What are Phytoplankton?

Written By Teacher: Teresa Pettitt-Kenney

Hi there! My name is Teresa and I just finished my Bachelor's degree in Environmental Science and am excited to pursue environmental education in the future! I am extremely passionate about climate change, equitable climate action, and how education can work to address these issues. 

How do you talk about phytoplankton with students when you can’t even see them without a microscope? Although they’re quite small, phytoplankton are an incredibly important part of the carbon cycle and the balance of the global environment. Use visuals to illustrate the impact of these organisms, like videos and interactive websites. Once your students understand what phytoplankton are, solidify their function and importance using experimental resources like labs and outdoor activities.

Written By: MIT Environmental Solutions Initiative

The MIT Climate Change Engagement Program, a part of MIT Climate HQ, provides the public with nonpartisan, easy-to-understand, and scientifically-grounded information on climate change and its solutions.

Phytoplankton are microscopic plant-like organisms that live in the oceans, lakes, and rivers. There are a billion billion billion phytoplankton in the world’s oceans—more than there are stars in the sky. Phytoplankton are hugely diverse, with likely 100 thousand different species. These include some bacteria (the cyanobacteria, sometimes called “blue-green algae”), algae like the diatoms that surround themselves with glass-like structures, and coccolithophores with calcium carbonate armor. The smallest and most abundant phytoplankton species is called Prochlorococcus and, given its tiny size, was only discovered in the 1980s. Depending on the conditions, and the species of phytoplankton, a teaspoon of ocean water could harbor several thousand individuals.

By taking up carbon dioxide (CO2) from the air, phytoplankton play a large role in the natural carbon cycle, helping to regulate the amount of CO2 in the atmosphere and keep the Earth’s climate in balance.

The “plants” of the ocean

Like plants, phytoplankton grow through photosynthesis: they use energy from the sun to combine CO2 and nutrients into carbohydrates, which form the phytoplankton’s cells. Phytoplankton typically grow faster than land plants, roughly doubling in mass every day by dividing into daughter cells.

Phytoplankton are the base of the aquatic food webs. They are eaten by zooplankton, the microscopic herbivores of the aquatic realm, which in turn are food for fish and other sea creatures. As phytoplankton die, and the creatures above them in the food web die or poop, a small fraction of the carbon the phytoplankton took in during their lifetimes sinks down below the sunlight layers of the ocean. As this organic matter sinks, it becomes food for deeper dwelling animals, but also nourishment for bacteria, which release it back into inorganic forms like CO2.