Until recently, the chrysophytes were an algal group with mixed affinities and variable morphologies, united largely by their golden brown pigmentation. However, morphological and genetic studies have progressively removed many of the unaffiliated taxa, leaving two closely related classes, the Chrysophyceae sensu stricto and the Synurophyceae.

Both the classes contain freshwater and marine species, although are considered more abundant and diverse in oligotrophic waters in terrestrial habitats. Cells may be solitary or colonial, having unequal flagella (heterokont), with flagellar swellings on one or both flagella acting as photoreceptors. Taxa covered by siliceous scales are present in both classes, although silicified bristles are only present in some synurophytes. Some chrysophytes may form cellulosic, chitinous, siliceous or calcareous loricae instead of possessing scales. All taxa from both classes produce resting stages known as stomatocysts, with some species forming them during both asexual and sexual phases. Since stomatocysts, and to a lesser extent scales and bristles, are preserved in the underlying sediments, their fossil record is relatively well known, with the oldest stomatocysts found in Early Cretaceous sediments.

Fossil stomatocysts have proven very useful in palaeolimnological studies, particularly when reconstructing past nutrient levels, whereas their marine counterparts are a proxy for past sea‐ice distribution.

Key Concepts:

• Genetic studies have shown that the Chrysophyceae sensu stricto and Synurophyceae are closely related, whereas the other taxa previously included in the Chrysophyceae sensu lato belong to different algal classes.
  
• Chrysophytes and synurophytes are heterokont algae, with a long ‘flimmer’ flagellum and a short ‘whiplash’ flagellum and with flagellar swellings on one or both flagella.
  
• Fucoxanthin is the main pigment responsible for their ‘golden brown’ colouration.
  
• Cells may possess silicified scales (but only synurophytes have bristles) produced by Golgi‐derived vesicles.
  
• Scale and bristle morphology appears to be species‐specific and is thus a key criterion in the identification of the taxa.
  
• Silicified resting stages bearing a plugged pore (stomatocysts) are unique to the Chrysophyceae and Synurophyceae, and may also be species‐specific.
  
• Stomatocysts are often preserved in the underlying sediments, and have been used as palaeoenvironmental indicators.
  
• The oldest stomatocysts were found in Early Cretaceous marine sediments from the Southern Ocean, whereas the oldest silicified scales and bristles were found in Middle Eocene freshwater sediments from northwest Canada.