[4] They are structurally diverse: most groups of chlorophytes are unicellular, such as the earliest-diverging prasinophytes, but in two major classes (Chlorophyceae and Ulvophyceae) there is an evolutionary trend toward various types of complex colonies and even multicellularity.

These contain chlorophylls a and b, and the carotenoids carotin, lutein, zeaxanthin, antheraxanthin, violaxanthin, and neoxanthin, which are also present in the leaves of land plants.

Some special carotenoids are present in certain groups, or are synthesized under specific environmental factors, such as siphonaxanthin, prasinoxanthin, echinenon, canthaxanthin, loroxanthin, and astaxanthin.

Among core chlorophytes, both unicellular groups can reproduce asexually through autospores,[18] wall-less zoospores,[19] fragmentation, plain cell division, and exceptionally budding.

[20] Multicellular thalli can reproduce asexually through motile zoospores,[21] non-motile aplanospores, autospores, filament fragmentation,[22] differentiated resting cells,[23] and even unmated gametes.

[36][37] Several species of Chlorophyta live in symbiosis with a diverse range of eukaryotes, including fungi (to form lichens), ciliates, forams, cnidarians and molluscs.

[41] With the exception of the three classes Ulvophyceae, Trebouxiophyceae and Chlorophyceae in the UTC clade, which show various degrees of multicellularity, all the Chlorophyta lineages are unicellular.

The Smith system, published in 1938 by American botanist Gilbert Morgan Smith, distinguished two classes: Chlorophyceae, which contained all green algae (unicellular and multicellular) that did not grow through an apical cell; and Charophyceae, which contained only multicellular green algae that grew via an apical cell and had special sterile envelopes to protect the sex organs.

[47] With the advent of electron microscopy studies, botanists published various classification proposals based on finer cellular structures and phenomena, such as mitosis, cytokinesis, cytoskeleton, flagella and cell wall polysaccharides.

[48][49] British botanist Frank Eric Round [nl] proposed in 1971 a scheme which distinguishes Chlorophyta from other green algal divisions Charophyta, Prasinophyta and Euglenophyta.

[45] The modern usage of the name 'Chlorophyta' was established in 2004, when phycologists Lewis & McCourt firmly separated the chlorophytes from the streptophytes on the basis of molecular phylogenetics.

[52] Below is a cladogram representing the current state of green algal classification:[53][52][54][55] Prasinodermophyceae Palmophyllophyceae Pyramimonadophyceae Mamiellophyceae Nephroselmidophyceae Pseudoscourfieldiales Picocystophyceae Chloropicophyceae Chlorodendrophyceae Trebouxiophyceae Chlorophyceae Ulvophyceae Pedinophyceae Mesostigmatophyceae Chlorokybophyceae Klebsormidiophyceae Charophyceae Coleochaetophyceae Zygnematophyceae Embryophyta (land plants) Currently eleven chlorophyte classes are accepted, here presented in alphabetical order with some of their characteristics and biodiversity: In February 2020, the fossilized remains of a green alga, named Proterocladus antiquus were discovered in the northern province of Liaoning, China.

In particular, the unicellular flagellate Chlamydomonas reinhardtii and the colonial organism Volvox carteri are object of interest due to sharing homologous genes that in Volvox are directly involved in the development of two different cell types with full division of labor between swimming and reproduction, whereas in Chlamydomonas only one cell type exists that can function as a gamete.

Other volvocine species, with intermediate characters between these two, are studied to further understand the transition towards the cellular division of labor, namely Gonium pectorale, Pandorina morum, Eudorina elegans and Pleodorina starrii.

[73] Chlorophyte microalgae are a valuable source of biofuel and various chemicals and products in industrial amounts, such as carotenoids, vitamins and unsaturated fatty acids.

Various genera (Chlorella, Scenedesmus, Haematococcus, Dunaliella and Tetraselmis) are used as cellular factories of biomass, lipids and different vitamins for either human or animal consumption, and even for usage as pharmaceuticals.