Ochrophyte


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 2017, 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, introduced by Finnish biologist 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 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, their swimming cells frequently display two markedly unequal flagella: an anterior flagellum with straw-like hollow tripartite hairs called mastigonemes, and an immature posterior smooth flagellum 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 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, whose chloroplasts have only two membranes. The two outer layers of ochrophyte plastids are contiguous with the endoplasmic reticulum, together composing the chloroplast endoplasmic reticulum, also known as the periplastidial endoplasmic reticulum, 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, between the second and third layers, is a separate region that in other algal groups 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

Ochrophyte chloroplasts contain chlorophylls a and c as photosynthetic pigments, in addition to fucoxanthin. Chlorophyll a binds to thylakoids, while the c pigment is present in the stroma. The most frequent accessory pigment in ochrophytes is the yellow β-carotene. The golden-brown or brown pigmentation in diatoms, brown algae, golden algae and others is conferred by the xanthophyll fucoxanthin. In the yellow-green or yellow-brown raphidophyceans, eustigmatophyceans and xanthophyceans, vaucheriaxanthin is dominant instead. These pigment combinations extend their photosynthetic ability beyond chlorophyll a alone. Additionally, xanthophylls protect the photosystems from high intensity light.

Storage products

Ochrophyte algae accumulate chrysolaminarin, a carbohydrate consisting of short chains of β-1,3-linked glucose molecules, as a storage product. It is stored in vesicles located within the cytoplasm, outside plastids, unlike other algae. Cytoplasmic lipid droplets are also common. They lack starch, which is the common storage product in green algae and plants.

Diversity

According to a 2024 survey, photosynthetic stramenopiles include 23,314 described species, with 490 species of uncertain position. However, they may amount to more than 100,000 estimated species, of which the majority are diatoms. They are divided into the following classes:
  • Aurearenophyceae – One species of unicellular marine algae, Aurearena cruciata. Cells alternate between a naked, swimming stage and a non-motile stage covered in a cell wall.
  • Bolidophyceae – 18 species of marine unicellular algae that may be naked and flagellated, or spherical and silicified, covered in silica "shields".
  • Chrysoparadoxophyceae – One enigmatic species of sand-dwelling unicellular algae, Chrysoparadoxa australica. Cells have been observed covered in cell walls, from which they escape as naked zoospores instead of dividing. They are exceptional for only having two chloroplast membranes.
  • Chrysophyceae – Commonly known as golden algae, they amount to 1,274 species of freshwater or terrestrial algae with diverse morphologies: unicellular, colonial, amoeboid, capsoid, and coccoid. A considerable portion are colorless heterotrophs. Cells may be naked or covered by cell walls, organic loricas, scales made of organic or siliceous material, or even be embedded in mucilage.
  • Diatomeae – The diatoms are the most species-rich ochrophyte group, with 14,684 described species that occupy freshwater, marine, and terrestrial ecosystems. Primarily unicellular, with some forming colonies consisting of chains of cells. They are known for their two overlapping silica frustules that cover each cell.
  • Dictyochophyceae – 217 species of mostly unicellular algae, which may be naked or covered in organic scales, and exhibit shapes from amoeboid to radially symmetrical. Some, known as silicoflagellates, grow basket-like silica skeletons that hold the cytoplasm.
  • Eustigmatophyceae – 218 species of unicellular coccoid algae covered by cell walls, present in freshwater, terrestrial, and less frequently marine habitats. They lack the girdle lamella characteristic of all other ochrophytes. They are known for their large eyespot, which lies outside the chloroplast.
  • Olisthodiscophyceae – Two marine unicellular flagellated species with flattened gliding cells that live close to the substrate. They have been observed in various shallow marine habitats.
  • Pelagophyceae – 31 species of marine flagellates characterised by a dense perforated theca. Many have highly reduced flagellar apparatuses.
  • Phaeophyceae – Commonly known as brown algae, they are the second most species-rich group, with 2,124 primarily marine species. They are multicellular algae that exhibit tissue differentiation and a complex polysaccharide metabolism. Cells are covered in cell walls composed of cellulose, alginates, and other polymers.
  • Phaeosacciophyceae – Eight species of unicellular, colonial, filamentous or thallic forms, typically covered with cell walls. They can be freshwater, marine, or soil-dwelling.
  • Phaeothamniophyceae – 31 species of filamentous, pseudofilamentous, coccoid or capsoid algae, previously classified as golden algae and xanthophytes. Cells are covered in cell walls and surrounded by a gelatinous envelope.
  • Picophagea – One species of tiny marine heterotrophic flagellates, Picophagus flagellatus.
  • Pinguiophyceae – Five species of either flagellated or nonmotile unicellular marine algae, with an unusually high amount of fatty acids.
  • Raphidophyceae – 58 species of marine and freshwater unicellular flagellated algae. Their cells are covered with multiple surface structures, such as mucocysts and trichocysts.
  • Schizocladiophyceae – One species of filamentous alga, Schizocladia ischiensis. It is similar to the filamentous Phaeophyceae, but distinguished by the absence of cellulose in its cell wall, and the absence of intercellular cytoplasmic connections.
  • Synchromophyceae – Five species of marine unicellular algae without flagella, that can join to form meroplasmodia. Some are colorless. One genus, Synchroma, has unique chloroplast complexes.
  • Xanthophyceae – 616 species of algae with an unusual yellow-green coloration. Some are macroscopic, either filamentous or siphonous, while others are single-celled and coccoid.