complex organic compounds (like carbohydrates, fats, and proteins) from non-living material, such as water, air, soil, sunlight or minerals. Photoautotrophs are autotrophs who get their energy from sunlight. Phytoplankton, for example, obtain their energy from sunlight and therefore are photoautotrophic. A heterotroph, on the other hand, cannot create energy from non-living material. They need to feed on organic matter. Take krill, for example. These tiny crustaceans are one of the most abundant types of heterotroph. They feed on phytoplankton, and in turn are eaten by other heterotrophs like squid, penguins, seals and whales.

What Do Phytoplankton Need For Growth?

Similar to land plants, phytoplankton rely on the availability of CO2, sunlight, and nutrients for metabolic growth. Like land plants, phytoplankton require inorganic nutrients such as nitrates, phosphates, and sulfur which they turn into proteins, fats, and carbohydrates. They also need tiny amounts of iron, whose scarcity hinders phytoplankton growth in many areas of the ocean. Other factors affecting phytoplankton growth, include water temperature, wind and salt content, as well as what predators they have to deal with.

But when conditions are right, phytoplankton communities can also experience explosive growth over a few weeks, or even days. This phenomenon is known as a bloom. Blooms in the ocean may cover hundreds of square kilometers and can be seen quite clearly on satellite images. A bloom may last several weeks, even though the life span of an individual phytoplankton is rarely more than a few days.

Unfortunately, a massive bloom is usually followed by a serious episode of ocean deoxygenation, as phytoplankton die and attract the attention of decomposers. These decomposers, such as bacteria, break down the bodies of the phytoplankton into basic nutrients, ready for recycling. But in the process, the bacteria use up all the oxygen in the water, suffocating other marine life and creating a dead zone.

What Are The Main Types Of Phytoplankton?

The term “phytoplankton”, from the Greek for “drifting plant” 6, embraces all photosynthesizing organisms in aquatic food webs. In total, there are about 5,000 known species. 4

Phytoplankton fall into two very different types. One consists of single-celled algae, such as diatoms and are most abundant near coasts. One teaspoon of seawater may contain as many as 50,000 tiny algae.

From time to time, the population of these algae explodes in response to excessive levels of nutrients in the water – a state known as eutrophication – caused by nitrogen fertilizer run-off, or excess phosphorus in waste-water run-off.

The huge numbers of algae form massive blooms that are sometimes visible from space. There are trillions of living diatoms, while the shells of dead diatoms can form a pile up to half a mile high on the ocean floor. Nobody knows exactly how many diatoms are out there, but conservative estimates suggest there are an astounding 100,000 to 200,000 different species! 7

The other basic type of phytoplankton is photosynthetic bacteria. These microorganisms are no more than a micron in diameter, but account for half of the ocean’s primary energy production and are the most abundant organisms in the sea – more populous than diatoms. One teaspoon of seawater may contain more than 1 million of these organisms.

This type also includes cyanobacteria, which date back to 3.5 billion years BC, from which chloroplasts – the special compartment inside plant cells where photosynthesis occurs – are thought to have originated. A modern example, called Prochlorococcusis, which was only discovered in 1988, is estimated to be responsible for 20 percent of all oxygen in the atmosphere.

In oceanic regions with low nutritional levels (such as the Sargasso Sea or the heavily polluted South Pacific Gyre), the most populous phytoplankton are small types like cyanobacteria. Within more nutritious marine environments – especially those characterized by thermohaline upwelling which brings large quantities of marine organisms from the ocean bed – larger phytoplankton like dinoflagellates tend to dominate. Dinoflagellates, whose bodies are covered with complex shells, are particularly interesting because they can create energy from both sunlight and from the ingestion of organic prey. 8

How Big Are Phytoplankton?

Phytoplankton are among the tiniest organisms in the sea. They range in size from the femtoplankton (up to 0.2 micron); (Note: 1 micron = one millionth of a meter); picoplankton (0.2-2 micron); nanoplankton (2-20 micron); microplankton (20-200 micron); mesoplankton (0.2-20 mm); all the way up to macroplankton (2-20 cm) and megaplankton (20 cm and up).

What Are Phycobiliproteins?

Because photosynthesis requires sunlight, most phytoplankton live near the surface of the ocean in what is known as the “photic zone” (“sun-lit” zone). The depth of this zone varies according to water transparency but is typically no deeper than 200 meters (660 feet), compared to an average ocean depth of 4,000 metres (13,000 feet).

Even so, it is more difficult for sunlight to penetrate water than air. In particular, yellow and red portions of light can only penetrate to a depth of 100 metres (330 feet), less than half the depth reached by blue and green portions. Normally, this would create a problem, because the chlorophyll in phytoplankton absorbs mostly red, orange and blue light. Happily, nature has created a set of chemical molecules called “phycobiliproteins”, which convert the green portion of sunlight to red which is ideal for chlorophyll. 9 Phycobiliproteins are found inside phytoplankton such as cyanobacteria and certain types of algae. 10

What is the maximum depth for photosynthesis?

It is difficult to set a precise limit on the photic zone for the simple reason that phytoplankton continue to find ways to extend it. Usually, as stated in the article, the maximum limit for photosynthesis – even with the help of phycobiliproteins is about 660 feet below the surface. But a red algae, called Corallinales, manages to photosynthesize at an incredible 886 feet below the ocean’s surface. This creature somehow manages to produce oxygen despite receiving only the tiniest fraction of sunlight.

