The Chrysophyta or chrysophytes, They are a highly diverse group of microscopic algae with more than 1,000 species described so far. They are generally found in plaktonic zones, although some of them can be found in the benthic region.
The Chrysophyta division includes three classes: golden algae, yellow-green algae, and diatoms. They are unicellular organisms that can swim freely in freshwater environments, although they can group together and form filamentous structures or colonies..
Most of its representatives are photosynthetic. The most relevant pigments of the group are chlorophylls a and c, beta carotene, fucoxanthin and certain xanthophylls. Pigments with brown hues mask the characteristic green of chlorophyll. However, there are certain species that lack pigments.
Their reproduction is mostly asexual, although there are some species that occasionally reproduce sexually by the union of two gametes..
Regarding nutrition, the group is not considered as truly autotrophic and some biologists prefer to consider them as facultative heterotrophic, since they can consume food particles when there is not enough solar radiation or when food is available in significant quantities.
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Chrysophytic algae are single-celled organisms that live in fresh water. In these medium or low productivity aquatic environments, they constitute a dominant or subdominant portion of the phytoplankton biomass..
They are golden algae, since they present in the chromatophores high concentrations of fucoxanthin, a brown or brown carotenoid pigment that gives them their peculiar coloration. The members of this division show important similarities with the members of the chlorophytes..
Chrysophytes are capable of producing resistance cysts, structures known as statospores or stomatocysts. Its shape is spherical or ellipsoidal, the size ranges from 4 to 20 µm and they are surrounded by a collar.
The fossil record is rich in these statospores, as they are highly resistant to degradation and attack by bacteria. In fact, the record is so good that they are often used as paleoecological indicators and serve to reconstruct ancient environments..
There are fossil records of this group since the Cretaceous, and according to the evidence, they reached their greatest diversity in the Miocene. The fossils are of silica or calcareous deposits.
Chrysophytes are a highly diverse group in terms of the appearance of their members. There are flagellate, plameloid, coccoid, filamentous, and thaloid forms. Each of these will be described below..
Individuals that present flagella to move in the aquatic environment are known as chrysomonates. In addition, they are capable of altering their locomotion mechanism in a notorious way..
The gender Ochromonas, for example, it has a shape reminiscent of a pear, from which two heterogeneous flagella emanate - one is almost six times larger than the other.
These types of uneven flagella are known as heterocontial flagella. Typically the long flagellum has rigid extensions called mastigonemes, giving it a feather-like appearance..
On some occasions the individual can eliminate the flagella and take an amoeboidal shape with the presence of rhizopods. It is common for the amoeba form to develop into a thick-walled cyst. This genus is nutritionally very versatile, and can feed on blue-green algae.
On the other hand, the planktonic form Mallomonas It possesses a silica wall decorated with fine and long needle-shaped structures. It is speculated that these processes may participate in the flotation process of the cell. There are also forms with a single flagellum, the Silicoflagellineae.
These forms are usually very common. The gender Synura it is characterized by forming colonial structures in the planktonic region of aquatic environments. These individuals are similar to those of the genus Mallomonas, mentioned in the previous section, with the exception that these are held together thanks to a substance of gelatinous consistency.
The gender Hydrurus it forms layers in the rocks, with irregular branches and with the gelatinous substance. Finally, in Dinobryon, the cells are elongated and covered with cellulose. Typically found in freshwater and saltwater environments.
Phaeothamnion It is a genus of filamentous algae that normally grows attached to rocks. With respect to the talose forms, they are rare. Among them we can mention Chrysothallus.
Chrysophytes are such a large and variable group that few characteristics are common to all of their individuals..
They are included within a large group called Stramenopiles, whose main characteristic is the structure of the processes that are present in the flagellum. This group also includes the Oomycotes, the feophyte algae, among other protists..
There are other classification systems, such as the Ochrophyta, which purports to include the chrysophyte division. There is no doubt that Chrysophyta is a paraphyletic group, as they share a common ancestor with the Oomycot lineage, which is not included within the chrysophytes..
The Chrysophyta division includes three classes: the Chrysophyceae, which are the golden algae, the Xanthophyceae class, which are the greenish-yellow algae, and the Bacillariophyceae class, commonly known as diatoms..
In most cases, chrysophytes reproduce asexually, by longitudinal fissions (this phenomenon is important in unicellular individuals with flagella).
However, copulation processes have been observed in some flagellates. For example, in the genre Synura there are colonies divided by sex, that is, male or female colonies. Sex cells are indistinguishable from the cells that make up organisms.
Male gametes are able to swim and fuse with female gametes from another colony in isogamic fertilization, since the gametes are identical. In humans, for example, we can differentiate the male gamete, a small and mobile cell thanks to a flagellum, from the female gamete, a large, oval cell..
These algae have an immense diversity of life cycles, indicating a transition between different types exhibiting key adaptations in the evolution of the group. Chrysophytes are organisms widely used in the laboratory to conduct research on how life cycles work at the molecular level..
The cycle begins with the germination of a nonmotile cell from a cyst. Not long after, this cell develops a flagellum that begins to move through the water and generates a sphere with a gelatinous texture, being able to move within it.
By undergoing successive binary longitudinal divisions, cells can feed on bacteria that inhabit the sphere.
The sphere reaches a maximum size of about 500 µm in diameter. At this point the gelatinous substance begins to disintegrate and the cells can escape through the ruptures that are formed..
Cells are grouped into "swarms" of five to forty. In these associations, cells undergo cannibalism events, resulting in giant cells that have the ability to form statospores..
This formation is not affected by environmental conditions or other factors, such as changes in the availability of nutrients or changes in temperatures. The formation of statospores begins with cell division, about 15 or 16 times after germination.
Most chrysophytes are autotrophic, that is, they can obtain energy from sunlight, through photosynthesis. However, some individuals are classified as mixotrophic, since depending on the conditions they can be autotrophagous or phagotrophic..
A phagotrophic organism is capable of capturing food particles from its environment and "engulfing" them with its plasma membrane. They can feed on small organisms such as bacteria and diatoms.
If conditions warrant, the algae stops photosynthesizing and develops extensions in its membrane called pseudopods that allow them to trap their food..
There are chrysophytes that lack any type of pigments and plastids, so they are forced to live a heterotrophic life. They must get their energy source actively, engulfing potential food.
On the other hand, chrysophytes prefer to use as a source of reserve certain fats, and not starch as occurs in green algae.
Chrysophytes have a vital ecological role, as they are important components of plankton. They not only participate as primary producers, they also do so as consumers. They are the main food of many fish and crustaceans.
In addition, they contribute to the flow of carbon in freshwater environments, being essential members in these aquatic ecosystems..
However, they have been little studied organisms due to intrinsic difficulties of the group, mainly due to difficulties in their cultivation and preservation. In addition, there is a tendency to study lakes that have suffered from environmental impact, where chrysophytes are scarce..
In contrast, one species in particular, Prymnesium parvum, it is responsible for the production of toxins that have as a consequence the death of the fish fauna. The algae only has negative effects on aquatic communities, as it appears to be harmless to humans and livestock..
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