Algae are among the oldest organisms on Earth, with an appearance that dates back to about 3 billion years ago, and have maintained their basic form throughout time. Algae can vary in size from microscopic single cells, also known as protophytes, to large multicellular algae that can reach several meters. Algae belong to different evolutionary lines, so they are defined mainly according to their ecological properties.

Algae are considered "aquatic plants" but lack some of the familiar characteristics of plants, such as roots, peonies, tips and beauty. They also differ from normal plants in their reproductive mechanism, but share with plants their ability to carry out photosynthesis. In fact, over 70% of photosynthesis in the world is carried out by algae, and a significant part of the oxygen in the atmosphere is produced by them. The algae, and especially the blue ones from the plankton, play a cardinal role in photosynthesis and nitrogen fixation, and are therefore essential to the biosphere.

In most cases, algae are phototrophic - that is, they produce their organic molecules through the process of photosynthesis. A small part of them are mixotrophic, which combine external food sources with photosynthesis, while another small part is heterotrophic, completely dependent on external food sources, with a limited photosynthesis process.