of relatedness among protists, Krylov and co-workers proposed that the morphology of mitochondrial cristae remained relatively unchanged over time, and would define large sectors of protistan diversity. Subsequent molecular studies have confirmed this, with the stramenopiles being most closely related to Alveolates and Rhizaria - all with tubular mitochondrial cristae and collectively referred to as the SAR clade. Stramenochromes The stramenopiles with plastids (stramenochromes) have plastids with an off-green, orange, golden or brown color because of the occurrence of chlorophyll a, chlorophyll c, and fucoxanthin. This form of plastid is referred to as a chromoplast (as opposed to chloroplast which if used narrowly refers to the chlorophyll B containing plastids in green algae, some euglenids, and the land plants). The most significant autotrophic stramenopiles are the brown algae (wracks and many other seaweeds), and the diatoms. The latter are among the most significant primary producers in marine and freshwater ecosystems. Chromoplasts are surrounded by four membranes. The outermost is continuous with the chloroplast endoplasmic reticulum, or cER. The second membrane presents a barrier between the lumen of the cER and the primary endosymbiont or chloroplast. The symbiont was the source of the two innermost membranes within which the thylakoid membranes are found. Most molecular analyses suggest that the most basal stramenopiles are colorless heterotrophs. This suggests that the stramenopiles arose as heterotrophic organisms, diversified, and only at a later stage in their evolutionary history did they acquire chromoplasts. Some lineages (such as the axodine lineage that included the chromophytic pedinellids, colourless ciliophryids, and colourless actinophryid heliozoa) have secondarily reverted to heterotrophy. Significance Some stramenopiles are significant as autotrophs and as heterotrophs in natural ecosystems. Blastocystis is a parasite of humans; opalines and proteromonads live in the intestines of cold-blooded vertebrates; oomycetes include some significant plant pathogens (including the agent that caused the potato blight famine in Ireland that resulted in approximately one million deaths and led to extensive emigration). Diatoms are major contributors to global carbon cycles because they are the most important autotrophs in most marine habitats. The brown algae (or kelp) are major autotrophs of the intertidal and subtidal marine habitats. Some of the bacterivorous stramenopiles, such as Cafeteria are common and widespread consumers of bacteria, and thus play a major role in recycling carbon and nutrients within microbial food webs. Gallery References External links Tree of Life Web Project: Stramenopiles SAR supergroup