The biological pump, also called the marine biological carbon pump, is a process that moves carbon from the air and land into the deep ocean and ocean floor sediments. This process is driven by living things, which move carbon to the deep ocean, away from the air and land. The biological pump is part of the larger marine carbon pump, which includes both physical and biological processes. It helps move organic matter made by phytoplankton when they do photosynthesis, as well as calcium carbonate, which forms shells in organisms like plankton and mollusks.

Scientists calculate the biological pump's activity by comparing how much carbon settles to the ocean floor versus how much is released back into the air.

The biological pump is not one single process but the result of several steps that all work together. Overall, it moves about 10.2 billion tons of carbon each year into the deep ocean, totaling about 1,300 billion tons over 127 years. This keeps carbon away from the atmosphere for thousands of years or more. If the ocean didn't have a biological pump, carbon dioxide levels in the air would be about 400 parts per million higher than today.

Overview

The element carbon is essential for life on Earth and plays a key role in the planet's climate. Carbon moves between different parts of Earth, including the land, ice, air, living things, and water. This movement is called the Earth's carbon cycle. It is also closely connected to the movement of other elements and compounds. The ocean is very important in the carbon cycle because it helps control the amount of carbon dioxide (CO₂) in the air. The biological pump is a process that moves organic carbon from the surface of the ocean to the deep ocean, and it is a major part of the ocean's carbon cycle.

The biological pump works by moving organic carbon that is produced in the sunlit surface area of the ocean. This carbon is made by tiny ocean plants called phytoplankton during photosynthesis. These plants use CO₂, nitrogen, phosphorus, and other elements to create sugars, fats, and proteins. Some plankton, like coccolithophores and foraminifera, use calcium and carbonates to form a hard shell made of calcium carbonate.

Once this carbon is stored in the bodies of these organisms, the organisms either stay near the surface and are recycled in the ocean's nutrient cycle or sink to the deep ocean after they die. As they sink, these particles often join together to form larger groups, which helps them sink faster and avoid being eaten or broken down in the water.

When these sinking particles reach the deep ocean, bacteria break them down, turning the organic carbon back into inorganic carbon. This inorganic carbon can be used again by plants in the surface ocean. Some particles that are not broken down are buried in the ocean floor and can stay there for millions of years. This long-term storage of carbon helps reduce the amount of CO₂ in the air.

The biological pump involves the work of living things, physical processes, and gravity to move organic carbon into the deep ocean. This includes the process of turning inorganic carbon into organic matter through photosynthesis, the movement of carbon through food chains, the mixing of water, and the sinking of particles due to gravity. These processes are all part of the biological pump.

The biological pump changes dissolved inorganic carbon (DIC) into organic matter and moves it into the deep ocean as either particles or dissolved material. Phytoplankton use DIC during photosynthesis to create organic matter, which is then released as dissolved organic matter (DOM) or eaten by zooplankton. Larger zooplankton, like copepods, produce waste that can sink or join other organic material to form larger, faster-sinking groups. Some DOM is used by bacteria and released as carbon dioxide, while the rest moves into the deep ocean. Particles and DOM that reach the deep ocean are broken down, returning carbon to the deep ocean's DIC. Only about 1% of the particles that leave the surface ocean reach the seabed and are either used, released as carbon dioxide, or buried in sediment. This stored carbon remains in the sediment for millions of years. These processes remove carbon from the surface ocean and return it to DIC in the deep ocean, keeping a balance between the surface and deep ocean. Over very long periods, ocean currents move deep-ocean DIC back to the atmosphere.

Primary production

The first step in the biological pump is the creation of both organic and inorganic carbon compounds by phytoplankton in the top, sunlit layers of the ocean. Organic compounds, such as sugars, carbohydrates, lipids, and proteins, are made during photosynthesis. This process can be shown as:

CO₂ + H₂O + light → CH₂O + O₂

In addition to carbon, organic matter in phytoplankton contains nitrogen, phosphorus, and small amounts of trace metals. The ratio of carbon to nitrogen and phosphorus varies depending on the location, but the average is close to 106C:16N:1P, known as the Redfield ratio. Trace metals like magnesium, cadmium, iron, calcium, barium, and copper are present in much smaller amounts in phytoplankton compared to carbon, nitrogen, and phosphorus. However, these metals are still needed for certain metabolic processes and can act as limiting nutrients in photosynthesis because they are less common in ocean water.

