Phytoplankton are microscopic, plant-like organisms that drift in the upper, sunlit layers of the ocean. These tiny entities are immensely abundant, forming the foundational layer of nearly all marine and freshwater food webs. Their collective activity drives major global cycles, particularly the movement of carbon throughout the atmosphere and oceans. Understanding how these organisms utilize carbon is foundational to understanding the planet’s climate system and the health of aquatic ecosystems.
Converting Inorganic Carbon to Organic Matter
The first and most widespread method of carbon use by phytoplankton is the assimilation of inorganic carbon from the water. This process, known as primary production, involves the organisms taking in dissolved carbon dioxide (CO2) or bicarbonate (HCO3-) from the surrounding seawater. Using sunlight as an energy source, they chemically convert these simple inorganic molecules into complex organic compounds, such as sugars and carbohydrates.
This conversion process, photosynthesis, represents the initial capture of carbon dissolved in the ocean. The resulting organic matter is referred to as particulate organic carbon (POC), which forms the biomass of the phytoplankton cell itself. This assimilation acts as a powerful biological sink, drawing carbon from the water and, indirectly, from the atmosphere. The efficiency of this carbon fixation is dependent on factors like available light and the presence of other nutrients, such as nitrogen and phosphorus.
Using Fixed Carbon for Growth and Respiration
Once phytoplankton fix inorganic carbon into organic matter, the carbon takes on two distinct metabolic fates within the cell. A significant portion of this organic carbon is directed toward building the structural and functional components of the cell.
This growth process allows the phytoplankton population to rapidly multiply and form vast blooms, incorporating carbon into the organism’s physical structure. The second metabolic use is respiration, which is the breakdown of some fixed organic carbon to release energy (ATP).
This energy powers all cellular processes, including nutrient uptake and movement. This metabolic breakdown releases a fraction of the carbon back into the surrounding water as CO2, representing an internal cycling loop within the surface ocean.
The Role of Phytoplankton in Carbon Export and Storage
The second major way phytoplankton utilize carbon is by facilitating its long-term storage and export away from the surface layer. Certain species, such as coccolithophores, use inorganic carbon in a process called calcification to form external shells made of calcium carbonate (CaCO3). This structural use of carbon is distinct from the organic carbon fixed through photosynthesis.
These calcium carbonate shells act as a ballast, increasing the density of the cells. When these phytoplankton die or are consumed by zooplankton and excreted as dense fecal pellets, the carbon-rich material begins to sink. This downward movement of both organic (POC) and inorganic (CaCO3) carbon is known as the biological carbon pump.
The pump effectively transfers carbon from the sunlit surface waters into the deep ocean, where it can be sequestered for hundreds to thousands of years. The efficiency of this export mechanism is enhanced by the structural components of species like diatoms, whose silica shells contribute to the density of sinking particles.
Only a small percentage of the total carbon produced at the surface reaches the deep seabed, but this process is responsible for the ocean’s ability to act as a major carbon reservoir.
Global Impact on Carbon Cycling
The combined actions of carbon fixation and export by phytoplankton exert a profound influence on the Earth’s climate system. Phytoplankton are responsible for roughly half of the planet’s total photosynthetic activity, making them the largest biological carbon sink on Earth. Through primary production, they draw down an estimated 50 gigatonnes of carbon annually from the ocean’s dissolved inorganic carbon pool.
This massive scale of carbon uptake lowers the CO2 concentration in the surface water, allowing the ocean to absorb more CO2 from the atmosphere to maintain equilibrium. The subsequent biological carbon pump transfers approximately 10 gigatonnes of carbon per year into the deep ocean, physically removing it from circulation with the atmosphere. Without this continuous biological mechanism, atmospheric CO2 levels would be significantly higher, demonstrating the fundamental role of these microorganisms in regulating global climate.