Carbon is the foundation of nearly every biological and chemical process in the ocean. It fuels marine food webs, builds the shells and skeletons of sea creatures, regulates ocean chemistry, and gets stored in deep water and sediments for centuries to millennia. The ocean holds roughly 37,000 gigatons of dissolved inorganic carbon alone, making it the largest carbon reservoir on Earth accessible on human timescales, holding more than 50 times as much carbon as the pre-industrial atmosphere.
Fueling the Marine Food Web
Carbon enters the living ocean primarily through photosynthesis. Phytoplankton, the microscopic algae floating in sunlit surface waters, pull dissolved carbon dioxide from seawater and convert it into organic carbon using the same core enzyme that land plants rely on. This process is the starting point for virtually all marine life. Small zooplankton graze on phytoplankton, fish eat zooplankton, and so on up the food chain. Without this constant conversion of inorganic carbon into living tissue, the ocean would be largely lifeless.
Not all of that organic carbon stays inside cells. A large pool of dissolved organic carbon, roughly 600 gigatons at any given time, floats freely in seawater. Bacteria consume this dissolved carbon, forming what scientists call the microbial loop. These bacteria become food for tiny grazers, which in turn feed larger organisms. In some environments, this dissolved carbon pathway is the primary energy source. In certain coastal groundwater caves, for example, carbon from decomposing vegetation supports an entire food web of bacteria and cave-adapted shrimp, with no sunlight involved at all.
Building Shells, Skeletons, and Reefs
Marine organisms also use carbon in its inorganic form to construct hard body parts. Corals, coralline algae, foraminifera (single-celled organisms with tiny shells), and even fish all produce calcium carbonate structures. Corals and fish typically build with a form called aragonite, while coralline algae and foraminifera favor calcite. Both are crystalline forms of calcium carbonate, just arranged differently at the molecular level.
This biological calcification has shaped the ocean for hundreds of millions of years. Coral reefs are the most visible result: enormous geologic structures built by generations of organisms pulling carbon and calcium from seawater and locking them into solid rock. But even the smallest calcifiers matter. Foraminifera are so abundant that their accumulated shells form thick layers of sediment on the ocean floor, effectively storing carbon in the earth’s crust for millions of years. Fish use calcium carbonate to form otoliths, small structures in their inner ears essential for balance and hearing.
Absorbing Atmospheric Carbon Dioxide
The ocean acts as a massive carbon sink, absorbing about 31% of the carbon dioxide humans release into the atmosphere. During the 2010s, global fossil fuel emissions averaged 9.5 billion tons of carbon per year, meaning the ocean was taking up roughly 3 billion tons annually. This absorption happens through simple gas exchange at the sea surface: when atmospheric CO2 concentrations rise, more gas dissolves into seawater, the same way a carbonated drink absorbs more fizz under pressure.
Once dissolved, carbon dioxide reacts with seawater to form bicarbonate and carbonate ions. This dissolved inorganic carbon pool, at around 37,000 gigatons, dwarfs the organic carbon pool by a factor of about 60. It acts as a chemical buffer that has historically kept ocean pH relatively stable, though that system is now under strain from the sheer volume of CO2 being absorbed.
Transporting Carbon to the Deep Ocean
Carbon doesn’t stay at the surface. A process often called the biological pump moves organic carbon from sunlit waters into the deep ocean. When phytoplankton die or get eaten, their remains clump together with other debris into particles known as marine snow. These particles sink, carrying carbon downward.
The journey isn’t straightforward. Bacteria colonize sinking particles and break them down, and the speed of that degradation determines how deep the carbon travels. Most particles last around 30 days below the surface before being consumed, though some persist for over 200 days. Roughly half of sinking particles get recycled in the upper 1,000 meters. The other half can reach depths beyond 2,000 meters, where the carbon stays locked away on timescales of decades to millennia. Whatever reaches the seafloor and gets buried in sediment can remain stored for millions of years.
Storing Carbon in Coastal Ecosystems
Coastal habitats play an outsized role in long-term carbon storage. Mangrove forests, seagrass meadows, and salt marshes, collectively known as blue carbon ecosystems, trap organic carbon in their soils at remarkable densities. Seagrass meadows in the Caribbean, for instance, store an average of 241 metric tons of organic carbon per hectare in just the top meter of soil. These deposits accumulate over centuries, with some seagrass carbon stores dating back 2,000 years.
What makes these ecosystems so effective is the waterlogged, low-oxygen conditions of their soils. Decomposition slows dramatically without oxygen, so carbon that sinks into mangrove mud or seagrass sediment stays put far longer than carbon in a typical forest floor. Mangrove-derived material also contributes significantly to nearby seagrass carbon stocks, with mangrove matter accounting for about 27% of stored carbon in some Caribbean seagrass beds. The two ecosystems work as a connected carbon storage system.
How Carbon Shapes Ocean Chemistry
The carbon dissolved in seawater does more than feed organisms and build structures. It controls the ocean’s acidity. When CO2 dissolves, it produces hydrogen ions, which lower pH. Historically, the ocean’s vast reserves of carbonate ions have neutralized much of that acidity. But as CO2 absorption accelerates, the balance is shifting. Excess hydrogen ions bond with carbonate ions, pulling them out of circulation and leaving fewer available for marine organisms that need them.
This is the core problem of ocean acidification. With fewer carbonate ions in the water, corals, shellfish, and plankton have to work harder to build and maintain their calcium carbonate structures. If pH drops far enough, existing shells and skeletons can begin to dissolve. The carbon chemistry of seawater, in other words, doesn’t just support life. It sets the boundary conditions for which organisms can survive and where. The same element that builds a coral reef can, in excess, start to erode it.