The Earth’s atmosphere naturally contains carbon dioxide, a gas that plays a significant role in regulating our planet’s temperature. Human activities have greatly increased the amount of carbon dioxide released, leading to concerns about global climate shifts. The ocean serves as a vast natural reservoir, absorbing a substantial portion of this atmospheric carbon dioxide and helping to regulate the planet’s climate.
Ocean Organisms Leading Carbon Absorption
The most significant direct absorbers of carbon dioxide from the atmosphere are microscopic organisms known as phytoplankton. These single-celled organisms perform photosynthesis, using sunlight, water, and dissolved carbon dioxide to create food and grow, releasing oxygen. Phytoplankton absorb an estimated 40 to 50 billion metric tons of carbon annually, making them the primary drivers of carbon fixation in marine environments.
Beyond these tiny organisms, larger forms of marine plant life also contribute to carbon absorption, particularly in coastal regions. Macroalgae, commonly known as seaweeds, absorb dissolved carbon dioxide through photosynthesis. These diverse organisms grow rapidly and form extensive underwater forests, particularly in cooler, nutrient-rich waters. Their biomass stores carbon, which can then be transported to deeper waters or buried in sediments when they die.
Coastal marine plants like seagrasses and mangroves are effective carbon sinks, often referred to as “blue carbon” ecosystems. Seagrass meadows, found in shallow coastal waters worldwide, absorb carbon dioxide and store it in their leaves, roots, and underlying sediments. Mangrove forests, located in tropical and subtropical intertidal zones, similarly capture significant amounts of carbon in their dense vegetation and oxygen-poor soils beneath them. These ecosystems are efficient at long-term carbon sequestration due to slow decomposition rates in their saturated sediments.
Carbon’s Journey: The Biological Pump
The carbon absorbed by surface-dwelling organisms, primarily phytoplankton, embarks on a journey through the ocean, a process known as the biological pump. This mechanism transports carbon from sunlit surface waters to the deep ocean, effectively removing it from the atmosphere for extended periods. The initial step involves zooplankton, small marine animals, consuming phytoplankton.
As zooplankton graze on phytoplankton, they package the carbon-rich organic matter into dense fecal pellets. When these organisms die, their remains also contribute to the downward flux of organic material. These fecal pellets and dead organic matter are relatively heavy, causing them to sink rapidly through the water column. This efficient sinking minimizes the time carbon spends in the upper ocean where it could be released back into the atmosphere.
During their descent, some of these sinking particles are consumed by other organisms or broken down by marine bacteria in a process called remineralization. This decomposition releases some carbon back into the dissolved inorganic carbon pool at intermediate depths. However, a significant portion of the organic matter escapes this remineralization and continues its descent, eventually reaching the deep ocean or seafloor. Carbon that reaches the deep ocean can remain sequestered for hundreds to thousands of years, making the biological pump an important component of Earth’s long-term carbon cycle.
Other Marine Life and Carbon Storage
While photosynthetic organisms are the primary absorbers of atmospheric carbon dioxide, other marine life also plays a role in the ocean’s carbon cycle and storage. Corals, for instance, contribute to carbon sequestration through calcification, forming their skeletons from calcium carbonate. This chemical reaction removes dissolved inorganic carbon from seawater and locks it into their structures. Many corals also host symbiotic algae, called zooxanthellae, which perform photosynthesis and directly absorb carbon dioxide from the water.
Fish and marine mammals, such as whales, store carbon within their biomass. When these animals die, their bodies can sink to the deep ocean, transporting carbon to the seafloor. Additionally, some fish and zooplankton undertake daily vertical migrations, moving between surface waters at night and deeper waters during the day. This movement helps to redistribute carbon within the water column. While not directly absorbing atmospheric carbon dioxide, these organisms contribute to the overall carbon cycling and storage within the marine ecosystem.