The ocean absorbs approximately 30% of human-caused atmospheric carbon dioxide emissions annually. This function relies on both physical processes, like gas dissolving directly into cold surface water, and biological mechanisms driven by marine life. The biological component, often referred to as the biological carbon pump, involves organisms that capture carbon from the surface and transfer it into the deep ocean. This article focuses on the specific marine organisms that absorb and transport carbon, contributing to regulating the planet’s climate.
The Ocean’s Primary Carbon Absorbers
The largest volume of carbon absorption in the ocean is carried out by organisms that photosynthesize, primarily microscopic algae and cyanobacteria collectively known as phytoplankton. These single-celled organisms float in the sunlit surface layer of the ocean, where they convert dissolved carbon dioxide into organic carbon in their soft tissues. This process, known as primary production, forms the base of the marine food web and is the initial step in sequestering carbon from the atmosphere.
The carbon is fixed via the Calvin cycle, where the dissolved inorganic carbon in the water is transformed into carbohydrates, proteins, and fats that make up the cell biomass. Phytoplankton have a rapid turnover rate, meaning they grow and die quickly, but their massive scale of production makes them highly effective absorbers. This conversion of inorganic carbon into organic matter is a significant draw-down of carbon from the surface layer, initiating the flow of carbon into the deep ocean.
Carbon Storage Through Hard Shell Formation
A distinct method of biological carbon storage involves calcification, the process where certain marine organisms build hard shells or skeletons made of calcium carbonate (\(CaCO_3\)). Organisms like coccolithophores, foraminifera, pteropods, and mollusks draw dissolved inorganic carbon compounds from the seawater to construct these rigid structures.
This mechanism differs from photosynthesis because it uses carbonate and bicarbonate ions already dissolved in the water, binding them with calcium ions to form a solid mineral. Although the chemical reaction of calcification can locally release a small amount of carbon dioxide back into the water, the ultimate fate of the shell is what matters for long-term storage. When these calcifying organisms die, their dense shells sink, creating a vertical flux of inorganic carbon that can eventually become incorporated into deep-sea sediments.
The Role of Larger Marine Life in Carbon Export
Once carbon is fixed by surface organisms, larger marine life becomes responsible for actively transporting this organic material to deeper waters. Zooplankton, which are tiny marine animals like copepods and krill, graze heavily on phytoplankton and repackage the soft-tissue carbon into dense, fast-sinking fecal pellets. These pellets are a major component of “marine snow,” the shower of organic detritus that constantly drifts from the surface to the deep ocean.
Larger organisms also contribute to carbon export through active migration and waste. Many zooplankton, and even some fish, participate in the largest daily migration on Earth, swimming up to the surface at night to feed and then descending hundreds of meters during the day to avoid predators. By consuming carbon-rich food near the surface and then respiring or excreting waste at depth, they physically move carbon into the ocean’s twilight zone. A seasonal version of this occurs in colder regions, where zooplankton like copepods migrate to depths exceeding 500 meters to overwinter, injecting carbon into the deep ocean through respiration and death.
Deep Ocean Carbon Sequestration
The final stage of this biological process is the long-term sequestration of carbon in the deep ocean, where it is isolated from the atmosphere for centuries or millennia. The organic and inorganic particles that sink, including soft tissue remnants, fecal pellets, and calcium carbonate shells, fall below the thermocline. The thermocline is the layer of water where temperature rapidly decreases, effectively acting as a barrier that slows the mixing of surface and deep waters.
As the marine snow descends, some organic matter is consumed or broken down by microbes, releasing carbon back into the deep-sea water as dissolved inorganic carbon. However, a portion of the material reaches the abyssal plain and becomes incorporated into the marine sediment. This burial process locks the carbon away, forming geological carbon pools that are removed from the active global carbon cycle.