The ocean regulates Earth’s climate by absorbing atmospheric carbon dioxide. It functions as a major carbon sink, taking up about 25% of human-produced carbon dioxide emissions in recent decades. The continuous exchange of carbon between the atmosphere and the ocean helps to slow the rate of climate change. Its ability to absorb carbon dioxide makes it the second largest carbon reservoir on the planet, holding approximately 50 times more carbon than the atmosphere.
Microscopic Powerhouses: Phytoplankton
Phytoplankton are tiny, plant-like organisms that primarily absorb carbon dioxide. These diverse microscopic entities include photosynthesizing bacteria, diatoms, and coccolithophores. They are responsible for nearly half of the world’s primary production, converting sunlight into energy and organic matter. They initiate the transfer of atmospheric carbon dioxide into the ocean’s biological systems.
Phytoplankton absorb dissolved carbon dioxide through photosynthesis, using sunlight, water, and inorganic carbon to create organic compounds. This process fixes carbon into their cells, effectively removing it from the upper ocean layers. As they grow, their demand for carbon dioxide increases, creating a sink that draws more atmospheric carbon dioxide into surface waters.
They form the base of the marine food web, providing sustenance for everything from microscopic zooplankton to large whales. The quality and quantity of organic matter produced by phytoplankton regulate the energy flow through marine food chains. Their contribution to carbon fixation and position at the start of the food web make them fundamental to the ocean’s ability to sequester carbon.
Shell-Builders and Reef-Formers
Beyond photosynthesis, many marine organisms absorb carbon by building shells and skeletal structures. This process, known as calcification, involves converting dissolved calcium and bicarbonate ions from seawater into calcium carbonate. Organisms like corals, mollusks, foraminifera, and coccolithophores are prominent calcifiers. These hard tissues provide protection, support, and shelter.
Coccolithophores are single-celled phytoplankton that produce intricate calcium carbonate scales called coccoliths. While their calcification consumes dissolved inorganic carbon, it also releases some carbon dioxide. The overall process of shell formation sequesters carbon into solid structures, which can eventually sink to the seafloor when the organisms die. This mechanism represents another pathway for carbon incorporation into marine structures.
Coastal Carbon Sinks: Marine Plants
Coastal ecosystems feature marine plants that are highly effective at absorbing and storing carbon. These include seagrasses, mangroves, and salt marsh plants, recognized for their “blue carbon” sequestration capabilities. Similar to land plants, they absorb carbon dioxide from the atmosphere through photosynthesis. These ecosystems cover a small percentage of the ocean’s area but account for a significant portion of blue carbon and carbon sequestration in ocean sediments.
These coastal plants sequester large amounts of carbon in their roots and underlying sediments. The submerged, oxygen-poor conditions of these soils slow the decomposition of organic matter, allowing carbon to be stored for hundreds or even thousands of years. Seagrass meadows store carbon in their leaves and roots, with much residing in the soil. Mangrove forests store considerable carbon, primarily below ground in their root systems and associated sediments.
The Ocean’s Carbon Conveyor Belt
Once absorbed by marine organisms, carbon’s journey continues through the “biological pump,” transporting it from the surface to the deep ocean. Phytoplankton, after fixing carbon via photosynthesis, become part of the marine food web. When these organisms die or are consumed, their organic matter, including zooplankton fecal pellets, sinks through the water column as “marine snow.”
This downward movement of organic carbon sequesters it away from the atmosphere for extended periods. Most of the carbon is recycled in surface waters as organisms consume and decompose, but a fraction reaches the ocean depths. Upon reaching the deep ocean or seafloor, the carbon can be stored in sediments for thousands of years. This continuous process regulates atmospheric carbon dioxide concentrations over geological timescales.