Plankton play an essential part in the marine life. They consist of many micro living-organisms drifting on the surface of soft or salt waters. Their sizes are usually expressed in micron. Many marine animals live on plankton to subsist. There are three types of plankton: the phytoplankton, the zooplankton and the bacterioplankton.

Phytoplankton live near the water surface, no deeper than 15 meters below it. They broadly refer to all the marine plants and algae proliferating in the sea. They're auto-trophic, which means that they produce organic compounds from mineral carbon (CO2) and mineral salts through the process of photosynthesis (light is the energy needed to synthesize carbons into compounds). This mode of nutrition is common to all the chlorophylls. Roughly, phytoplankton absorb CO2 and water, produce organic compounds trough light, and throw out oxygen.

Zooplankton live on living organisms, like phytoplankton! They're hiding from predators deep into the ocean, where the temperature is low. Poor zooplankton can't stand too much light and suffer badly from “light pollution”, a urban phenomena due to the multiplication of housings alongside the coasts.

Phytoplankton, which make up 1% of the total biomass, produce 45% of the “primary production”.
The term refers to the organic compounds which constitute the very first link in an ecosystem chain food. Plankton are at the basis of the marine life: they're swallowed by small fishes, which are in turn eat by bigger fishes and so on. That's evolution baby!! Phytoplankton are the main producer of oxygen, before trees and other terrestrial plants.