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 . In particular, they are characterized by photosynthetic or enclosed by four cell membrane, with membrane-bound compartments called organized in piles of three, chlorophyll a and c as their photosynthetic pigments, and additional pigments such as β-carotene and . Ochrophytes are one of the most diverse lineages of eukaryotes, containing ecologically important algae such as brown algae and . They are classified either as phylum Ochrophyta, Heterokontophyta or as subphylum Ochrophytina withing phylum Gyrista. Their plastids are of Red algae origin.
Description
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 (brown algae, seaweeds) develop as large multicellular
thalli with differentiated tissues.
All ochrophytes uniformly have tubular
crista.
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
.
Flagella
As stramenopiles (=
heterokonts), their swimming cells frequently display two markedly unequal flagella: an anterior flagellum ("tinsel") with straw-like hollow tripartite hairs called
, and an immature posterior smooth flagellum ("whiplash") 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
(
) per cell.
Some groups contain species with
, 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
cell membrane: 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 (
, red algae and
green algae), whose chloroplasts have only two membranes.
The two outer layers of ochrophyte plastids are continguous with the endoplasmic reticulum (ER), together composing the chloroplast endoplasmic reticulum (CER),
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.
The periplastid compartment (PC), between the second and third layers, is a separate region that in other algal groups (i.e. and chlorarachniophytes) contains a nucleomorph, the vestigial cell 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 and chromerid algae:
Commonly, within the plastid stroma, three stacked differentiate into the "girdle lamella", which runs around the periphery of the plastid, beneath the innermost membrane.
Pigmentation
Ochrophyte chloroplasts contain
chlorophylls chlorophyll a and
chlorophyll 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
beta-carotene. The golden-brown or brown pigmentation in
,
brown algae,
golden algae and others is conferred by the
xanthophyll fucoxanthin. In the yellow-green or yellow-brown
, eustigmatophyceans and
,
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
are also common.
They lack
starch, which is the common storage product in
green algae and plants.
Reproduction
Ochrophytes are capable of asexual reproduction by fragmentation,
, vegetative
cell division,
sporogenesis or zoosporogenesis. In addition, they are capable of sexual reproduction through
gametes, by three different modes:
isogamy,
anisogamy or
oogamy.
Ecology
Ochrophytes are present in nearly all environments.
Some classes are more common in marine habitats, while others are more frequent in freshwater or soil.
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 by environmental sequencing.
Regarding nutrition, various ochrophytes are
mixotrophic, usually through
phagocytosis.
Marine
Several classes of heterokont algae are exclusively known from marine habitats, such as
Bolidophyceae,
Pelagophyceae,
Pinguiophyceae and Schizocladiophyceae. The brown algae (Phaeophyceae) are almost exclusively marine, with very few freshwater genera.
Freshwater
Chrysophyceae, Phaeothamniophyceae and
Xanthophyceae are predominantly freshwater classes. In
lotic habitats (rivers, streams), golden algae (Chrysophyceae) and yellow-green algae (Xanthophyceae) are common and occasionally abundant. The golden algal
genus Hydrurus, in particular, can be widespread in some
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.
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 produce extracellular 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.
Brown algae (Phaeophyceae), although highly diversified, contain only seven
species present in rivers. These lack any complex multicellular thalli, and instead exist as
benthic filamentous forms that have evolved independently from
marine life ancestors.
Harmful algae
Two main lineages of photosynthetic stramenopiles include many toxic species. 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. Freshwater
Gonyostomum 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 like
Chaetoceros 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 genus
Pseudonitzschia. More than a dozen species of
Pseudonitzschia are capable of producing a
neurotoxin,
domoic acid, the cause of amnesiac shellfish poisoning.
Evolution
External
The ochrophytes constitute a highly
biodiversity clade within
Stramenopila, a eukaryotic supergroup that also includes several heterotrophic lineages of
protists such as
oomycetes,
hyphochytrids,
,
opalines and
.
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.
The total group of ochrophytes is estimated to have evolved between 874 and 543 million years ago (Ma) through molecular clock inference. However, the earliest fossil remains, assigned to the billion-year-old xanthophyte Palaeovaucheria,
Internal
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 class Schizocladiophyceae is the sister lineage to brown algae, followed by a clade of closely related classes
Xanthophyceae, Phaeosacciophyceae
and Chrysoparadoxophyceae.
This is in turn the sister lineage to a clade containing
Aurearenophyceae and Phaeothamniophyceae,
which are sometimes treated as one class Aurophyceae.
The
Raphidophyceae are the most basal within the SI. The SII lineage contains the golden algae or
Chrysophyceae, as well as smaller classes
Synurophyceae, Eustigmatophyceae,
Pinguiophyceae and
Picophagea (also known as Synchromophyceae). Both clades, SI and SII, compose the
Chrysista lineage. The remaining classes are grouped within the sister lineage
Diatomista, equivalent to the SIII lineage; these are the
diatoms or Bacillariophyceae,
Bolidophyceae,
Dictyochophyceae (including the
) and
Pelagophyceae.
A new class of algae, Olisthodiscophyceae, was described in 2021 and recovered as part of the SII lineage.
One group of heterotrophic protists, Actinophryida,
Systematics
Taxonomic history
In hierarchical classifications, where
(kingdom,
phylum, class, order...) are utilized, the heterokont algae are commonly regarded as an entire phylum (or division in botanical nomenclature) by the name of
Ochrophyta, within the Stramenopila or
Heterokonta.
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).
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.
Gyrista and
Bigyra compose the two main branches of stramenopiles, which are regarded as the superphylum
Heterokonta within the kingdom
Chromista. However, this classification system is in disuse due to the kingdom's non-
monophyletic nature.
While Ochrophyta is the preferred name by general and protozoologists, the name Heterokontophyta is considered more familiar among .
As opposed to the hierarchical classification, the cladistic classification only recognizes clades as valid groups, rejecting the use of paraphyletic or polyphyletic 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 higher Stramenopiles group, within the SAR supergroup.
Classification
As of 2024, ochrophytes amount to 23,314 described species, with 490 species of uncertain position.
However, it is estimated that they amount to more than 100,000 species, of which the majority are diatoms.
Below is the present classification of ochrophytes according to the 2019 revision of eukaryotic classification,
with the inclusion of classes of algae described in posterior years
as well as the number of described species for each class.
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.
History of knowledge
Multicellular
, in the class
Phaeophyceae, were described in early Chinese (around 3000 BC), Greek (300 BC, such as
Theophrastus) and Japanese (ca. 500 AD) writings. Knowledge of them likely predates recorded history, being used as
food,
, and for medicinal purposes. The first formal description of a stramenopile alga was that of the genus
Fucus, by
Linnaeus in his 1753 work
Species Plantarum. Shortly after, unicellular
were described by Otto Friedrich Müller. During this first era of scientific discovery, brown algae were described as
plants, while microscopic algae were treated as
animals under the name of
infusoria.
During the 20th century, evolutionary and phylogenetic discussions began including heterokont algae. Transmission electron microscopy and 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.
Notes
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