Phytoplankton are microscopic organisms that live in the ocean. They use sunlight to grow and make food. They also produce the oxygen we breathe. Just as plants form the basis for the ecosystem on land, phytoplankton form the basis for the ocean ecosystem. Every ocean animal, from the tiniest fish to the largest whale, depends on phytoplankton for survival. Even if an animal doesn't eat phytoplankton, it eats another animal that does, or animal that eats that animal, and so on.

Image credits: main image, Sean Murtha.