This document discusses plankton, which are microscopic plant and animal organisms that float in oceans, seas, lakes, and rivers. Plankton are carried by water currents and include phytoplankton such as algae, and zooplankton such as krill and protozoa. Phytoplankton come in thousands of species and some can affect water color, while diatoms are an important component. Zooplankton are eaten by whales, fish, and other marine life and play a key role in ocean ecosystems. Examples are given of different plankton types like radiolarians, foraminifera, and their fossilized shells in rocks.