Algae are a diverse group of eukaryotic organisms that contain chlorophyll and carry out photosynthesis. Some contain other photosynthetic pigments which gives them their characteristic colour - most algae are green, but some are red. They occur in a wide range of forms from microscopic to macroscopic e.g. seaweeds, some of which are up to 30 metres long and are not considered to be micro-organisms. Microscopic algae exist either as single cells e.g. diatoms, in colonies e.g. Volvox or in filaments e.g. Spirogyra.

Most algae are free living although some form symbiotic relationships with other organisms. Lichens are an example of symbiosis between algae and fungi. Algae usually live in aquatic habitats of all types; some are found in soil or vegetation where there is enough moisture. They are rarely parasitic.

Some algae are motile. Single-celled algae often have flagellae and move in a spiralling manner. Others do not have flagellae but secrete a sticky substance which attach them to the substrate, enabling them to glide along leaving a trail of slime behind.

Algae can reproduce asexually by fragmentation, binary fission and spore formation. They can also reproduce sexually.