How Do Phytoplankton Fit Into The Carbon Cycle?

Biological Carbon Pump

As phytoplankton photosynthesize, they consume carbon dioxide from the atmosphere on a scale equivalent to forests and other terrestrial plants. The carbon is incorporated into the body of the phytoplankton, just like carbon is absorbed into the branches, leaves, trunk and bark of a tree. (This transfer of CO2 is known as the “biological carbon pump”.) However, as in plants, much of the CO2 absorbed by phytoplankton is released again during respiration, either by the phytoplankton itself or by the zooplankton or bacteria that feed on phytoplankton or it’s remains. Near-surface respiration puts CO2 back into the water, from where it can return to the atmosphere via the continuous ocean-atmosphere exchange mechanism.

When phytoplankton die, about 30 percent of this carbon is carried down to the deep ocean, while some is dispersed among different layers of the ocean as phytoplankton are consumed by fish and other creatures, which themselves generate waste, and die. Most carbon, however, returns to the near-surface, although it may take tens of thousands of years to arrive. But see also: Marine Microbes Drive the Aquatic Food Web.

NOTE: It’s not clear exactly how much carbon makes it to the ocean floor. For example, compare the article text with this sentence from the American Chemical Society (ACS).  “Like inorganic carbon, most organic carbon ultimately ends up in sediments on the ocean floor.” [Ocean Chemistry. ACS Climate Science Toolkit. Oceans, Ice, and Rocks. American Chemical Society. acs.org/]

Ocean-Atmosphere Exchange

The continuous ocean-atmosphere exchange mechanism is the main pathway in the fast carbon cycle for the movement of CO2 between sea and air. CO2 moves across the air-sea boundary by molecular diffusion according to relative CO2 pressure (pCO2). For example, when the atmospheric pCO2 is higher than the surface ocean, CO2 diffuses across the air-sea boundary into the sea water. Several factors influence this mechanism, chiefly temperature. Generally speaking, at the tropics CO2 moves from ocean to air; while in polar oceans CO2 moves from the air into the ocean.

Solubility Carbon Pump

Once CO2 diffuses from the air into cold polar waters, a significant proportion of it is carried to the bottom in ocean currents powered by thermohaline circulation. This downwelling occurs mostly in the Greenland and Labrador seas in the Arctic, and the Ross and Weddell seas off Antarctica. The process is known as the “solubility carbon pump”. Thanks to the solubility pump, the deep ocean contains around 38,000 billion tonnes of carbon, most of which will remain locked up for thousands of years. In contrast, the surface zone of the ocean contains only 1,000 billion tonnes. 11

What’s the difference between phytoplankton and zooplankton?

Phytoplankton, like diatoms and algae, are regarded as aquatic plants, whereas zooplankton are tiny fish, crustaceans and other aquatic animals. Both types are so small that they drift along with the ocean currents. Phytoplankton make their own food vis photosynthesis; zooplankton eat other organisms like phytoplankton. Phytoplankton give off oxygen when photosynthesizing; zooplankton don’t photosynthesize. Phytoplankton live near the surface of the sea in order to photosynthesize sunlight; zooplankton live in the darker and colder depths.

Why Are Phytoplankton Important?

  • Phytoplankton are the foundation of the marine food chain. All other marine creatures depend on them directly or indirectly for food or oxygen. Krill, for example – arguably the most important link between plankton and larger marine creatures – is heavily dependent upon phytoplankton for its main source of food.
  • Phytoplankton generate somewhere between 50 and 80 percent of the planet’s oxygen. 12
  • Phytoplankton are the main contributor in the biological carbon pump, taking carbon dioxide out of the atmosphere via photosynthesis and carrying it into the ocean depths when they die.
  • Phytoplankton are active in other marine biogeochemical cycles, as well. They absorb, transform, and recycle elements needed by other creatures in the ocean. Photosynthetic bacteria are especially important in the nutrient-poor ocean spaces, where they scavenge and release scarce vitamins and other micronutrients that help sustain other marine life. 13
  • Plankton (including phytoplankton and zooplankton) are the foundation of the oceanic food web and remain highly sensitive to physical and chemical factors in the hydrosphere, including levels of nutrients, salt content, heavy metals and temperature. These factors are affected by both natural and man-made fluctuations in climate and hydrography. Because of their limited lifecycles, plankton communities react rapidly to these fluctuations. As a result, plankton-based indicators have the potential to detect variations in the ocean at an early stage.
  • One group of phytoplankton, known as coccolithophorids, release significant amounts of dimethyl sulfide (DMS) into the atmosphere. DMS reacts with oxygen to form sulfate which, in parts of the atmosphere containing low concentrations of aerosol particles, promotes the formation of clouds leading to a cooling effect due to higher cloud albedo, according to a theory (the CLAW Hypothesis) first propounded by Robert Jay Charlson, James Lovelock, Meinrat Andreae and Stephen G. Warren. The theory suggests that coccolithophorids are responsive to variations in global warming, and that they act to stabilise the temperature of the Earth’s atmosphere. 14

What Is The Microbial Loop?

The microbial loop describes the interconnected life cycles of competing phytoplankton, zooplankton and decomposers (bacteria, fungi, worms and the like) at the base of the marine food web. These organisms create energy from sunlight, eat other organisms, reproduce, die (or are eaten), decay, get demineralized or broken down by decomposers, and get recycled. The organic matter in their bodies, waste and remains, provides the nutritional base for the oceanic food chain, and is an important part of the ‘biological carbon pump’.


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