Ochrophytes, also known as heterokontophytes or stramenochromes, are a phylum of algae. They are the photosynthetic stramenopiles, a group of eukaryotes, organisms with a cell nucleus, characterized by the presence of two unequal flagella, one of which has tripartite hairs called mastigonemes. In particular, they are characterized by photosynthetic organelles or plastids enclosed by four membranes, with membrane-bound compartments called thylakoids organized in piles of three, chlorophyll a and c as their photosynthetic pigments, and additional pigments such as β-carotene and xanthophylls. Ochrophytes are one of the most diverse lineages of eukaryotes, containing ecologically important algae such as brown algae and diatoms. They are classified either as phylum Ochrophyta, Heterokontophyta or as subphylum Ochrophytina within phylum Gyrista. Their plastids are of red algal origin.
Etymology Throughout history, different names have been used to describe photosynthetic stramenopiles. The more widely used name is the phylum —or division, in botanical nomenclature— Ochrophyta, based on the golden alga Ochromonas. This name was first coined by evolutionary biologist Thomas Cavalier-Smith in 1986 as Ochrista, later renamed to Ochrophyta to comply with the recommendations of the International Code of Botanical Nomenclature. In 2018, the same author lowered it to a subphylum inside phylum Gyrista, and modified the name to Ochrophytina to match the -phytina suffix used for botanical subdivisions. Despite this, Ochrophyta is preferred over Ochrophytina by the scientific community. The alternative name Heterokontophyta is more familiar among phycologists. The origin of this name is the class Heterokontæ (from Greek hetero 'different' and kontos 'pole'), introduced by Finnish biologist Alexander Ferdinand Luther in 1899 to include yellow-green freshwater algae, now part of Xanthophyceae and Raphidophyceae. This name referenced, among other traits, the two unequal flagella characteristic of all stramenopiles, also known as heterokonts. Eventually it was expanded to include more algae and became the division Heterokontophyta, coined by Christiaan van den Hoek in 1978 and used to describe all photosynthetic stramenopiles.
Characteristics Ochrophytes are eukaryotic organisms composed of cells that are either naked or covered by scales, lorica or a cell wall. They can be single-celled, colonial, coenocytic or multicellular. Some Phaeophyceae (brown algae, seaweeds) develop as large multicellular thalli with differentiated tissues. All ochrophytes uniformly have tubular mitochondrial cristae. This is a common trait shared with their relatives, heterotrophic stramenopiles, as well as other closely related groups such as Rhizaria, Telonemia and Alveolata. As primarily photosynthetic eukaryotes, they are considered algae, distinguished from other groups of algae by specific morphological and ultrastructural traits, such as their flagella, chloroplasts and pigments.
Flagella As stramenopiles (=heterokonts), their swimming cells frequently display two markedly unequal flagella: an anterior flagellum ("tinsel") with straw-like hollow tripartite hairs called mastigonemes, and an immature posterior smooth flagellum ("whiplash") lacking these hairs. The ciliary transition zone of the flagellum generally has a transitional helix.
Chloroplasts The ochrophytes are mostly photosynthetic. As such, they may possess one or more photosynthetic plastids (chloroplasts) per cell. Some groups contain species with leucoplasts, chloroplasts that have lost photosynthetic capacity and pigments but presumably continue to play a role in the synthesis of amino acids, lipids and heme groups. Ochrophytes have a distinct plastid ultrastructure in comparison to other algal groups. Their chloroplasts originate from an event of secondary endosymbiosis from a red alga, which lead to four surrounding membranes: two inner membranes, corresponding to the primary plastid membranes; a third membrane, corresponding to the plasma membrane of the red alga; and an outermost layer, corresponding to the phagosome membrane. This characteristic differentiates them from archaeplastid algae (glaucophytes, red algae and green algae), whose chloroplasts have only two membranes. The two outer layers of ochrophyte plastids are contiguous with the endoplasmic reticulum (ER), together composing the chloroplast endoplasmic reticulum (CER), also known as the periplastidial endoplasmic reticulum (PER), which is often connected to the nuclear envelope. The tripartite flagellar hairs, characteristic of stramenopiles, are produced within either the PER or the nuclear envelope. The periplastid compartment (PC), between the second and third layers, is a separate region that in other algal groups (i.e. cryptomonads and chlorarachniophytes) contains a nucleomorph, the vestigial nucleus of the secondary endosymbiont; however, no nucleomorphs are known within the ochrophytes. Instead, other structures have been observed within the PC, similarly to those seen in haptophytes and chromerid algae: "blob-like structures" where PC proteins are localized, and a vesicular network. Within the CER, there is a prominent region of tight direct contacts between the periplastid membrane and the inner nuclear envelope, where lipid transfers might occur, and perhaps exchange of other molecules. Commonly, within the plastid stroma, three stacked thylakoids differentiate into the "girdle lamella", which runs around the periphery of the plastid, beneath the innermost membrane. The remaining thylakoids are arranged in stacks of three. In synchromophytes and aurearenophytes, a consortium of several plastids, each surrounded by two or three inner membranes respectively, is enveloped by a shared outer membrane.
Pigmentation
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