Phytoplankton ( / ˌ f aɪ t oʊ ˈ p l æ ŋ k t ə n / ) are the self-feeding parts of the plankton group and an important part of ocean and freshwater ecosystems. The name comes from Ancient Greek words: φυτόν (phutón), meaning "plant," and πλαγκτός (planktós), meaning "drifter, wanderer, roamer," which together mean "plant drifter."
Phytoplankton get their energy through photosynthesis, just like trees and other plants on land. This means they need sunlight, so they live in the sunlit surface layers (euphotic zone) of oceans and lakes. Compared to land plants, phytoplankton are found over a much larger area, change less with the seasons, and grow and replace themselves much faster (days instead of decades). Because of this, phytoplankton quickly respond to changes in the climate worldwide.
Phytoplankton are the base of food webs in the ocean and freshwater environments and play a major role in the global carbon cycle. They are responsible for about half of all photosynthesis on Earth and at least half of the oxygen produced, even though they make up only about 1% of the planet’s plant life.
Phytoplankton are very different from one another and include photosynthesizing bacteria (cyanobacteria) and many types of single-celled protists (such as diatoms). Most phytoplankton are too small to be seen without a microscope. However, when they are in large numbers, some types can create colored patches on the water’s surface because of the chlorophyll in their cells and other pigments like phycobiliproteins or xanthophylls.
Types
Phytoplankton are tiny, microscopic organisms that use sunlight to make their own food. They live in the top layer of oceans and freshwater, where sunlight can reach. Like plants on land, phytoplankton take in carbon dioxide from water and create organic compounds. They form the base of the aquatic food web and play a key role in Earth's carbon cycle.
Phytoplankton include photosynthesizing bacteria called cyanobacteria and single-celled protists, such as diatoms. Some organisms once classified as phytoplankton, like coccolithophores and dinoflagellates, are no longer grouped in this category. These organisms can both make their own food and consume other organisms, so they are now called mixoplankton. This change affects how scientists understand the structure and function of planktonic food webs.
Ecology
Phytoplankton get their energy from photosynthesis, which means they need to live in the top layer of the ocean, sea, lake, or other water bodies where there is enough sunlight. This top layer is called the euphotic zone. Phytoplankton are responsible for about half of all photosynthesis on Earth. Their ability to convert sunlight into energy stored in carbon compounds is the foundation for most ocean and freshwater food webs, except in cases where organisms use chemical energy instead of sunlight (chemosynthesis).
Most phytoplankton are photoautotrophs, meaning they rely on sunlight for energy. However, some species can use both sunlight and other food sources (mixotrophic), and a few are heterotrophic, meaning they eat other organisms or detritus. These heterotrophic species are often classified as zooplankton. Well-known examples include certain types of dinoflagellates, such as Noctiluca and Dinophysis, which take in organic carbon by consuming other organisms or detritus.
Phytoplankton live in the photic zone of the ocean, where sunlight can reach them. During photosynthesis, they take in carbon dioxide and release oxygen. If sunlight is too strong, phytoplankton may be damaged by a process called photodegradation. Different species of phytoplankton have various pigments that allow them to absorb different colors of light underwater. This means they can use different wavelengths of light efficiently. Light is not just one resource but many, depending on its color. Changes in the color of light can affect phytoplankton communities even if the total light intensity remains the same.
For growth, phytoplankton also need nutrients that enter the ocean through rivers, weathering of land, and melting ice at the poles. They release dissolved organic carbon into the ocean. Since phytoplankton form the base of marine food webs, they are eaten by zooplankton, fish larvae, and other organisms. They can also be broken down by bacteria or viruses. While some phytoplankton, like dinoflagellates, can move up and down in the water column, they cannot swim against ocean currents. As a result, they slowly sink and eventually provide nutrients to the seafloor.
Phytoplankton need several nutrients to survive. These include macronutrients like nitrate, phosphate, and silicic acid, which are needed in large amounts. The availability of these nutrients in the ocean depends on the balance between the biological pump (which moves nutrients from the surface to the deep ocean) and upwelling (when deep, nutrient-rich water rises to the surface). The way phytoplankton use nutrients is connected to the Redfield ratio, which describes the balance of nutrients in the ocean. They also need trace metals like iron, manganese, zinc, cobalt, cadmium, and copper. A lack of these metals can change phytoplankton communities. In areas like the Southern Ocean, phytoplankton often struggle due to low iron levels. Some scientists suggest adding iron to the ocean to help phytoplankton grow and absorb carbon dioxide from the atmosphere. However, experiments on this idea are still debated because of concerns about changing ecosystems.
Phytoplankton also need B vitamins to survive. Some areas of the ocean lack enough B vitamins, which affects phytoplankton growth.
Changes in global temperatures caused by humans are being studied for their effects on phytoplankton. Warmer temperatures can alter ocean layers, the speed of biological processes, and how nutrients reach the surface. These changes may impact future phytoplankton growth.
Ocean acidification, caused by increased carbon dioxide, also affects phytoplankton. For example, coccolithophores, a type of phytoplankton, have shells made of calcium carbonate that are sensitive to acidification. Some phytoplankton may adapt quickly to changes in ocean pH over months or years.