Oceanic primary production is responsible for about half of all carbon fixation on Earth. Each year, marine phytoplankton fix approximately 50–60 petagrams of carbon, even though they make up less than 1% of Earth's total photosynthetic biomass. Most of this carbon fixation (about 80%) happens in the open ocean, while the rest occurs in highly productive upwelling regions. Although upwelling areas produce 2 to 3 times more fixed carbon per unit area, the open ocean covers more than 90% of Earth's ocean surface and is the larger contributor overall.

Forms of carbon

Phytoplankton supports all life in the ocean by changing inorganic compounds into organic materials. This process creates the base of the marine food web. In the diagram below, arrows show how dissolved organic matter (DOM) is produced and removed. Arrows pointing toward the DOM pool show production, while arrows pointing away show removal. Dashed arrows show the main biological processes that move DOM through the ocean. Because of these processes, the amount of labile DOM (easily broken down) decreases quickly as depth increases, while the refractory DOM (hard to break down) becomes more common as it moves to the deep ocean. DOM is short for dissolved organic matter.

The marine biological pump depends on several key pools, parts, and processes that affect its function. There are four main carbon pools in the ocean.

• Dissolved inorganic carbon (DIC) is the largest pool. It contains about 38,000 Pg C and includes dissolved carbon dioxide (CO₂), bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and carbonic acid (H₂CO₃). The balance between carbonic acid and carbonate determines the pH of seawater. CO₂ dissolves easily in water, and its solubility decreases as temperature increases. During photosynthesis, dissolved CO₂ is used, which lowers the amount of CO₂ in seawater and helps remove CO₂ from the atmosphere. The opposite process, respiration, releases CO₂ back into water, increasing its levels and allowing CO₂ to return to the atmosphere. Organisms like coccolithophores create calcium carbonate, which releases CO₂ into water.
• Dissolved organic carbon (DOC) is the second-largest pool, containing about 662 Pg C. DOC can be classified as refractory (hard to break down), semi-labile (moderately breakable), or labile (easily broken down). The labile part is about 0.2 Pg C, is available for life, and is produced quickly (~15–25 Pg C per year). The refractory part is the largest (~642 Pg C ± 32) but has a very slow turnover rate (0.043 Pg C per year). Refractory DOC may stay in the ocean for over 1,000 years.
• Particulate organic carbon (POC) is about 2.3 Pg C and is smaller compared to DIC and DOC. Despite its size, POC is highly active, with the fastest turnover rate of any organic carbon pool. It is driven by primary production and produces about 50 Pg C per year globally. POC can be divided into living (e.g., phytoplankton, zooplankton, bacteria) and non-living (e.g., detritus) materials. Phytoplankton carbon is especially important because it supports marine primary production and serves as food for larger organisms in the ocean.
• Particulate inorganic carbon (PIC) is the smallest pool, containing about 0.03 Pg C. It exists as calcium carbonate (CaCO₃) in solid form and affects the ocean’s carbonate system and pH. PIC production is estimated at 0.8–1.4 Pg C per year, with about 65% dissolving in the upper ocean and the rest contributing to deep sediments. Coccolithophores and foraminifera are major sources of PIC in the open ocean. PIC is important because it helps move carbon to the deep ocean through the carbonate pump, where PIC sinks and settles in ocean sediments.

Particulate inorganic carbon (PIC) is usually calcium carbonate (CaCO₃) and plays a major role in the ocean’s carbon cycle. This carbon is used as a protective layer by many plankton (coccolithophores, foraminifera) and larger marine animals (mollusk shells). Fish also release calcium carbonate during osmoregulation, and it can form in events called whiting. Although PIC is not directly taken from the atmosphere, it forms from dissolved carbonate in equilibrium with CO₂ and helps remove carbon through long-term storage.

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
Ca²⁺ + 2HCO₃⁻ → CaCO₃ + CO₂ + H₂O

This process fixes a large amount of carbon but uses two units of alkalinity for every unit of carbon stored. The formation and sinking of CaCO₃ creates a difference in alkalinity between the ocean’s surface and deep layers, which raises the pH of surface water. This changes the form of dissolved carbon, increasing the amount of dissolved CO₂ in surface water and raising atmospheric CO₂ levels. When CaCO₃ is buried in sediments, it lowers overall ocean alkalinity, which can increase pH and atmospheric CO₂ levels unless new alkalinity from weathering balances it. Carbon permanently buried on the seafloor becomes part of Earth’s geological record. Calcium carbonate often forms large deposits, such as the White Cliffs of Dover in England, which are made almost entirely of plates from buried coccolithophores.

