Ochrophyte

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Ochrophytes
Temporal range: Middle Proterozoic[1] 1000–0 Ma
Dense kelp forest with understorey at Partridge Point near Dave's Caves, Cape Peninsula
Dense kelp forest with understory at Partridge Point near Dave's Caves, Cape Peninsula
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Diaphoretickes
Clade: SAR
Clade: Stramenopiles
Phylum: Gyrista
Subphylum: Ochrophytina
Cavalier-Smith 1986 emend. 1996[2]
Type genus
Linnaeus
, 1753
Classes[4]

Incertae sedis:

  • Actinophryida
Diversity
23,314 described species[5]
>100,000 estimated species[6]
Synonyms

Ochrophytes, also known as heterokontophytes or stramenochromes, are a group of

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 or Heterokontophyta, or as subphylum Ochrophytina within phylum Gyrista. Their plastids are of red algal
origin.

Description

Ochrophytes are

chloroplasts and pigments.[10]

mitochondria (in orange); n, nucleus (in purple, nucleolus in darker purple); p, plastid (stroma in light green, thylakoids in dark green); pc, periplastidial compartment (in pink); per, periplastidial endoplasmic reticulum (in blue); pf, posterior flagellum; v, vacuole
.

Flagella

As stramenopiles (=heterokonts), their swimming cells frequently display two markedly unequal flagella: an anterior flagellum ("tinsel") with straw-like hollow tripartite hairs called

ciliary transition zone of the flagellum generally has a transitional helix.[7]

Chloroplasts

The ochrophytes are mostly photosynthetic. As such, they may possess one or more photosynthetic

archaeplastid eukaryotes, whose chloroplasts have only two membranes.[13][page needed][18] The two outer layers of ochrophyte plastids are continguous with the endoplasmic reticulum (ER), together composing the chloroplast endoplasmic reticulum (CER),[14] 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.[10]

The periplastid compartment (PC), between the second and third layers, is a separate region that in other algal groups (i.e.

chromerid algae:[14] "blob-like structures" where PC proteins are localized, and a vesicular network.[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.[17]

Commonly, within the plastid

synchromophytes and aurearenophytes, a consortium of several plastids, each surrounded by two or three inner membranes respectively, is enveloped by a shared outer membrane.[14]

Pigmentation

Chemical structure of fucoxanthin
Chemical structure of fucoxanthin

Ochrophyte chloroplasts contain

photosystems from high intensity light.[10]

Storage products

Ochrophyte algae accumulate chrysolaminarin, a carbohydrate consisting of short chains of β-1,3-linked glucose molecules, as a storage product.[10][19] It is stored in vesicles located within the cytoplasm, outside plastids, unlike other algae.[13] Cytoplasmic lipid droplets are also common.[10] They lack starch, which is the common storage product in green algae and plants.[7]

Reproduction

Ochrophytes are capable of

gametes, by three different modes: isogamy, anisogamy or oogamy.[7]

Ecology

Ochrophytes are present in nearly all environments.

mixotrophic, usually through phagocytosis.[19]

Marine

Several classes of heterokont algae are exclusively known from marine habitats, such as

Pinguiophyceae and Schizocladiophyceae. The brown algae (Phaeophyceae) are almost exclusively marine, with very few freshwater genera.[19]

Freshwater

benthic filamentous forms that have evolved independently from marine ancestors.[22]

Harmful algae

Two main lineages of photosynthetic stramenopiles include many toxic species. Within the class

Pseudonitzschia. More than a dozen species of Pseudonitzschia are capable of producing a neurotoxin, domoic acid, the cause of amnesiac shellfish poisoning.[23]

Evolution

External

The ochrophytes constitute a highly

secondary endosymbiosis where a red alga was incorporated into the chloroplast of the common ancestor of ochrophytes.[4][26][27]

The

chrysophyte scales, and valves resembling the modern centric diatom valves, have been found in 800–700 million-years-old sediments.[28]

Internal

 Ochrophyta 
 
Chrysista
 
 SI 

Schizocladiophyceae

Phaeophyceae
(brown algae)

Aurearenophyceae

Phaeothamniophyceae

Raphidophyceae

 ? 

Actinophryida

 SII 

Olisthodiscophyceae[3]

Chrysophyceae
(golden algae)

Synurophyceae

Synchromophyceae/Picophagea

Eustigmatophyceae

Pinguiophyceae

 
Diatomista
/SIII 

Dictyochophyceae

Pelagophyceae

Bolidophyceae

Diatomeae
(diatoms)

 ? 

Actinophryida

Pseudofungi

Evolutionary relationships between all ochrophyte classes based on the latest phylogenetic analyses,[30][27][29][3] and the approximate number of species in each class.[4]

Relationships among many classes of ochrophytes remain unresolved, but three main clades (called SI, SII and SIII) are supported in most

Olisthodiscophyceae, was described in 2021 and recovered as part of the SII lineage.[3]

One group of heterotrophic

nuclear genome of this actinophryid, implying that its common ancestor with ochrophytes may have already begun domesticating plastids.[33]

Systematics

Taxonomic history

In hierarchical classifications, where

monophyletic nature.[25]

While Ochrophyta is the preferred name by general

Michael Guiry, Øjvind Moestrup and Robert Andersen validly published Heterokontophyta as a phylum in 2023.[7]

As opposed to the hierarchical classification, the

Diatomista is fully accepted by the scientific community and backed up by phylogenetic analyses.[25]

Classification

As of 2024, ochrophytes amount to 23,314 described species, with 490 species of uncertain position.[5] However, it is estimated that they amount to more than 100,000 species, of which the majority are diatoms.[6] Below is the present classification of ochrophytes according to the 2019 revision of eukaryotic classification,[25] with the inclusion of classes of algae described in posterior years[15][29][3] as well as the number of described species for each class.[5] According to the aforementioned 2019 revision by protistologists, the diatoms (Diatomeae) do not form a single class Bacillariophyceae, but numerous classes to reflect the phylogenetic advances over the previous decade.[25]

Dinobryon (Chrysophyceae)
Pelvetiopsis (Phaeophyceae)

History of knowledge

Multicellular

animals under the name of infusoria.[19]

During the 20th century, evolutionary and phylogenetic discussions began including heterokont algae.

molecular phylogenetic analysis led to the description of many new groups and several classes well into the 21st century. The sequencing of the first ochrophyte genome, belonging to Thalassiosira pseudonana, began in 2002.[19]

Notes

  1. ^ The only known exception is Chrysoparadoxa, which contains chloroplasts surrounded by two membranes as opposed to four.[15][16]

References

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  8. ^ Cavalier-Smith, T. (1986). The kingdom Chromista, origin and systematics. In: Round, F.E. and Chapman, D.J. (eds.). Progress in Phycological Research. 4: 309–347.
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External links

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