Chlorophytes (Chlorophyta) are the most diverse and numerous group of algae, with approximately 10,000 species described, of which approximately 10% are marine. Chlorophytes are characterized by having chlorophylls a and b in their chloroplasts to perform photosynthesis, which gives them their characteristic green color and therefore their colloquial name, green algae. However, in addition to these chlorophylls, they also have β- and γ-carotenes, as well as various xanthophylls as accessory pigments.

Their similarity to terrestrial plants, both in molecules used to capture energy from the sun (chlorophylls), and energy storage molecules (starch), as well as genetic similarity, have led to the consideration of Chlorophytes as possible direct ancestors of terrestrial plants.

The vast majority of Chlorophytes species are benthic, i.e. they are attached to the sea floor by specialized structures (rhizoids). However, there are also species, mainly unicellular and often flagellated, i.e. planktonic, that live in suspension as part of the phytoplankton. There is a great variety of both shapes and sizes. Similarly, there is great variability in the longevity of green algae species; there are perennial species, such as Codium tomentosum, which can live for several years, although there are others that are seasonal, and if there are adequate light conditions and availability of nutrients, they can experience such rapid growth that they cause 'green tides'.

The sub-classification of the Chlorophytes' phylum is a field that has changed considerably in recent years. We currently find the following classes of Chlorophytes:

  • Charophyceae, is a diverse class of freshwater algae, although they are able to tolerate slight concentrations of salt. They are often found in waters with high hardness (high concentrations of calcium and magnesium carbonates) and very soft currents. Phylogenetic studies have shown that these algae are the closest group to terrestrial plants (Embriophyta).

  • Chlorophyceae, a class of unicellular algae predominantly found in fresh water. They have one or two flagella and the cells usually have a more or less calcified teak.

  • Prasinophytes, this is a heterogeneous class of primitive unicellular algae with mobility that may or may not have an outer cover formed by scales composed of polysaccharides. They are predominantly part of the marine plankton, although there are certain freshwater species. Ostreococcus tauri is less than 1 µm in diameter and is the smallest eukaryotic algae on record.

  • Ulvophyceae, we find in this class microscopic algae, unicellular giant cell species (Acetabularia acetabulum) and multicellular species. They are mostly marine, although there are certain freshwater species and others in humid subaerial habitats. The vast majority of filamentous green algae and large green seaweeds belong to this class.

Ecologically, they play a fundamental role in the trophic chain, since they are part of the base of the sea food chain, either forming the phytoplankton, or as a source of food for herbivorous species. Likewise, numerous species are of food interest to humans, such as some species of the genera Ulva, Monostroma, Enteromorpha and Caulerpa.