Oceanic carbon cycle

The marine carbon cycle involves three main processes, called pumps, that move carbon dioxide (CO₂) from the atmosphere into the ocean and distribute it throughout the oceans. These pumps are: (1) the solubility pump, (2) the carbonate pump, and (3) the biological pump. The total amount of carbon active on Earth’s surface for less than 10,000 years is about 40,000 gigatons of carbon (Gt C). A gigaton is one billion tons, which is roughly the weight of 6 million blue whales. About 95% of this carbon (about 38,000 Gt C) is stored in the ocean, mostly as dissolved inorganic carbon. The different forms of dissolved inorganic carbon in the ocean play a major role in controlling the ocean’s acid-base balance.

The biological pump works alongside the solubility pump. The solubility pump moves large amounts of dissolved inorganic carbon (DIC) from the ocean’s surface to its deeper layers. This process depends only on physical and chemical changes, not on living organisms.

The solubility pump is driven by two factors:
– Carbon dioxide dissolves more easily in colder seawater than in warmer seawater.
– Deep water forms in high-latitude regions where seawater is typically colder and denser, creating ocean currents that move water downward.

Because deep water forms in cold, high-latitude areas where carbon dioxide dissolves more readily, it holds more dissolved inorganic carbon than average surface water. This combination of processes moves carbon from the atmosphere into the ocean’s interior. When this deep water rises to the surface in warmer, equatorial regions, it releases carbon dioxide into the atmosphere because the gas dissolves less easily in warmer water.

The carbonate pump is sometimes called the "hard tissue" part of the biological pump. Some marine organisms, like coccolithophores, create hard structures made of calcium carbonate (a form of particulate inorganic carbon) by using bicarbonate. This process is an important part of the ocean’s carbon cycle. The chemical reaction involved is:
Calcium combines with bicarbonate to form calcium carbonate, releasing carbon dioxide and water.

The biological pump converts inorganic carbon (CO₂) into organic carbon in the form of sugar (C₆H₁₂O₆), while the carbonate pump converts inorganic bicarbonate into calcium carbonate, which releases carbon dioxide. In this way, the carbonate pump works in the opposite direction of the biological pump, reducing the amount of carbon dioxide taken up by the biological pump.

The continental shelf pump operates in shallow waters near the edges of continents, moving carbon (dissolved or in solid form) from coastal areas to the deep ocean. This process occurs where the solubility pump interacts with colder, denser water from the continental shelf, which flows down the slope into the deep ocean. The shallow depth of continental shelves limits the mixing of cooling water, allowing continental shelf waters to cool more than open ocean waters. This cooling increases the solubility of carbon dioxide, leading to more dissolved inorganic carbon being stored in these waters. The higher biological activity on continental shelves also adds more carbon. These dense, carbon-rich waters then sink to the ocean floor and mix into the deep ocean. As global sea levels rise due to climate change, the area of continental shelves will increase, making the shelf pump stronger.

Processes in the biological pump

In the diagram on the right, phytoplankton take in carbon dioxide (CO₂) that has dissolved from the air into the top layer of the ocean (90 gigatons per year). They use this CO₂ to create particulate organic carbon (POC) during photosynthesis (~50 gigatons of carbon per year). These tiny plants are then eaten by copepods, krill, and other small zooplankton, which are in turn eaten by larger animals. Any phytoplankton not eaten form clumps, and along with waste from zooplankton, these clumps sink quickly and leave the top layer of the ocean (<12 gigatons of carbon per year). Krill, copepods, zooplankton, and microbes in the surface ocean and deeper waters eat phytoplankton and sinking particles, turning POC back into CO₂ (dissolved inorganic carbon, DIC). This means only a small amount of carbon from the surface reaches the deep ocean (below 1000 meters). When krill and small zooplankton eat, they break particles into smaller, slower-sinking pieces, which reduces how much POC reaches the deep ocean. This process releases dissolved organic carbon (DOC) directly from cells or indirectly through bacteria (yellow circle around DOC). Bacteria then turn DOC back into DIC (CO₂) through a process called microbial gardening.