Phytoplankton are the foundation of aquatic food webs and support all aquatic life. Future changes in phytoplankton populations due to warming and acidification may affect how much they are eaten by zooplankton, which could change food chains. One example is the chain where phytoplankton feed krill, which in turn feed baleen whales.
The El Niño-Southern Oscillation (ENSO), a climate pattern in the Pacific Ocean, can influence phytoplankton. During ENSO events, changes in ocean chemistry and temperature can alter phytoplankton communities. For example, during El Niño phases, phytoplankton biomass and density may decrease. Because phytoplankton are sensitive to environmental changes, they are often used as indicators of ocean health. Scientists use satellite images to monitor changes in phytoplankton distribution globally.
Diversity
The term phytoplankton refers to all microorganisms in aquatic environments that use sunlight to produce their own food. Unlike on land, where most plants are the main producers, phytoplankton include a wide variety of organisms, such as protists, bacteria, and other single-celled life forms. Scientists have identified about 5,000 species of marine phytoplankton. It is not clear how this diversity developed, even though resources in the ocean are often limited.
The most important groups of phytoplankton include diatoms, cyanobacteria, and dinoflagellates, though many other types of algae are also present. One group, called coccolithophorids, releases large amounts of dimethyl sulfide (DMS) into the atmosphere. DMS is converted into sulfate, which can help form cloud condensation nuclei. In areas with few airborne particles, this process may increase cloud cover and cloud brightness, as explained by the CLAW hypothesis.
Different types of phytoplankton support different parts of food chains in various ecosystems. In areas with few nutrients, such as the Sargasso Sea or the South Pacific Gyre, small plankton cells called picoplankton and nanoplankton (also known as picoflagellates and nanoflagellates) dominate. These are mostly cyanobacteria, like Prochlorococcus and Synechococcus, and tiny eukaryotes such as Micromonas. In more nutrient-rich areas, where upwelling or land runoff adds more resources, larger dinoflagellates are more common and make up a greater share of the plankton population.
Growth strategies
In the early 1900s, Alfred C. Redfield discovered that the chemical makeup of phytoplankton closely matches the major dissolved nutrients found in deep ocean water. He suggested that the ratio of carbon to nitrogen to phosphorus (106:16:1) in the ocean is determined by the needs of phytoplankton. As phytoplankton break down, they release nitrogen and phosphorus into the water. This ratio, called the "Redfield ratio," is important for understanding marine ecosystems, chemical processes in the ocean, and the development of phytoplankton. However, this ratio is not always the same everywhere in the ocean. It can change due to factors like the amount of nutrients coming from outside the ocean and the activity of microbes, such as nitrogen fixation, denitrification, and anammox.
The changing ratios in single-celled algae show their ability to store nutrients inside their cells, switch between different enzymes that require different nutrients, and change the types of molecules they use to manage water balance. Different parts of a cell have different ratios of nutrients. For example, parts of the cell that help gather resources like light or nutrients, such as proteins and chlorophyll, have high nitrogen levels but low phosphorus levels. In contrast, parts of the cell involved in growth, like ribosomal RNA, have high levels of both nitrogen and phosphorus.
Based on how they use resources, phytoplankton are divided into three growth types: survivalist, bloomer, and generalist. Survivalist phytoplankton have a high nitrogen-to-phosphorus ratio (>30) and have many tools to collect resources, which helps them grow even when resources are scarce. Bloomer phytoplankton have a low nitrogen-to-phosphorus ratio (<10), have many parts for growth, and are adapted to grow rapidly. Generalist phytoplankton have a ratio similar to the Redfield ratio and have balanced tools for both collecting resources and growing.
Factors affecting abundance
The NAAMES study was a five-year scientific research program carried out from 2015 to 2019 by scientists from Oregon State University and NASA. It studied how phytoplankton, tiny ocean plants, change in ocean ecosystems and how these changes affect tiny particles in the air, clouds, and Earth’s climate. NAAMES stands for the North Atlantic Aerosols and Marine Ecosystems Study. The study focused on the sub-arctic part of the North Atlantic Ocean, where one of Earth’s largest repeated phytoplankton blooms happens. This area was chosen because scientists have studied it for a long time, and it is easier to reach than other locations, making it a good place to test existing scientific ideas about how phytoplankton affects Earth’s energy balance.
The NAAMES project studied specific times in the yearly cycle of phytoplankton: when their numbers are lowest, when they are highest, and during the times when their numbers are rising or falling. This helped scientists better understand when and how phytoplankton blooms form and repeat each year. The project also studied the amount, size, and types of tiny particles in the air created by phytoplankton to learn how their growth patterns influence cloud formation and Earth’s climate.
Factors affecting productivity
Phytoplankton play a major role in moving carbon from the ocean surface to the deep ocean. To predict future levels of carbon dioxide in the atmosphere, scientists must understand how phytoplankton respond to changes in their environment. The main factors that affect phytoplankton include temperature, sunlight, the amount of nutrients in the water, and the concentration of carbon dioxide. The balance of nutrients in phytoplankton is very important for other sea life, such as copepods, fish, and shrimp, because it affects how nutritious they are and how energy moves through ocean food chains. Climate change could change the types of phytoplankton in the ocean, which might cause big changes in ocean food webs and affect how much carbon is moved to the deep ocean.
