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.

[17] 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.

[10] Ochrophyte algae accumulate chrysolaminarin, a carbohydrate consisting of short chains of β-1,3-linked glucose molecules, as a storage product.

The golden algal genus Hydrurus, in particular, can be widespread in some drainage basins and is common in cold, clear, fast-flowing mountain streams, where it attaches to a firm substrate.

Xanthophycean genera commonly found in rivers include Vaucheria, Tribonema and Bumilleria, either freely floating or attached to filamentous algae and plants.

Others (e.g., Eunotia, Nitzschia) grow stalks or colonial tubes capable of reaching higher into the water column to acquire more nutrients.

These lack any complex multicellular thalli, and instead exist as benthic filamentous forms that have evolved independently from marine ancestors.

Within the class Raphidophyceae, strains of Heterosigma and Chattonella at high concentrations are responsible for fish mortality, although the nature and action of their toxins is not resolved.

Centric diatoms like Chaetoceros live as colonial chains of cells with long spines (setae) that can clog fish gills, causing their death.

More than a dozen species of Pseudonitzschia are capable of producing a neurotoxin, domoic acid, the cause of amnesiac shellfish poisoning.

[23] The ochrophytes constitute a highly diverse clade within Stramenopila, a eukaryotic supergroup that also includes several heterotrophic lineages of protists such as oomycetes, hyphochytrids, labyrinthuleans, opalines and bicosoecids.

[24][4][25] This lineage of stramenopiles originated from an event of secondary endosymbiosis where a red alga was transformed into the chloroplast of the common ancestor of ochrophytes.

[4][26][27] The total group of ochrophytes is estimated to have evolved between 874 and 543 million years ago (Ma) through molecular clock inference.

[28] Xanthophyceae Chrysoparadoxophyceae Phaeosacciophyceae[29] Schizocladiophyceae Phaeophyceae (brown algae) Aurearenophyceae Phaeothamniophyceae Raphidophyceae Actinophryida Olisthodiscophyceae[3] Chrysophyceae (golden algae) Synurophyceae Synchromophyceae/Picophagea Eustigmatophyceae Pinguiophyceae Dictyochophyceae Pelagophyceae Bolidophyceae Diatomeae (diatoms) Actinophryida Pseudofungi Relationships among many classes of ochrophytes remain unresolved, but three main clades (called SI, SII and SIII) are supported in most phylogenetic analyses.

The SI lineage, containing the diverse and multicellular class Phaeophyceae, or brown algae, experienced an evolutionary radiation during the late Paleozoic (around 310 million years ago).

The SII lineage contains the golden algae or Chrysophyceae, as well as smaller classes Synurophyceae, Eustigmatophyceae, Pinguiophyceae and Picophagea (also known as Synchromophyceae).

Although it lacks chloroplasts, plastidial genes have been found in the nuclear genome of this actinophryid, implying that its common ancestor with ochrophytes may have already begun domesticating plastids.

[24] The phylum was first described by protozoologist Thomas Cavalier-Smith in 1986, as Ochrista, later renamed to Ochrophyta in 1996 in accordance to recommendations of the International Code of Nomenclature for algae, fungi, and plants (ICN).

[2][34] It remained a phylum-level taxon until 2017, when the same author lowered it to subphylum level and modified the name to Ochrophytina to match the -phytina suffix in botanical nomenclature, which corresponds to subdivisions.

The phylum to which ochrophytes belong in his classification system is Gyrista, a clade that also contains some heterotrophic heterokonts, namely the Pseudofungi and the Bigyromonada.

[7] The origin of this name is the class Heterokontæ, introduced by Finnish biologist Alexander Ferdinand Luther [fi] in 1899[35] to include the orders Chloromonadales and Confervales, later separated into Xanthophyceae and Raphidophyceae.

After several electron microscopy discoveries, Christiaan van den Hoek introduced in 1978 the division Heterokontophyta for five algal classes: Chrysophyceae, Xanthophyceae, Bacillariophyceae, Phaeophyceae, and Chloromonadophyceae.

The latest revision of the International Society of Protistologists, in 2019, recognizes Ochrophyta as a valid taxon within the higher Stramenopiles group, within the SAR supergroup.