| Ochrophytes Temporal range: Middle Proterozoic[1]1000–0 Ma | |
|---|---|
 | |
| Densekelp forest withunderstory atPartridge Point nearDave's Caves,Cape Peninsula | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | SAR |
| Clade: | Stramenopiles |
| Phylum: | Gyrista |
| Subphylum: | Ochrophytina Cavalier-Smith 1986 emend. 1996[2] |
| Type genus | |
| Fucus Linnaeus, 1753 | |
| Classes[4] | |
| Diversity | |
| 23,314 described species[5] >100,000 estimated species[6] | |
| Synonyms | |
Ochrophytes, also known asheterokontophytes orstramenochromes, are a group ofalgae. They are thephotosyntheticstramenopiles, a group ofeukaryotes, organisms with acell nucleus, characterized by the presence of two unequalflagella, one of which has tripartite hairs calledmastigonemes. In particular, they are characterized by photosyntheticorganelles orplastids enclosed by fourmembranes, with membrane-bound compartments calledthylakoids organized in piles of three,chlorophylla andc as theirphotosynthetic pigments, and additional pigments such asβ-carotene andxanthophylls. Ochrophytes are one of the most diverse lineages of eukaryotes, containing ecologically important algae such asbrown algae anddiatoms. They are classified either as phylumOchrophyta orHeterokontophyta, or as subphylumOchrophytina within phylumGyrista. Their plastids are ofred algal origin.
Ochrophytes areeukaryotic organisms composed ofcells that are either naked or covered by scales,lorica or acell wall. They can besingle-celled,colonial,coenocytic ormulticellular. SomePhaeophyceae (brown algae, seaweeds) develop as large multicellularthalli withdifferentiated tissues.[7] All ochrophytes uniformly have tubularmitochondrial cristae.[10] This is a common trait shared with their relatives,heterotrophicstramenopiles, as well as other closely related groups such asRhizaria,Telonemia andAlveolata.[11][12] As primarilyphotosynthetic eukaryotes, they are consideredalgae, distinguished from other groups of algae by specificmorphological andultrastructural traits, such as theirflagella,chloroplasts andpigments.[10]
As stramenopiles (=heterokonts), their swimming cells frequently display two markedly unequal flagella: an anterior flagellum ("tinsel") with straw-like hollow tripartite hairs calledmastigonemes, and an immature posterior smooth flagellum ("whiplash") lacking these hairs.[13][10] Theciliary transition zone of the flagellum generally has a transitional helix.[7]
The ochrophytes are mostly photosynthetic. As such, they may possess one or more photosyntheticplastids (chloroplasts) per cell.[14] Some groups contain species withleucoplasts, chloroplasts that have lost photosynthetic capacity and pigments but presumably continue to play a role in the synthesis ofamino acids,lipids andheme groups.[10] Ochrophytes have a distinct plastidultrastructure in comparison to other algal groups.[14] Their chloroplasts originate from an event of secondary endosymbiosis from ared alga, which lead to four[a] surroundingmembranes: two inner membranes, corresponding to the primary plastid membranes; a third membrane, corresponding to theplasma membrane of the red alga; and an outermost layer, corresponding to thephagosome membrane.[17] This characteristic differentiates them fromarchaeplastid algae (glaucophytes, red algae andgreen algae), whose chloroplasts have only two membranes.[18] The two outer layers of ochrophyte plastids are continguous with theendoplasmic reticulum (ER), together composing the chloroplast endoplasmic reticulum (CER),[14] also known as the periplastidial endoplasmic reticulum (PER), which is often connected to thenuclear 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.cryptomonads andchlorarachniophytes) contains anucleomorph, the vestigialnucleus 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 inhaptophytes andchromerid 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 plastidstroma, three stackedthylakoids differentiate into the "girdle lamella", which runs around the periphery of the plastid, beneath the innermost membrane.[14] The remaining thylakoids are arranged in stacks of three.[10] Insynchromophytes andaurearenophytes, a consortium of several plastids, each surrounded by two or three inner membranes respectively, is enveloped by a shared outer membrane.[14]
Ochrophyte chloroplasts containchlorophyllsa andc asphotosynthetic pigments, in addition tofucoxanthin.[13] Chlorophylla binds to thylakoids, while thec pigment is present in the stroma.[10] The most frequentaccessory pigment in ochrophytes is the yellowβ-carotene. The golden-brown or brown pigmentation indiatoms,brown algae,golden algae and others is conferred by thexanthophyll fucoxanthin. In the yellow-green or yellow-brownraphidophyceans,eustigmatophyceans andxanthophyceans,vaucheriaxanthin is dominant instead. These pigment combinations extend their photosynthetic ability beyond chlorophylla alone. Additionally, xanthophylls protect thephotosystems from high intensity light.[10]