The figure shows the main environmental factors that influence how much phytoplankton grow. All of these factors are expected to change in the future ocean due to global changes. Scientists predict that ocean temperatures will rise, and the layers of the ocean will become more separated, which could change ocean currents and increase sunlight reaching the ocean surface. At the same time, less nutrients will be available in the ocean, along with increased ocean acidification and warming, because the mixing of deep water nutrients to the surface will decrease.
Role of phytoplankton
Phytoplankton affect several areas, including the gas in the air, inorganic nutrients, movement of tiny elements, and the transfer and recycling of organic material through biological processes (see figure). Carbon fixed through photosynthesis is quickly reused in the upper ocean. At the same time, some of this carbon is carried as sinking particles to the deep sea, where it undergoes further changes, such as being broken down into simpler forms.
Phytoplankton support the marine food web and the microbial loop. They form the base of the marine food web because they do not depend on other organisms for food, placing them at the first level of the food chain. Zooplankton and other organisms eat phytoplankton, which are then eaten by other animals, continuing up to the fourth level, where apex predators live. About 90% of the carbon is lost between each level of the food chain due to respiration, dead material, and dissolved organic matter. This highlights the role of phytoplankton and bacteria in breaking down materials and recycling nutrients to maintain efficiency.
Phytoplankton blooms occur when a species grows quickly under favorable conditions. These blooms can sometimes lead to harmful algal blooms (HABs).
Aquaculture
Phytoplankton are an important food source in both aquaculture and mariculture. These farming methods use phytoplankton to feed the animals being raised. In mariculture, phytoplankton naturally occur in the ocean and enter enclosures through seawater movement. In aquaculture, phytoplankton must be collected or grown and then added directly. Phytoplankton can be gathered from water or cultured, but gathering is rarely used. Phytoplankton is used as food for rotifers, which are then used to feed other animals. It is also used to feed many types of farmed mollusks, such as pearl oysters and giant clams. A 2018 study used satellite data to measure the nutritional value of natural phytoplankton worldwide, finding that its energy content varies by region and season.
Growing phytoplankton in controlled environments is a type of aquaculture. Phytoplankton is cultured for purposes such as food for other farmed animals and as a supplement for invertebrates in aquariums. Culture sizes range from small laboratory setups with less than one liter to large commercial systems with thousands of liters. For efficient growth, cultures need specific conditions. Most cultured plankton is marine, and seawater with a specific gravity of 1.010 to 1.026 is often used. This water must be sterilized, usually by high heat or ultraviolet light, to prevent contamination. Fertilizers are added to the culture to help plankton grow. The culture must be aerated or stirred to keep plankton suspended and provide carbon dioxide for photosynthesis. Most cultures are also mixed by hand regularly. Light is needed for phytoplankton growth, with a color temperature of about 6,500 K being ideal, though other temperatures between 4,000 K and 20,000 K or higher can also work. Phytoplankton typically need about 16 hours of light daily for the most efficient growth.
Anthropogenic changes
Marine phytoplankton take in about half of the carbon dioxide used in global photosynthesis (about 50 billion tons of carbon each year) and produce about half of the oxygen on Earth, even though they make up only about 1% of all plant life. Compared to land plants, phytoplankton live in a much larger area, experience less change with the seasons, and grow and die much faster than trees (days instead of decades). Because of this, phytoplankton respond quickly to changes in the climate. These traits are important when studying how much carbon they help remove from the atmosphere and how this might change with environmental changes. Predicting how climate change affects their productivity is difficult because their growth patterns depend on both bottom-up factors, such as the availability of nutrients and how water mixes, and top-down factors, such as being eaten by other organisms or affected by viruses. More sunlight, higher temperatures, and more freshwater entering the ocean can make ocean layers more separated, which limits the movement of nutrients from deep water to the surface, reducing their productivity. However, higher levels of carbon dioxide can increase their food production, but only if nutrients are not limited.
Some studies suggest that the number of phytoplankton in the ocean has decreased over the last century. However, these findings are debated because long-term data about phytoplankton is limited, methods for collecting data vary, and phytoplankton production naturally changes a lot each year and over decades. Other studies show that overall oceanic phytoplankton production may be increasing, with changes seen in certain regions or types of phytoplankton. The amount of sea ice is decreasing, which allows more sunlight to reach the ocean, possibly increasing food production. However, predictions about how changes in water mixing and nutrient availability will affect productivity in polar regions are unclear.
The impact of human-caused climate change on the variety of phytoplankton species is not well understood. If greenhouse gas emissions continue to rise to very high levels by 2100, some models predict that the number of different phytoplankton species in an area may increase due to warmer ocean temperatures. In addition to changes in species variety, phytoplankton are expected to move toward the Earth's poles. This shift could disrupt ecosystems because phytoplankton are eaten by zooplankton, which support fish populations. This movement might also reduce their ability to store carbon from human activities. Changes to phytoplankton caused by humans affect both natural processes and economic activities.