Phytoplankton, or microalgae, are plant-like organisms that live in the upper layers of almost every body of water on the planet. They photosynthesize just like terrestrial plants by absorbing carbon dioxide from their environment and expelling oxygen. Phytoplankton are responsible for half of all photosynthetic activity on the planet. Different species of phytoplankton require different conditions to live, requiring particular nutrients or temperatures to thrive. When conditions are ideal, phytoplankton can multiply rapidly, creating a bloom. These blooms can be small patches on ocean surfaces or huge streaks stretching hundreds of miles.