Golden algae

Lore & Background

Originally, the Chrysophyceae were taken to include all such forms of diatoms and multicellular brown algae, but they have since been divided into several groups (e.g., Haptophyceae, Synurophyceae) based on pigmentation and cell structure. Some heterotrophic flagellates such as bicosoecids and choanoflagellates were sometimes seen as related. The class is now usually restricted to a core group distinguished primarily by flagellar structure in motile cells, and membership may be revised further as more species are studied. The Chrysophyceae have been placed by some in the polyphyletic Chromista, but the broader monophyletic group to which they belong is the Stramenopiles.

Reader's Guide

The Chrysophyceae are significant for studies of food web dynamics in oligotrophic freshwater ecosystems and for assessment of environmental degradation resulting from eutrophication and acid rain. They contain the pigment fucoxanthin, and because of this were once considered a specialized form of cyanobacteria. However, many chrysophyte precursor fossils entirely lacked any photosynthesis-capable pigment, and the most primitive stramenopiles are regarded as heterotrophic. The ancestors of the Chrysophyceae were likely heterotrophic flagellates that obtained photosynthesis from an endosymbiotic relationship with fucoxanthin-containing cyanobacteria. Their siliceous cysts provide a relatively complete fossil record, allowing modern biologists to confirm they are not derived from cyanobacteria but from a non-photosynthetic ancestor. Classification of the group has varied widely among authors, with different orders recognized by Pascher, Smith, Bourrely, Starmach, Kristiansen, Margulis, van den Hoek, Preisig, and Guiry and Guiry, reflecting ongoing uncertainty about membership and relationships.

Did You Know?

• The Chrysophyceae produce an endogenous siliceous cyst called a statospore, stomatocyst, or statocyst, which is usually globose with a single pore.
• Most members are unicellular flagellates with either two visible flagella (e.g., Ochromonas) or sometimes one (e.g., Chromulina), but a short second flagellum or second basal body is always present.
• Some members are amoeboid with long branching cell extensions, and one species, Myxochrysis paradoxa, has a complex life cycle with a multinucleate plasmodial stage similar to slime molds.
• The ancestors of the Chrysophyceae were likely heterotrophic flagellates that obtained photosynthesis from an endosymbiotic relationship with fucoxanthin-containing cyanobacteria.