Diatoms belong to a large group called the heterokonts, which include both autotrophs such as golden algae and kelp; and heterotrophs such as water moulds. The classification of heterokonts is still unsettled: they may be designated a division, phylum, kingdom, or something intermediate to those. Consequently, diatoms are ranked anywhere from a class, usually called Diatomophyceae or Bacillariophyceae, to a division (=phylum), usually called Bacillariophyta, with corresponding changes in the ranks of their subgroups.
Genera and species An estimated 20,000 extant diatom species are believed to exist, of which around 12,000 have been named to date according to Guiry, 2012 (other sources give a wider range of estimates). Around 1,000-1,300 diatom genera have been described, both extant and fossil, of which some 250-300 exist only as fossils.
Classes and orders
Overview For many years the diatoms—treated either as a class (Bacillariophyceae) or a phylum (Bacillariophyta)—were divided into just 2 orders, corresponding to the centric and the pennate diatoms (Centrales and Pennales; alternative names Biddulphiales and Bacillariales, as used e.g. in Lee, 1989). This classification was extensively overhauled by Round, Crawford and Mann in 1990 who treated the diatoms at a higher rank (division, corresponding to phylum in zoological classification), and promoted the major classification units to classes, maintaining the centric diatoms as a single class Coscinodiscophyceae, but splitting the former pennate diatoms into 2 separate classes, Fragilariophyceae and Bacillariophyceae (the latter older name retained but with an emended definition), between them encompassing 45 orders, the majority of them new. Today (writing at mid 2020) it is recognised that the 1990 system of Round et al. is in need of revision with the advent of newer molecular work, however the best system to replace it is unclear, and current systems in widespread use such as AlgaeBase, the World Register of Marine Species and its contributing database DiatomBase, and the system for "all life" represented in Ruggiero et al., 2015, all retain the Round et al. treatment as their basis, albeit with diatoms as a whole treated as a class rather than division/phylum, and Round et al.'s classes reduced to subclasses, for better agreement with the treatment of phylogenetically adjacent groups and their containing taxa. (For references refer the individual sections below). One proposal, by Linda Medlin and co-workers commencing in 2004, is for some of the centric diatom orders considered more closely related to the pennates to be split off as a new class, Mediophyceae, itself more closely aligned with the pennate diatoms than the remaining centrics. This hypothesis—later designated the Coscinodiscophyceae-Mediophyceae-Bacillariophyceae, or Coscinodiscophyceae+(Mediophyceae+Bacillariophyceae) (CMB) hypothesis—has been accepted by D.G. Mann among others, who uses it as the basis for the classification of diatoms as presented in Adl. et al.'s series of syntheses (2005, 2012, 2019), and also in the Bacillariophyta chapter of the 2017 Handbook of the Protists edited by Archibald et al., with some modifications reflecting the apparent non-monophyly of Medlin et al. original "Coscinodiscophyceae". Meanwhile, a group led by E.C. Theriot favours a different hypothesis of phylogeny, which has been termed the structural gradation hypothesis (SGH) and does not recognise the Mediophyceae as a monophyletic group, while another analysis, that of Parks et al., 2018, finds that the radial centric diatoms (Medlin et al.'s Coscinodiscophyceae) are not monophyletic, but supports the monophyly of Mediophyceae minus Attheya, which is an anomalous genus. Discussion of the relative merits of these conflicting schemes continues by the various parties involved.
Round et al., 1990 Based on the fact that pennate diatoms either do or do not have a longitudinal groove in the valve, called a raphe, a 1990 classification by Round, Crawford & Mann divides the diatoms (as Bacillarophyta) into three classes, centric (22 orders); pennate without a raphe (12 orders); and pennate with a raphe (11 orders), as follows:
Phylum Bacillarophyta (diatoms) Class Coscinodiscophyceae Round & R.M.Crawford (centric diatoms) Subclass Biddulphiophycidae Round & R.M.Crawford Anaulales Round & R.M.Crawford Biddulphiales Krieger Hemiaulales Round & R.M.Crawford Triceratiales Round & R.M.Crawford Subclass Chaetocerotophycidae Round & R.M.Crawford Chaetocerotales Round & R.M.Crawford Leptocylindrales Round & R.M.Crawford Subclass Corethrophycidae Round & R.M.Crawford Corethrales Round & R.M.Crawford Subclass Coscinodiscophycidae Round & R.M.Crawford Arachnoidiscales Round Asterolamprales Round Aulacoseirales R.M.Crawford Chrysanthemodiscales Round Coscinodiscales Round Ethmodiscales Round Melosirales R.M.Crawford Orthoseirales R.M.Crawford Paraliales R.M.Crawford Stictocyclales Round Stictodiscales Round & R.M.Crawford Subclass Cymatosirophycidae Round & R.M.Crawford Cymatosirales Round & R.M.Crawford Subclass Lithodesmiophycidae Round & R.M.Crawford Lithodesmiales Round & R.M.Crawford Subclass Rhizosoleniophycidae Round & R.M.Crawford Rhizosoleniales Silva Subclass Thalassiosirophycidae Round & R.M.Crawford Thalassiosirales Glezer & Makarova Class Fragilariophyceae Round (pennate diatoms without a raphe (araphids)) Subclass Fragilariophycidae Round Ardissoneales Round Climacospheniales Round Cyclophorales Round & R.M.Crawford Fragilariales P.C.Silva Licmophorales Round Protoraphidales Round Rhabdonematales Round & R.M.Crawford Rhaphoneidales Round Striatellales Round Tabellariales Round Thalassionematales Round Toxariales Round Class Bacillariophyceae Haeckel, 1878, emend. D.G.Mann (pennate diatoms with a raphe (raphids)) Subclass Bacillariophycidae D.G.Mann Achnanthales P.C.Silva Bacillariales Hendey Cymbellales D.G.Mann Dictyoneidales D.G.Mann Lyrellales D.G.Mann Mastogloiales D.G.Mann Naviculales Bessey Rhopalodiales D.G.Mann Surirellales D.G.Mann Thalassiophysales D.G.Mann Subclass Eunotiophycidae D.G.Mann Eunotiales P.C.Silva
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