The concept of chromists, at its most expansive, includes the heterokonts (stramenopiles), alveolates, rhizarians, heliozoans, telonemians, haptophytes and cryptophytes. There is mounting evidence that this grouping is not valid. Even in the narrowest sense (the heterokonts), chromists include very diverse forms, exhibiting a great variety of trophic mechanisms. This great diversity in form and feeding make it difficult to identify any unifying features, but molecular phylogenetic studies have shown that this group of organisms is indeed monophyletic. The distribution of morphological characters over reconstructed trees allows for the identification of potential synapomorphic characters that have been secondarily lost or modified across the group. These include a combination of mitochondria with tubular cristae; the biflagellate heterokont condition; and, if photosynthetic, then with chlorophyll c, girdle lamellae and four membranes around the chloroplast, the outer continuous with the nuclear envelope. Heterotrophy appears to be ancestral but is also occasionally a derived state from autotrophic forms.

Key Concepts:

• There is no consistency in the ranking of the various eukaryotic taxa, making reference to particular forms and their relationships awkward.
  
• Molecular studies, particularly over the last decade, indicate an ever-increasing delimitation of the Chromista to include very diverse forms.
  
• The chromalveolate theory, at the root of the concept of Chromista, has been fiercely debated and most recent evidence points to multiple independent events involving red algal endosymbionts in diverse eukaryotic hosts.
  
• Even the original grouping of Chromista (heterokonts, haptophytes and cryptophytes) is tenuous, making it more sensible to equate chromists with the heterokonts (=stramenopiles).
  
• Heterokonts enjoy robust support from molecular phylogenetic analyses, but there are no universal morphological and physiological characters.
  
• The most universal character is the biflagellate condition of swimming cells, with one tinsel (hairy) and one smooth flagellum. The hairy flagellum is invested with two opposite rows of tri-partite, tubular hairs that are responsible for reversing thrust.
  
• Plastids of autotrophic heterokonts have consistent features, including two additional surrounding membranes (the periplastidial membrane and the RER), girdle lamellae and thylakoids stacked in groups of three.
  
• The classification within the heterokonts also has a chequered history, but multigene phylogenetic analyses are providing a clearer idea of groupings.
  
• Heterotrophic forms are rooted deeply in phylogenetic trees reconstructed using molecular markers, but some are secondarily derived from autotrophic forms.
  
• Heterokonts play an important role both ecologically and economically.