The biological carbon pump is a major factor in how carbon is spread vertically in the ocean and affects the level of CO₂ in the ocean’s surface, which influences the exchange of CO₂ between the ocean and the atmosphere. It includes phytoplankton, the animals that eat them, and bacteria that process their waste. This system moves carbon from the atmosphere to the deep ocean, where it is stored for centuries. Photosynthesis by phytoplankton lowers CO₂ levels in the upper ocean, creating a stronger gradient that helps the ocean absorb more CO₂ from the air. This process also creates POC in the sunlit layer of the ocean (0–200 meters deep). POC is broken down by microbes, zooplankton, and their predators into fecal pellets, organic clumps ("marine snow"), and other forms, which then sink or are carried deeper by zooplankton and fish. While both dissolved organic carbon (DOC) and POC are produced during primary production, only POC leads to effective carbon export to the deep ocean. Most DOC is recycled by bacteria, but some DOC in the sunlit layer is more resistant to breakdown and accumulates as semi-labile DOC. This semi-labile DOC eventually sinks to the deep ocean, contributing to the biological carbon pump. The efficiency of this process varies by region, being stronger in subtropical oceans. Overall, the biological carbon pump’s effectiveness depends mostly on how much POC is exported.

Most carbon in organic and inorganic materials forms at the ocean’s surface and begins sinking to the ocean floor. The deep ocean receives nutrients from the upper layers in the form of "marine snow," which includes dead organisms, fecal matter, sand, and other materials. A single phytoplankton cell sinks about one meter per day. Since the average ocean depth is about 4,000 meters, it can take over 10 years for these cells to reach the ocean floor. However, when phytoplankton clump together or are packed into predator waste, they form larger clumps called marine snow, which sink much faster—reaching the deep ocean in days.

In the diagram on the right, phytoplankton use sunlight to convert CO₂ into POC in the sunlit layer of the ocean. This POC is broken down by microbes, zooplankton, and their predators into organic clumps (marine snow), which then sink or are carried deeper by zooplankton and fish. Export flux refers to the movement of carbon out of the surface layer (around 100 meters deep), while sequestration flux refers to the movement of carbon out of the mesopelagic zone (around 1000 meters deep). Some POC is turned back into CO₂ in the deep ocean by microbes and zooplankton, maintaining a vertical difference in CO₂ levels. This deep ocean CO₂ returns to the atmosphere over thousands of years through ocean currents. Between 1% and 40% of the primary production moves out of the sunlit layer, decreasing rapidly as it sinks, with only about 1% reaching the ocean floor.

Out of the 50–60 petagrams of carbon produced each year, about 10% leaves the ocean’s surface layer, and less than 0.5% reaches the ocean floor. Most carbon is reused in the sunlit layer, and a large portion is broken down during the sinking process. Carbon that leaves the surface layer is sometimes considered "sequestered," meaning it is removed from the atmosphere for centuries. However, in areas like the Southern Ocean, much of this carbon can return to the atmosphere within decades.

Calculations of the biological carbon pump rely on the balance between carbon sinking to deeper layers and carbon being released back into the atmosphere. It is estimated that about 25% of the carbon captured by phytoplankton in the surface ocean sinks to deeper layers. Around 20% of this sinking carbon (5% of total surface production) is buried in ocean sediments, often due to the presence of minerals that help particles sink. During the sinking process, these organic particles become centers of microbial activity and play a key role in breaking down organic matter and redistributing nutrients. Observations show that the movement of minerals like calcium carbonate, opal, and rock particles in the deep ocean is closely linked to the movement of organic carbon. Much of the organic matter in the ocean exists as marine snow aggregates (>0.5 mm) made of phytoplankton, dead matter, minerals, and fecal pellets. These aggregates drive the biological carbon pump by moving organic matter from the surface to the deep ocean and sediments. The amount of organic matter that leaves the surface layer depends on how fast these aggregates sink and how quickly microbes break them down. Recent studies show that the movement of minerals and organic carbon is closely related in the deep ocean, leading to the idea that the presence of minerals in sinking particles helps move more organic carbon to the deep ocean.

Minerals like calcium carbonate and opal help about 60% of the particulate organic carbon (POC) in the North Atlantic and 40% in the Southern Ocean sink to deeper layers. Strong links between minerals and POC movement are also seen in the deep ocean, suggesting that minerals help more organic carbon reach the deep ocean. This happens because minerals make organic particles heavier, helping them sink faster.

Quantification

The geological part of the carbon cycle works slowly compared to other parts of the global carbon cycle. It is one of the most important factors that influence how much carbon is in the atmosphere, which affects global temperatures.

The biological pump plays a key role in Earth's carbon cycle, so scientists work hard to measure how strong it is. However, the processes that form the biological pump are hard to study because they involve complex ecological interactions that happen deep in the ocean. One common method to estimate the strength of the biological pump is to measure primary production powered by nutrients like nitrate and ammonium. These nutrients come from different sources linked to the breakdown of sinking material. From these measurements, scientists calculate the f-ratio, which is a way to estimate the local strength of the biological pump. Using findings from local studies to understand the entire ocean is difficult because ocean currents affect different regions in unique ways.