Ochrophyte algae accumulatechrysolaminarin, acarbohydrate consisting of short chains of β-1,3-linkedglucose molecules, as a storage product.[10][19] It is stored invesicles located within thecytoplasm, outside plastids, unlike other algae.[13] Cytoplasmiclipid droplets are also common.[10] They lackstarch, which is the common storage product ingreen algae and plants.[7]
Ochrophytes are capable ofasexual reproduction byfragmentation,propagules,vegetativecell division,sporogenesis or zoosporogenesis. In addition, they are capable ofsexual reproduction throughgametes, by three different modes:isogamy,anisogamy oroogamy.[7]
Ochrophytes are present in nearly all environments.[19] Some classes are more common in marine habitats, while others are more frequent in freshwater or soil.[10] Among the ochrophyte lineages are the diatoms, the most abundant photosynthetic eukaryotes worldwide in marine habitats; multicellular seaweeds, such as brown algae (e.g.,kelp) and golden algae; and an array of microscopic single-celled lineages that are also abundant, as evidenced byenvironmental sequencing.[14] Regarding nutrition, various ochrophytes aremixotrophic, usually throughphagocytosis.[19]
Several classes of heterokont algae are exclusively known from marine habitats, such asBolidophyceae,Pelagophyceae,Pinguiophyceae andSchizocladiophyceae. The brown algae (Phaeophyceae) are almost exclusively marine, with very few freshwater genera.[19]
Chrysophyceae,Phaeothamniophyceae andXanthophyceae are predominantly freshwater classes. Inlotic habitats (rivers, streams), golden algae (Chrysophyceae) and yellow-green algae (Xanthophyceae) are common and occasionally abundant. The golden algalgenusHydrurus, in particular, can be widespread in somedrainage basins and is common in cold, clear, fast-flowing mountain streams, where it attaches to a firm substrate. Xanthophycean genera commonly found in rivers includeVaucheria,Tribonema andBumilleria, either freely floating or attached to filamentous algae and plants.[20] Diatoms are more diverse, with more than 60 genera commonly found in rivers. Many river diatoms have developed different strategies to attach to the substrate to avoid being displaced by water currents. The most basic strategy is to produceextracellular polymeric substances, varied carbohydrate structures formed from the cell membrane. In faster-flowing waters, some diatoms (e.g.,Cocconeis) grow directly attached to the substrate through adhesive films. Others (e.g.,Eunotia,Nitzschia) grow stalks or colonial tubes capable of reaching higher into the water column to acquire more nutrients.[21] Brown algae (Phaeophyceae), although highly diversified, contain only sevenspecies present in rivers. These lack any complex multicellular thalli, and instead exist asbenthic filamentous forms that have evolved independently frommarine ancestors.[22]
Two main lineages of photosynthetic stramenopiles include many toxic species. Within the classRaphidophyceae, strains ofHeterosigma andChattonella at high concentrations are responsible for fish mortality, although the nature and action of their toxins is not resolved. FreshwaterGonyostomum species are capable of mucilage secretion at high amounts detrimental to fish gills. Within the diatoms (Bacillariophyta), harmful effects can be due to physical damage or to toxin production. Centric diatoms likeChaetoceros live as colonial chains of cells with long spines (setae) that can clog fish gills, causing their death. Among diatoms, the only toxin producers have been found among pennate diatoms, almost entirely within the genusPseudonitzschia. More than a dozen species ofPseudonitzschia are capable of producing aneurotoxin,domoic acid, the cause of amnesiac shellfish poisoning.[23]
The ochrophytes constitute a highlydiverseclade withinStramenopila, aeukaryotic supergroup that also includes several heterotrophic lineages ofprotists such asoomycetes,hyphochytrids,labyrinthuleans,opalines andbicosoecids.[24][4][25] This lineage of stramenopiles originated from an event ofsecondary endosymbiosis where a red alga was transformed into the chloroplast of the common ancestor of ochrophytes.[4][26][27]
Thetotal group of ochrophytes is estimated to have evolved between 874 and 543 million years ago (Ma) throughmolecular clock inference. However, the earliest fossil remains, assigned to the billion-year-oldxanthophytePalaeovaucheria,[1] suggest that ochrophytes had appeared by 1000 Ma. Other early putative representatives of photosynthetic stramenopiles areJacutianema (750 Ma),Germinosphaera (750–700 Ma) and thebrown algaMiaohephyton (600–550 Ma). Scales similar to modernchrysophyte scales, and valves resembling the modern centric diatom valves, have been found in 800–700 million-years-old sediments.[28]
