Golden algae

Golden algae
Dinobryon divergens, a tree like sessile form with cells in the cup-like shells
Scientific classification
Domain:	Eukarya
Kingdom:	Chromalveolata
Phylum:	Heterokontophyta
Class:	Chrysophyceae
Orders
Chromulinales Chrysamoebales Ochromonadales Chrysosphaerales Hibberdiales Hydrurales Phaeothamniales

The golden algae or chrysophytes are a large group of algae, found mostly in freshwater.^[1] Golden algae is also commonly used to refer to a single species, Prymnesium parvum, which causes fish kills.^[2]

The term "chrysophyceae" should not be confused with the term Chrysophyta, which is more ambiguous.

Members

Originally they were taken to include all such forms except the diatoms and multicellular brown algae, but since then they have been divided into several different groups (e.g., Haptophyceaes,^[3] Synurophyceae) 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.

Traits, locomotion, and classification

A "primary" cell contains two specialized flagella. The active, "feathered" (with mastigonemes) flagellum is oriented toward the moving direction. The smooth passive flagellum, oriented toward the opposite direction, may be present only in rudimentary form in some species.

• Most members are unicellular flagellates, with either two visible flagella, as in Ochromonas, or sometimes one, as in Chromulina. The Chromulinales 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 Dinobryon, which is sessile and grows in branched colonies. Most forms with silicaceous scales are now considered a separate group, the synurids, but a few belong among the Chromulinales proper, such as Paraphysomonas.

• Some members are generally amoeboid, 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 multinucleate plasmodial stage, similar to those found in slime molds. These were originally treated as the order Chrysamoebales. The superficially similar Rhizochromulina 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 mucilage, such as Chrysosaccus, or coccoid and surrounded by a cell wall, as in Chrysosphaera. A few are filamentous or even parenchymatous 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

Chrysophytes contain the pigment fucoxanthin.^[4] Because of this, they were once considered to be a specialized form of cyanobacteria.^{[citation needed]} 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 photosynthesize from an endosymbiotic relationship with fucoxanthin-containing cyanobacteria.

References

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