Stramenopile

Stramenopiles, also known as heterokonts, are a major group of protists defined by having stiff, three-part external hairs. These hairs are usually found on flagella, though in some species they appear elsewhere on the cell surface, and in a few cases they have been lost entirely—in which case the organism’s relationship to stramenopile ancestors is inferred from other shared cell features or genetic similarities. Along with Alveolata and Rhizaria, stramenopiles make up one of the three main branches of the SAR supergroup.

These organisms are eukaryotes, mostly single-celled, but some are multicellular—including large seaweeds like brown algae. The group contains a wide mix: algal protists, heterotrophic flagellates, opalines and their close relatives the proteromonad flagellates (all of which live inside other organisms), actinophryid Heliozoa, and oomycetes. The defining tripartite hairs have been lost in some members, such as most diatoms.

Many stramenopiles are unicellular flagellates, and most others produce flagellated cells at some stage in their life cycle, such as gametes or zoospores. In the majority of flagellated heterokonts, there are two flagella: the front one carries one or two rows of stiff hairs (mastigonemes), while the rear flagellum is smooth, usually shorter, and in a few cases does not even extend from the cell.

The term "heterokont" has been used both as an adjective—describing a cell with two unlike flagella—and as the name of a taxonomic group. However, the groups included under that taxon have varied widely over time, creating what is known as the "heterokont problem." This confusion has been resolved by the definition of the stramenopiles.

**Nomenclature**

The name "stramenopile" was introduced by D. J. Patterson in 1989 for a group that overlapped with the poorly defined heterokonts. The term was later discussed by J. C. David. In 2005, the International Society of Protistologists formalized it as the taxonomic name Stramenopiles, which has since become the most widely accepted name for this group. Several alternative names have been proposed since Patterson’s publication, including Straminipila, Straminopiles, and Stramenopila—the last being the valid name under the PhyloCode.

**The heterokont problem**

The word "heterokont" serves both as an adjective for cells with two dissimilar flagella and as a taxon name. The taxon Heterokontae was introduced in 1899 by Alexander Luther for algae now considered Xanthophyceae. But the same term was later used for other groupings. For instance, in 1956, Copeland applied it to include the xanthophytes (under the name Vaucheriacea), along with what became known as chrysophytes, silicoflagellates, and hyphochytrids. He also included the unrelated collar flagellates (choanoflagellates), placing the bicosoecids within them, as well as the distantly related haptophytes. Because so many different concepts have been attached to the taxon "heterokont," its meaning can only be clarified by specifying the usage—for example, Heterokontae sensu Luther 1899 or Heterokontae sensu Copeland 1956. Such contextual clarification is rarely provided, so when the name is used, it is unclear what is meant. As a result, "heterokont" has lost its usefulness in critical discussions about the identity, nature, and relationships of the group. The term "stramenopile" was introduced to identify a clade (a monophyletic and holophyletic lineage) by pointing to a defining innovative characteristic, or apomorphy, following the approach of transformed cladists.

Over time, the scope of the group changed, especially when ultrastructural studies in the 1970s revealed greater diversity among algae with chromoplasts (chlorophylls a and c) than previously recognized. At the same time, a protistological perspective was replacing the 19th-century view that divided unicellular eukaryotes into animals and plants. One result was that a range of heterotrophic organisms—many not previously considered heterokonts—were seen as related to the "core heterokonts" (those with stiff hairs on the front flagellum). Newly recognized relatives included the parasitic opalines, proteromonads, and actinophryid Heliozoa. They joined other heterotrophic protists, such as bicosoecids, labyrinthulids, and oomycete fungi, which some had included as heterokonts and others had excluded. Rather than continue using a name whose meaning had shifted over time and become ambiguous, the name "stramenopile" was introduced for the clade of protists that possess tripartite stiff hairs (usually on flagella) and all their descendants. Molecular studies confirm that the genes coding for the proteins of these hairs are exclusive to stramenopiles.

**Characteristics**

The presumed apomorphy of tripartite flagellar hairs in stramenopiles is well understood. The base of the hair is flexible and inserts into the cell membrane; the middle section is a long, stiff tube (the "straw" or "stramen"); and the tip is covered with many delicate hairs called mastigonemes. The proteins that code for these mastigonemes appear to be unique to the stramenopile clade and are present even in taxa, such as diatoms, that no longer have the hairs.

Most stramenopiles have two flagella near the apex, typically supported by four microtubule roots in a distinctive arrangement. Inside the flagellum, there is a transitional helix where the beating axoneme—with its characteristic pattern of nine peripheral doublets around two central microtubules—transitions into the nine-triplet structure of the basal body.

**Plastids**

Many stramenopiles possess plastids that allow them to photosynthesize, using light to produce their own food. These plastids are colored off-green, orange, golden, or brown.