Phytoplankton are microscopic, plant-like organisms that drift in the sunlit upper layers of the world’s oceans and freshwater bodies. These single-celled algae are foundational to nearly all marine ecosystems, serving as primary producers that convert inorganic substances into food. Like terrestrial plants, they require carbon dioxide (CO2) to live and grow, drawing it directly from the surrounding water. This biological process is globally significant in the cycling of carbon, influencing ocean chemistry and the atmosphere.
Converting Carbon Dioxide into Organic Matter
The fundamental purpose for which phytoplankton use carbon dioxide is to fuel their growth through photosynthesis. This process, often termed the “soft tissue pump,” transforms light energy, water, and inorganic carbon into organic matter like sugars and carbohydrates. The reaction consumes dissolved CO2 and releases oxygen back into the water, accounting for approximately half of all photosynthetic activity on Earth. This organic material constitutes the cell’s biomass, providing the energy and building blocks necessary for cell division and population growth.
In seawater, the primary form of inorganic carbon is bicarbonate (HCO3-) and carbonate (CO32-) ions, not gaseous CO2. To overcome this scarcity, many species have evolved sophisticated Carbon Concentrating Mechanisms (CCMs). These adaptations involve specialized transport proteins that actively pump bicarbonate into the cell. This active transport maintains a high internal concentration of CO2, ensuring a steady supply for the photosynthetic machinery.
The organic carbon fixed forms the base of the marine food web. Zooplankton graze on the phytoplankton, transferring the stored carbon up to larger organisms like fish and whales. This transfer cycles the captured carbon through the ecosystem, supporting life at every trophic level.
Specialized Use for Structural Shell Building
A specialized use of carbon is found in certain groups of phytoplankton, most notably the coccolithophores. These organisms utilize dissolved inorganic carbon to construct intricate external plates composed of calcium carbonate (CaCO3), referred to as coccoliths. This distinct process is called calcification, and it is separate from the photosynthetic fixation of carbon into organic biomass. The coccoliths form a spherical shell, or coccosphere, around the cell, which may serve functions such as protection or assisting with buoyancy.
The formation of these plates occurs inside a specialized compartment, allowing phytoplankton to control the chemical environment for precipitation. The carbon source for this calcification is derived from the pool of dissolved inorganic carbon in the seawater, which is linked to atmospheric CO2. Producing these hard structures represents a cellular energy investment, but offers an evolutionary advantage in environments with high calcium concentrations.
This structural carbon use is often referred to as the “carbonate pump.” Calcification locks carbon into a dense mineral form, contrasting with the soft organic tissue created by photosynthesis. Not all phytoplankton species possess this ability, making it a specialized carbon pathway. The mineralized plates contribute unique mineral matter to the ocean environment and play a role in the ultimate fate of carbon.
The Role in Global Carbon Sequestration
The various ways phytoplankton utilize carbon dioxide collectively drive a massive planetary process known as the Biological Pump. This pump transfers carbon from the surface ocean and atmosphere into the deep ocean, where it can be stored for long periods. The process begins with the photosynthetic fixation of carbon in the sunlit surface layer, or euphotic zone, creating particulate organic carbon (POC) in the form of biomass.
The fate of this fixed carbon determines the pump’s efficiency. While a large portion is recycled immediately in the surface waters through respiration, a fraction sinks downward. This sinking material includes dead phytoplankton cells, aggregates of organic matter called “marine snow,” and dense fecal pellets produced by zooplankton. Coccoliths from calcifying species also contribute to this sinking flux, acting as a ballast that increases the density and sinking speed.
When this carbon-rich material sinks below the top layer, it is effectively removed from contact with the atmosphere. Carbon is considered sequestered when it reaches depths greater than about 500 meters, where it is unlikely to return to the surface for hundreds or thousands of years. This long-term storage in the deep ocean and seafloor sediments is known as carbon sequestration. The Biological Pump continuously draws down atmospheric CO2 into the ocean interior, regulating climate.