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.

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. 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. (Diatomeae)|upright=0.9]] (Phaeophyceae)]] 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. Both chlorophyll c and fucoxanthin is absent. 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.

Pre-Linnean

The first recorded stramenopile algae in history were the multicellular brown algae, such as kelp and other seaweeds. Their descriptions date back to early China (ca. 3000 BC), Japan (ca. 500 BC) and Greece (300 BC, such as Theophrastus). Knowledge of them likely predates recorded history, as they were potentially used by humans as a source for food, dyes, and medicine. They likely played a role in the early dispersal of humans along seashores, particularly from East Asia to the Americas, where brown algae may have formed a corridor rich in aquatic resources after deglaciation. This is known as the kelp highway hypothesis. The remaining stramenopile algae were not included in historical works, due to being microscopic. , from 1703¹. ]] In the late 17th century, Antony van Leeuwenhoek became the first person to observe microbes, but he did not record microscopic stramenopiles. The first unmistakeable descriptions of diatoms belong to two illustrations published anonymously in England in 1703, 80 years before the first taxonomic description of a diatom. They are attributed to Charles King of Staffordshire.

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