The heterokonts or stramenopiles are a major line of eukaryotes. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton. However some are colorless, most notably the parasitic water moulds, which superficially resemble fungi. The exact circumscription and treatment of the group varies considerably.

Chloroplasts

Heterokont algae have chloroplasts surrounded by four membranes, the last of which is continuous with the endoplasmic reticulum. These suggest that they were derived from a symbiotic eukaryote, presumably a red alga. The chloroplasts characteristically contain chlorophyll a and c, and usually the accessory pigment fucoxanthin, giving them a golden-brown or brownish-green color.

Most basal heterokonts are colorless, suggesting they branched off before the appearance of chloroplasts within the group. However, fucoxanthin-containing chloroplasts are also found among the haptophytes, and there is some evidence the two groups share a common origin, and possibly the cryptomonads as well. In that case the ancestral heterokont was an alga, and all colorless groups arose through chloroplast loss.

Motile cells

Many heterokonts are unicellular flagellates, and most others produce flagellate cells at some point in their life-cycle, for instance as gametes or zoospores. The name heterokont refers to the characteristic form of these cells, which typically have two unequal flagella. The anterior or tinsel flagellum is covered with lateral bristles or mastigonemes, while the other flagellum is whiplash, smooth and usually shorter, or sometimes reduced to a basal body. The flagella are inserted subapically or laterally, and are usually supported by four microtubule roots in a distinctive pattern.

Mastigonemes are manufactured from glycoproteins in the cell's endoplasmic reticulum before being transported to its surface. When the tinsel flagellum moves, these create a backwards current, pulling the cell through the water or bringing in food. The mastigonemes have a peculiar tripartite structure, which may be taken as the defining characteristic of the group, thereby including a few protists that do not produce cells with the typical heterokont form. They have been lost in a few lines, most notably the diatoms.

Classification

As noted above, classification varies considerably. Originally the heterokont algae were treated as two divisions, first within the kingdom Plantae and later the Protista:

Division Chrysophyta
    Class Chrysophyceae (golden algae)
    Class Bacillariophyceae (diatoms)
Division Phaeophyta (brown algae)

In this scheme, however, the Chrysophyceae are paraphyletic to both other groups. As a result, various members have been given their own classes and often divisions. Recent systems often treat these as classes within a single division, called the Heterokontophyta, Chromophyta or Ochrophyta. This is not universal, however - for instance Round et al. treat the diatoms as a division.

The discovery that water molds and hypochytrids are related to these algae, rather than fungi as previously thought, has led many authors to include them among the heterokonts. Should it turn out that they evolved from colored ancestors, the group would be paraphyletic in their absence. Once again, however, usage varies. Patterson named this extended group the stramenopiles, characterized by the presence of tripartite mastigonemes, mitochondria with tubular cristae, and open mitosis. He used the stramenopiles as a prototype for a classification without Linnaean ranks. Their composition has been essentially stable, but their use within ranked systems varies.

Cavalier-Smith treats the heterokonts as identical in composition with the stramenopiles; this is the definition followed here. He has proposed giving them a separate kingdom Chromista, together with the haptophytes and cryptomonads. This is one of the most common revisions to the five-kingdom system, but has not been generally adopted, partly because some biologists doubt their monophyly.

External links

• Tree of Life Web Project: Stramenopiles (http://tolweb.org/tree?group=Stramenopiles&contgroup=Eukaryotes)
• Jacob Feldman: Stramenopila (http://www.personal.psu.edu/users/j/s/jsf165/Bio110.html)