According to their habitat algae are classified into seven groups.

 Hydrophilus algae:

These are aquatic, free floating or completely submerged algae.

 Edaphic algae:

Terestial algae are called Edaphic algae. They live upon or inside the surface of earth. Edaphic algae are classified into two types,

·        Saprophytes E.g. Mesotaemium, Botryduium

·        Crypyophytes E.g. Nostoc, Anabaena

Aerial algae:

These are aerial forms of algae. They are found upon trunks of trees, walls, fencing wire, rocks and animals. Aerial algae are classified into four types. They are,

·        Epiphyllophytes E.g. Trentepohlia

·        Epiphloephytes

·        Epizoophytes E.g. Chaetophorales

·        Lithophytes E.g. Sctonema, Vaucheria, Nostoc

 Cryophytic algae:

Algae living on ice and snow are called cryophytes or cryophytic algae.

Eg. Chlamydomonas, Ankistrodesmus and Mesotaenium.

 Symbionts or Endophytes:

Algae growing in symbiotic association with other plants are called symbionts. There are three types. They are,

·       Symbiotic with fungi E.g. Chroococcus, Nostoc, Chlorella and palmella

·       Lives inside the pteridophyte Azolla. Eg. Anabaena azollae.

·       Found in the corolloid roots of Cycas. Eg. Anabaena cicadae.

Endozoic algae:

Algae living inside the body of animals are called Endozoic algae.

·        Inside fresh water sponges

·        Inside Hydra

 Parasites

Algae live as parasites on other plants. Eg.Cephaleuros virescens.