Diatoms

Abstract Diatoms are single‐celled autotrophic organisms with highly ornate siliceous walls. They account for more than 20% of the world's primary production, are responsible for much of the petroleum humans use, and deposits of their shells are mined for numerous uses. The most taxonomically diverse groups of photosynthetic protists, diatoms are of ecological importance in nearly every freshwater and marine habitat. Their closest relatives are a poorly known group of microflagellates, some silicified and some not. Efforts to reconstruct diatom phylogeny are hampered by the fact that their true diversity and that of their closest relatives remains unknown. Genomic studies are creating tremendous new opportunities for study of this important group. Perhaps as much as a third of the diatom nuclear genome is now thought to be recently laterally transferred from bacteria. Diatom adaptation to such a wide range of environments may be a result of the resulting novel gene combinations. Key Concepts: Diatoms are a highly diverse group of protists, with perhaps more than 200 000 species, most of which remain undescribed. There are four major structural groups, radial centrics, polar centrics, araphid pennates and raphid pennates. Among the four major structural groups of diatoms, only the raphid pennates are robustly supported as monophyletic. Diatoms dominate the world's oceans, lakes and streams, conducting more than 20% of the world's photosynthesis. Diatoms are critical indicators of aquatic ecosystem conditions, past and present. Fossil diatoms have significant economic importance as stratigraphic indicators for mineral exploration, particularly petroleum and as a source of diatomite, which is used in numerous industrial applications. Diatoms make a diverse number of nanoscale siliceous structures, an order of magnitude smaller than that presently attainable by human technology. Understanding how diatoms make their shells may lead to breakthroughs in nanotechnology. Nearly a third of the diatom nuclear genome appears to be the result of relatively recent horizontal gene transfer from bacteria. Horizontal gene transfer may play a role in the apparent rapid adaptation and diversification of diatoms.