Effects of climate change

Changes in how land is used, burning fossil fuels, and making cement have caused more carbon dioxide (CO₂) to enter the atmosphere. Right now, about one-third (around 2 petagrams of carbon per year) of human-made CO₂ emissions may be entering the ocean, but scientists are not certain. Some research suggests that higher CO₂ levels might affect the ocean’s ability to support life by increasing the growth of plants and algae in the ocean.

Climate change could change how the ocean moves carbon in the future. Warmer temperatures and layers forming in the ocean’s surface might reduce the amount of nutrients reaching the top layer of the ocean, which could lower the growth of plants and algae there. Also, ocean acidification (when the ocean becomes more acidic) might affect the ability of certain sea creatures, like those that make hard shells, to grow strong shells. This could weaken a part of the biological pump that depends on these hard shells. This change might also affect another part of the biological pump that involves soft tissues, because calcium carbonate (a material in hard shells) helps sink organic matter to the deep ocean.

A diagram shows how melting sea ice and thawing permafrost (frozen soil) in the Arctic might affect how carbon moves. On land, plants take in carbon, while tiny organisms in the soil release methane and CO₂. Lakes are major sources of methane, and both organic and inorganic carbon flow from land to the ocean through rivers and streams. In the ocean, methane can be released from frozen soil under the sea, and CO₂ is absorbed because ocean water has less CO₂ than the air above it. Many processes are connected to sea ice. Scientists estimate the amount of carbon moving between the ocean and the air in units called teragrams of carbon per year. Note that the methane estimate for lakes covers a larger area than the estimates for other land areas. Uncertainty ranges are shown in brackets where available. The arrows in the diagram do not show the size of each process.

The biological pump is believed to have helped control CO₂ levels in the atmosphere during past ice ages and warmer periods. However, scientists are not sure how it will respond to future climate changes. To understand this, it is important to study phytoplankton, tiny ocean plants that are a key part of the biological pump. Because phytoplankton come in many different types, they may respond to climate changes in different ways. For example, scientists predict that diatoms (a type of phytoplankton) may become less common in the future because the ocean might become more layered, which could reduce the nutrients they need. Diatoms help move carbon to the deep ocean by forming large, sinking groups, so fewer diatoms could mean less carbon is moved to the deep ocean.

Lower ocean pH (a sign of ocean acidification) might make it harder for coccolithophores (another type of phytoplankton) to build their hard shells, which could affect the biological pump. However, some species of coccolithophores may be more affected than others. Changes in the number of these or other phytoplankton types could change how much carbon the ocean can store, which would affect ocean life and how carbon is stored in the ocean.

A study from 2015 found that coccolithophore levels in the North Atlantic have increased ten times since the 1960s. Scientists think this increase is likely because the ocean has absorbed more CO₂ and temperatures have risen.

In 2017, scientists used computer models to predict where two important phytoplankton species, the diatom Chaetoceros diadema and the coccolithophore Emiliania huxleyi, might live in the future. They used data from a climate scenario called IPCC Representative Concentration Pathway 8.5, which describes how much the Earth’s climate might change by 2100. Their models predicted that the areas covered by these two species might decrease by 8% and 16%, respectively, under this scenario. If these changes happen, the ability of these species to help store carbon through the biological pump might be reduced.

A 2019 study suggested that if ocean acidification continues at current rates, Antarctic phytoplankton might become smaller and less effective at storing carbon by the end of the century.

Monitoring

Tracking the biological pump is important for understanding how the Earth's carbon cycle is changing. Many different methods are used to track the biological pump, and these can be used from different places like ships, autonomous vehicles, and satellites. Right now, satellite remote sensing is the only method that can observe the entire surface ocean over large areas and over time.

Needed research

To fully understand the biological pump in the deep ocean, scientists need to study multiple aspects of the deep water column:

• Physics: The way water layers are arranged (stratification) influences how fast particles sink. To learn where particles come from and how long carbon remains in the deep ocean after particles break down, scientists must measure how water moves (advection) and mixes.
• Biogeochemistry: The movement of organic matter (both solid particles and dissolved substances) from the ocean surface to deeper layers determines how much easily broken-down organic matter reaches the seafloor. This material is either used by life on the seafloor for energy or stored in the sediment for longer periods.
• Biology and ecosystems: Tiny animals (zooplankton) and microbes in the water break down and convert sinking particles into nutrients. Organic matter that reaches the deep ocean supports all life in the water and on the seafloor, including zooplankton, seafloor animals, and microbes, helping maintain their numbers, variety, and overall health.