Chlorophyta

Chlorophytes are eukaryotic organisms composed of cells with a variety of coverings or walls, and usually a single green chloroplast in each cell. 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.

Chloroplasts. Chlorophyte cells contain green chloroplasts surrounded by a double-membrane envelope. 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, echinenone, canthaxanthin, loroxanthin, and astaxanthin. They accumulate carotenoids under nitrogen deficiency, high irradiance of sunlight, or high salinity. In addition, they store starch inside the chloroplast as carbohydrate reserves. The thylakoids can appear single or in stacks. In contrast to other divisions of algae such as Ochrophyta, chlorophytes lack a chloroplast endoplasmic reticulum.

Flagellar apparatus. …

Several species of Chlorophyta live in symbiosis with a diverse range of eukaryotes, including fungi (to form lichens), ciliates, forams, cnidarians and molluscs. Some species of Chlorophyta are heterotrophic, either free-living or parasitic. Others are mixotrophic bacterivores through phagocytosis. Two common species of the heterotrophic green alga Prototheca are pathogenic and can cause the disease protothecosis in humans and animals. 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. Some members of the group form symbiotic relationships with protozoa, sponges, and cnidarians. Others form symbiotic relationships with fungi to form lichens, but the majority of species are free-living. All members of the clade have motile flagellated swimming cells. Monostroma kuroshiense, an edible green alga cultivated worldwide and most expensive among green algae, belongs to this group.

Chlorophyte microalgae are a valuable source of biofuel and various chemicals and products in industrial amounts, such as carotenoids, vitamins and unsaturated fatty acids. The genus Botryococcus is an efficient producer of hydrocarbons, which are converted into biodiesel. 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. Some of their pigments are employed for cosmetics.

The first mention of Chlorophyta belongs to German botanist Heinrich Gottlieb Ludwig Reichenbach in his 1828 work Conspectus regni vegetabilis. Under this name, he grouped all algae, mosses ('musci') and ferns ('filices'), as well as some seed plants (Zamia and Cycas). This usage did not gain popularity. In 1914, Bohemian botanist Adolf Pascher modified the name to encompass exclusively green algae, that is, algae which contain chlorophylls a and b and store starch in their chloroplasts. Pascher established a scheme where Chlorophyta was composed of two groups: Chlorophyceae, which included algae now known as Chlorophyta, and Conjugatae, which are now known as Zygnematales and belong to the Streptophyta clade from which land plants evolved. During the 20th century, many different classification schemes for the Chlorophyta arose. 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. With the advent of electron microscopy studies, botanists published various classification proposals based on finer cellular structures and phenomena, such as mitosis, cytokines …