Phytoplankton are the foundation of nearly every marine food web and produce roughly half of all the oxygen on Earth. These microscopic organisms, most invisible to the naked eye, drive processes that sustain ocean life from the sunlit surface to the deepest seafloor, while simultaneously regulating the planet’s carbon cycle and even influencing cloud formation.
Oxygen Production and Photosynthesis
Like plants on land, phytoplankton use sunlight to convert carbon dioxide and water into energy, releasing oxygen as a byproduct. NOAA estimates that about half of Earth’s oxygen production comes from the ocean, with the vast majority generated by drifting plankton, algae, and photosynthetic bacteria. One species alone, a tiny cyanobacterium called Prochlorococcus, is the smallest photosynthetic organism on the planet yet produces up to 20% of the oxygen in the entire biosphere.
This oxygen production isn’t just a bonus for life on land. It sustains dissolved oxygen levels throughout the upper ocean, supporting fish, invertebrates, and marine mammals. Roughly the same amount of oxygen that the ocean produces is consumed by marine life within it, making phytoplankton essential for keeping that cycle in balance.
The Base of the Marine Food Web
Phytoplankton sit at the very bottom of the ocean’s food chain as primary producers. Herbivorous zooplankton, the tiny animals drifting through the water column, graze directly on phytoplankton. Krill feed on them in enormous quantities. Filter feeders like oysters, mussels, sponges, and tube worms pull plankton from the surrounding water. Small fish eat the zooplankton and krill, and larger predators eat those fish, building a pyramid of energy that ultimately traces back to phytoplankton.
This means that the productivity of nearly every marine species, from anchovies to blue whales, depends on how much phytoplankton the ocean can grow. When phytoplankton populations shift, the effects ripple upward through the entire food web.
The Main Groups of Phytoplankton
Not all phytoplankton do the same thing. The two most abundant and ecologically important groups in coastal waters are diatoms and dinoflagellates. Diatoms are encased in glassy silica shells and serve as a primary food source for zooplankton and aquaculture organisms. Dinoflagellates are more versatile. Many are mixotrophic, meaning they photosynthesize when nutrients are plentiful but switch to consuming prey when conditions deteriorate. Cyanobacteria, the group that includes Prochlorococcus, dominate the open ocean and are responsible for a disproportionate share of global oxygen production.
Each group responds differently to temperature, nutrient availability, and light, which means the composition of phytoplankton communities shifts with the seasons and with changing ocean conditions.
Carbon Sequestration and the Biological Pump
Phytoplankton play a central role in pulling carbon dioxide out of the atmosphere. Through a process called the biological carbon pump, the ocean captures an estimated 5 to 12 gigatons (billion metric tons) of CO2 per year. Here’s how it works: phytoplankton absorb CO2 at the surface during photosynthesis. When they die, clump together, or get eaten and excreted, they sink as particles known as marine snow, carrying that carbon down into the deep ocean.
This sinking material is critical for deep-sea life. Marine snow is the primary food source for organisms living hundreds or thousands of meters below the surface, where no sunlight reaches. The composition and size of these sinking particles depend heavily on what kind of phytoplankton bloom produced them, which in turn determines how much carbon actually reaches the deep ocean versus being recycled near the surface.
The ratio of carbon, nitrogen, and phosphorus that phytoplankton incorporate into their cells, known as the Redfield ratio (roughly 106 parts carbon to 16 parts nitrogen to 1 part phosphorus), has shaped our understanding of ocean chemistry. This ratio varies by region: phytoplankton in nutrient-poor subtropical waters tend to have higher carbon-to-nutrient ratios, while those in cold, nutrient-rich polar waters have lower ratios. These differences affect how efficiently different ocean regions store carbon.
Influence on Cloud Formation and Climate
Phytoplankton affect climate in a way that surprises most people: they help create clouds. Many phytoplankton species produce a sulfur compound called DMSP, which acts as a kind of internal antifreeze and salt regulator. When phytoplankton die, get eaten by zooplankton, or are broken open by viruses, DMSP spills into the seawater. Bacteria then convert it into a gas called dimethyl sulfide, or DMS, which escapes into the atmosphere.
Once airborne, DMS reacts with oxygen to form tiny sulfate particles. These particles act as seeds around which water vapor condenses, forming cloud droplets. More phytoplankton activity means more DMS, more cloud seeds, and potentially brighter, more reflective clouds that bounce sunlight back into space. This feedback loop, first proposed in the late 1980s as the CLAW hypothesis, suggests that phytoplankton may partially regulate Earth’s temperature by modulating how much sunlight reaches the surface.
Nutrient Cycling in the Ocean
Phytoplankton don’t just consume nutrients; they regulate their availability throughout the ocean. By absorbing nitrogen, phosphorus, iron, and other elements from surface waters during growth, they deplete nutrients at the surface and transport them to depth when they sink. Deep ocean currents eventually return those nutrients to the surface through upwelling, completing a cycle that can take hundreds to thousands of years.
This cycling controls where and when nutrients are available for new growth. Some phytoplankton species can also fix atmospheric nitrogen directly, converting it into a form other organisms can use. The balance between nitrogen fixation and nutrient uptake by phytoplankton shapes the overall productivity of entire ocean basins.
Threats From Warming Oceans
Rising ocean temperatures pose a direct threat to phytoplankton populations. Prochlorococcus, which thrives in warm tropical waters, has division rates that increase with temperature up to about 28°C, then drop sharply. As tropical ocean temperatures push past that threshold by the end of this century, models project a 17 to 51% reduction in Prochlorococcus production across tropical oceans. Because this single genus produces such a large share of the ocean’s oxygen and organic matter, that decline could trigger cascading effects through open ocean food webs.
Warmer water also tends to form a more stable layer at the surface, reducing the mixing that brings nutrients up from deeper water. This starves phytoplankton of the nitrogen and phosphorus they need to grow, potentially shrinking populations in regions that are already nutrient-poor.
Harmful Algal Blooms
Not all phytoplankton growth is beneficial. When conditions align, certain species multiply explosively into harmful algal blooms. These blooms are most likely in water that is warm, slow-moving, and loaded with nutrients from fertilizer runoff, sewage, or urban stormwater. In saltwater, dinoflagellates and diatoms are the most common culprits. They can produce toxins that sicken or kill fish, shellfish, marine mammals, and people who eat contaminated seafood or swim in affected water.
These blooms appear to be getting worse. Warmer waters expand the conditions that favor rapid algal growth, and increased nutrient pollution from agriculture and development feeds the problem. A bloom can devastate local fisheries, close beaches, and contaminate drinking water supplies, turning the same organisms that sustain the marine food web into a public health hazard when their growth spirals out of control.