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| 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 mostphylogenetic analyses. The SI lineage, containing the diverse and multicellular classPhaeophyceae, or brown algae, experienced anevolutionary radiation during the latePaleozoic (around 310 million years ago). The classSchizocladiophyceae is the sister lineage to brown algae, followed by a clade of closely related classesXanthophyceae,Phaeosacciophyceae[29] andChrysoparadoxophyceae.[15] This is in turn the sister lineage to a clade containingAurearenophyceae andPhaeothamniophyceae,[4] which are sometimes treated as one class Aurophyceae.[27] TheRaphidophyceae are the most basal within the SI. The SII lineage contains the golden algae orChrysophyceae, as well as smaller classesSynurophyceae,Eustigmatophyceae,Pinguiophyceae andPicophagea (also known as Synchromophyceae). Both clades, SI and SII, compose theChrysista lineage. The remaining classes are grouped within the sister lineageDiatomista, equivalent to the SIII lineage; these are thediatoms or Bacillariophyceae,Bolidophyceae,Dictyochophyceae (including thesilicoflagellates) andPelagophyceae.[4] A new class of algae,Olisthodiscophyceae, was described in 2021 and recovered as part of the SII lineage.[3]
One group of heterotrophicheliozoan protists,Actinophryida,[31] is included in some classifications as the sister lineage to theraphidophytes, and both groups are treated as one classRaphidomonadea on the basis of18S rDNA phylogenetic analyses.[32] However, a recentphylogenomic study places one actinophryid,Actinophrys sol, as the probablesister group to ochrophytes. Although it lacks chloroplasts, plastidial genes have been found in thenuclear genome of this actinophryid, implying that its common ancestor with ochrophytes may have already begun domesticating plastids.[33]
In hierarchical classifications, wheretaxonomic ranks (kingdom,phylum,class,order...) are utilized, the heterokont algae are commonly regarded as an entire phylum (or division inbotanical nomenclature) by the name ofOchrophyta, within the Stramenopila orHeterokonta.[24] The phylum was first described byprotozoologistThomas Cavalier-Smith in 1986, asOchrista, later renamed to Ochrophyta in 1996 in accordance to recommendations of theInternational 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 toOchrophytina to match the-phytina suffix in botanical nomenclature, which corresponds to subdivisions. The phylum to which ochrophytes belong in his classification system isGyrista, a clade that also contains some heterotrophic heterokonts, namely thePseudofungi and theBigyromonada.[27] Gyrista andBigyra compose the two main branches of stramenopiles, which are regarded as the superphylumHeterokonta within the kingdomChromista. However, this classification system is in disuse due to the kingdom's non-monophyletic nature.[25]
While Ochrophyta is the preferred name by generalprotistologists and protozoologists, the nameHeterokontophyta is considered more familiar amongphycologists.[7] The origin of this name is the classHeterokontæ, introduced by Finnish biologistAlexander Ferdinand Luther [fi] in 1899[35] to include the ordersChloromonadales andConfervales, later separated intoXanthophyceae andRaphidophyceae. This name referenced, among other traits, the two unequal flagella characteristic of allStramenopiles, also known as heterokonts. After severalelectron microscopy discoveries, Christiaan van den Hoek introduced in 1978 the divisionHeterokontophyta for five algal classes:Chrysophyceae,Xanthophyceae,Bacillariophyceae,Phaeophyceae, andChloromonadophyceae.[36] Several other names were used to group heterokont algae withambiregnal organisms, such asChromophyta,Heterokonta and lastlyStramenopiles, which is not a validly published name under theICN.[7] Several phycologists currently advocate the use of Heterokontophyta as the phylum name for heterokont algae. However, the original use by Hoek in 1978 did not provide aLatin description, which was a requirement for valid publication under the ICN until 2011. PhycologistsMichael Guiry,Øjvind Moestrup and Robert Andersen validly published Heterokontophyta as a phylum in 2023.[7]
As opposed to the hierarchical classification, thecladistic classification only recognizesclades as valid groups, rejecting the use ofparaphyletic orpolyphyletic groups. This method of classification is preferred among protistologists. The latest revision of the International Society of Protistologists, in 2019, recognizes Ochrophyta as a valid taxon within the higherStramenopiles group, within theSAR supergroup.[25] The subdivision of ochrophytes betweenChrysista andDiatomista is fully accepted by the scientific community and backed up by phylogenetic analyses.[25]
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]
Multicellularseaweeds, in the classPhaeophyceae, were described in early Chinese (around 3000 BC), Greek (300 BC, such asTheophrastus) and Japanese (ca. 500 AD) writings. Knowledge of them likely predates recorded history, being used asfood,dyes, and for medicinal purposes. The first formal description of a stramenopile alga was that of the genusFucus, byLinnaeus in his 1753 workSpecies Plantarum. Shortly after, unicellularchrysophytes were described byOtto Friedrich Müller. During this first era of scientific discovery, brown algae were described asplants, while microscopic algae were treated asanimals under the name ofinfusoria.[19]
During the 20th century, evolutionary and phylogenetic discussions began including heterokont algae.Transmission electron microscopy andmolecular 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 toThalassiosira pseudonana, began in 2002.[19]
