Phytoplankton

Phytoplankton are the autotrophic components of the plankton community. The term "autotrophic" means they are self-feeding organisms. Their name comes from the Ancient Greek words "phutón," meaning plant, and "planktós," meaning drifter or wanderer.

These microscopic organisms are essential to both ocean and freshwater ecosystems. They act as the primary producers at the base of aquatic food webs. Even though they represent only about 1% of global plant biomass, they are incredibly powerful. They account for roughly half of all global photosynthetic activity. They also produce at least half of the Earth's oxygen.

To survive, phytoplankton rely on a process called photosynthesis. They use sunlight to convert carbon dioxide, which is dissolved in the water, into organic compounds. Because they require light, they live in the well-lit surface layers of water known as the euphotic zone.

During this process, they assimilate carbon and release oxygen as a byproduct. However, they must balance their light intake carefully. If solar radiation becomes too high, they may suffer from photodegradation. Different species use various photosynthetic pigments to absorb different wavelengths of light. This allows different communities to thrive depending on the spectral composition of the water.

Growth requires more than just light; phytoplankton also need specific nutrients. They depend on macronutrients, which are required in large quantities. These include nitrate, phosphate, and silicic acid. They also require micronutrients, which are trace metals needed in much smaller amounts. These metals include iron, manganese, zinc, cobalt, cadmium, and copper.

The availability of these nutrients is often controlled by the balance between upwelling and the biological pump. In some regions, like the Southern Ocean, a lack of iron can limit how much phytoplankton can grow. This has led to controversial experiments involving iron fertilization to try and remove carbon dioxide from the atmosphere.

Phytoplankton are a highly diverse group of organisms. They include photosynthesizing bacteria known as cyanobacteria, such as Prochlorococcus and Synechococcus. They also include various unicellular protists, like the diatoms.

Some groups, such as coccolithophores, are even more specialized. Coccolithophores have shells made of calcium carbonate called a coccosphere. These shells are sensitive to ocean acidification. Scientists have also identified "mixoplankton." These are organisms like certain dinoflagellates that are not just phototrophic, meaning they use light, but can also eat other organisms. This distinction changes how we understand the functioning of the food web.

In the early twentieth century, scientist Alfred C. Redfield made a major discovery regarding these organisms. He noticed a similarity between the elemental composition of phytoplankton and the nutrients in the deep ocean. He proposed that the ratio of carbon, nitrogen, and phosphorus in the ocean is controlled by phytoplankton requirements. This specific ratio is known as the Redfield ratio, which is 106:16:1.

This ratio is a fundamental principle for understanding marine ecology and biogeochemistry. It helps scientists track how nutrients move through the entire ocean system.

Phytoplankton play a vital role in the global carbon cycle. They perform primary production by fixing carbon into organic matter. This process supports a vast chain of life. For example, phytoplankton sustain krill, which are tiny crustaceans. These krill then serve as the primary food source for massive baleen whales.

When phytoplankton die, they can sink to the seafloor. This process eventually fertilizes the ocean floor with organic matter and detritus. This movement of matter is a key part of how the ocean functions.

Because they respond so quickly to environmental changes, phytoplankton are excellent indicators of ecosystem health. They have much faster turnover rates than land plants, which live for decades. Phytoplankton can respond to climate variations on a global scale in just days.

Changes in temperature, ocean acidification, and nutrient supply can all alter their populations. Scientists use satellite ocean color observations to monitor these changes from space. By watching the color of the water, researchers can track the health and distribution of these tiny but mighty drifters across the entire planet.