Golden algae
Encyclopedia
The golden algae or chrysophytes are a large group of algae
, found mostly in freshwater.
The term "chrysophyceae" should not be confused with the term Chrysophyta
, which is more ambiguous.
s and multicellular brown algae
, but since then they have been divided into several different groups based on pigmentation and cell structure. They are now usually restricted to a core group of closely related forms, distinguished primarily by the structure of the flagella
in motile cells, also treated as an order Chromulinales. It is possible membership will be revised further as more species are studied in detail.
Chrysophytes contain the pigment fucoxanthin
. Because of this, they were once considered to be a specialized form of cyanobacteria. Because many of these organisms had a silica capsule, they have a relatively complete fossil record, allowing modern biologists to confirm that they are, in fact, not derived from cyanobacteria, but rather an ancestor that did not possess the capability to photosynthesize. Many of the chrysophyta precursor fossils entirely lacked any type of photosynthesis-capable pigment. Most biologists believe that the chrysophytes obtained their ability to photosynthesis from an endosymbiotic relationship with fucoxanthin-containing cyanobacteria.
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, found mostly in freshwater.
The term "chrysophyceae" should not be confused with the term Chrysophyta
Chrysophyta
Chrysophyta is a term used to refer to certain heterokonts.It can be used to refer to:* golden algae* golden algae and diatoms together....
, which is more ambiguous.
Members
Originally they were taken to include all such forms except the diatomDiatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s and multicellular brown algae
Brown algae
The Phaeophyceae or brown algae , is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments, both as food and for the habitats they form...
, but since then they have been divided into several different groups based on pigmentation and cell structure. They are now usually restricted to a core group of closely related forms, distinguished primarily by the structure of the flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
in motile cells, also treated as an order Chromulinales. It is possible membership will be revised further as more species are studied in detail.
Traits, locomotion, and classification
They come in a variety of morphological types, originally treated as separate orders or families.- Most members are unicellular flagellateFlagellateFlagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
s, with either two visible flagella, as in Ochromonas, or sometimes one, as in Chromulina. The ChromulinalesChromulinalesChromulinales is an order of Chrysophyceae....
as first defined by Pascher in 1910 included only the latter type, with the former treated as the order Ochromonadales. However, structural studies have revealed that short second flagellum or at least a second basal body is always present, so this is no longer considered a valid distinction. Most of these have no cell covering. Some have loricae or shells, such as DinobryonDinobryonDinobryon is a type of microscopic algae....
, which is sessile and grows in branched colonies. Most forms with silicaceous scales are now considered a separate group, the synuridSynuridThe synurids are a small group of heterokont algae, found mostly in freshwater.-Characteristics:They are covered in silicate scales and spines. These are formed on the surface of the chloroplasts, of which there are usually two, but sometimes only one divided into two lobes...
s, but a few belong among the Chromulinales proper, such as Paraphysomonas.
- Some members are generally amoeboidAmoeboidAmoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
, with long branching cell extensions, though they pass through flagellate stages as well. Chrysamoeba and Rhizochrysis are typical of these. There is also one species, Myxochrysis paradoxa, which has a complex life cycle involving a multinucleateMultinucleateMultinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...
plasmodial stage, similar to those found in Slime molds. These were originally treated as the order Chrysamoebales. The superficially similar RhizochromulinaRhizochromulinaRhizochromulina is an unusual genus of marine heterokont algae, with one species, R. marina. They are colored amoeboids with a single flagellum, and produce distinctive spindle-shaped zoospores. These have a cell structure typical of the axodines...
was once included here, but is now given its own order based on differences in the structure of the flagellate stage.
- Other members are non-motile. Cells may be naked and embedded in mucilageMucilageMucilage is a thick, gluey substance produced by most plants and some microorganisms. It is a polar glycoprotein and an exopolysaccharide.It occurs in various parts of nearly all classes of plant, usually in relatively small percentages, and is frequently associated with other substances, such as...
, such as Chrysosaccus, or coccoid and surrounded by a cell wall, as in Chrysosphaera. A few are filamentous or even parenchymaParenchymaParenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
tous in organization, such as Phaeoplaca. These were included in various older orders, most of the members of which are now included in separate groups. Hydrurus and its allies, freshwater genera which form branched gelatinous filaments, are often placed in the separate order Hydrurales but may belong here.
Evolution
Fucoxanthin
Fucoxanthin is a xanthophyll, with formula C42H58O6. It is found as an accessory pigment in the chloroplasts of brown algae and most other heterokonts, giving them a brown or olive-green color...
. Because of this, they were once considered to be a specialized form of cyanobacteria. Because many of these organisms had a silica capsule, they have a relatively complete fossil record, allowing modern biologists to confirm that they are, in fact, not derived from cyanobacteria, but rather an ancestor that did not possess the capability to photosynthesize. Many of the chrysophyta precursor fossils entirely lacked any type of photosynthesis-capable pigment. Most biologists believe that the chrysophytes obtained their ability to photosynthesis from an endosymbiotic relationship with fucoxanthin-containing cyanobacteria.