Editing Diatom (section)

===Classes and orders===
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]]). 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.<ref>{{cite journal |doi=10.5091/plecevo.2010.418 |title=The limits of nuclear-encoded SSU rDNA for resolving the diatom phylogeny |journal=European Journal of Phycology |volume=4 |issue=3 |pages=277–290 |year=2009 |last1=Theriot |first1=Edward C. |last2=Cannone |first2=Jamie J. |last3=Gutell |first3=Robin R. |last4=Alverson |first4=Andrew J.|pmid=20224747 |pmc=2835975 }}</ref><ref>{{cite journal |doi=10.1080/09670260902749159 |title=A preliminary multigene phylogeny of the diatoms (Bacillariophyta): challenges for future research |journal=Plant Ecology and Evolution |volume=143 |issue=3 |year=2010 |last1=Theriot |first1=Edward C. |last2=Ashworth |first2=Matt |last3=Ruck |first3=Elizabeth |last4=Nakov |first4=Teofil |last5=Jansen |first5=Robert K. |pages=277–290 |pmid=20224747 |pmc=2835975 |doi-access=free }}</ref><ref>{{cite journal |doi=10.1093/molbev/msx268 |title=Signal, uncertainty, and conflict in phylogenomic data for a diverse lineage of microbial eukaryotes (Diatoms, Bacillariophyta) |journal=Molecular Biology and Evolution |volume=35 |issue=1 |pages=80–93 |year=2018 |last1=Parks |first1=Matthew B. |last2=Wickett |first2=Norman J. |last3=Alverson |first3=Andrew J. |pmid=29040712 |pmc=5850769 |doi-access=free }}</ref><ref>{{cite journal |title=Review of the phylogenetic reconstruction of the diatoms using molecular tools with an analysis of a seven gene data set using multiple outgroups and morphological data for a total evidence approach |journal=Phycologia |volume=in press |year=2020 |last1=Medlin |first1=L.K. |last2=Desdevises |first2=Y. |url=http://plymsea.ac.uk/id/eprint/8938/1/PH20-10_R1.pdf |access-date=20 July 2020 |archive-date=20 July 2020 |archive-url=https://web.archive.org/web/20200720012627/http://plymsea.ac.uk/id/eprint/8938/1/PH20-10_R1.pdf |url-status=live }}</ref>

====Adl et al., 2019 treatment====
In 2019, Adl ''et al.''<ref name="Adl 2018">{{Cite journal |last1=Adl |first1=Sina M. |last2=Bass |first2=David |last3=Lane|first3=Christopher E. |last4=Lukeš |first4=Julius |last5=Schoch |first5=Conrad L. |last6=Smirnov |first6=Alexey |last7=Agatha |first7=Sabine |last8=Berney |first8=Cedric |last9=Brown |first9=Matthew W. | display-authors = 6 | date=2018-09-26 |title=Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes|journal=Journal of Eukaryotic Microbiology |volume=66 |issue=1 |pages=4–119 |doi=10.1111/jeu.12691|pmid=30257078 |pmc=6492006 }}</ref> presented the following classification of diatoms, while noting: "This revision reflects numerous advances in the phylogeny of the diatoms over the last decade. Due to our poor taxon sampling outside of the Mediophyceae and pennate diatoms, and the known and anticipated diversity of all diatoms, many clades appear at a high classification level (and the higher level classification is rather flat)." This classification treats diatoms as a phylum (Diatomeae/Bacillariophyta), accepts the class Mediophyceae of Medlin and co-workers, introduces new subphyla and classes for a number of otherwise isolated genera, and re-ranks a number of previously established taxa as subclasses, but does not list orders or families. Inferred ranks have been added for clarity (Adl. et al. do not use ranks, but the intended ones in this portion of the classification are apparent from the choice of endings used, within the system of botanical nomenclature employed).
* Clade [[Diatomista]] Derelle et al. 2016, emend. Cavalier-Smith 2017 (diatoms plus a subset of other ochrophyte groups)
** Phylum Diatomeae Dumortier 1821 [= [[Bacillariophyta]] Haeckel 1878] (diatoms)
*** Subphylum Leptocylindrophytina D.G. Mann in Adl et al. 2019
**** Class [[Leptocylindrophyceae]] D.G. Mann in Adl et al. 2019 (''[[Leptocylindrus]]'', ''[[Tenuicylindrus]]'')
**** Class [[Corethrophyceae]] D.G. Mann in Adl et al. 2019 (''[[Corethron]]'')
*** Subphylum Ellerbeckiophytina D.G. Mann in Adl et al. 2019 (''[[Ellerbeckia]]'')
*** Subphylum Probosciophytina D.G. Mann in Adl et al. 2019 (''[[Proboscia]]'')
*** Subphylum Melosirophytina D.G. Mann in Adl et al. 2019 (''[[Aulacoseira]]'', ''[[Melosira]]'', ''[[Hyalodiscus]]'', ''[[Stephanopyxis]]'', ''[[Paralia (diatom genus)|Paralia]]'', ''[[Endictya]]'')
*** Subphylum Coscinodiscophytina Medlin & Kaczmarska 2004, emend. (''[[Actinoptychus]]'', ''[[Coscinodiscus]]'', ''[[Actinocyclus]]'', ''[[Asteromphalus]]'', ''[[Aulacodiscus]]'', ''[[Stellarima]]'')
*** Subphylum Rhizosoleniophytina D.G. Mann in Adl et al. 2019 (''[[Guinardia]]'', ''[[Rhizosolenia]]'', ''[[Pseudosolenia]]'')
*** Subphylum Arachnoidiscophytina D.G. Mann in Adl et al. 2019 (''[[Arachnoidiscus]]'')
*** Subphylum Bacillariophytina Medlin & Kaczmarska 2004, emend.
**** Class [[Mediophyceae]] Jouse & Proshkina-Lavrenko in Medlin & Kaczmarska 2004
***** Subclass Chaetocerotophycidae Round & R.M. Crawford in Round et al. 1990, emend.
***** Subclass Lithodesmiophycidae Round & R.M. Crawford in Round et al. 1990, emend.
***** Subclass Thalassiosirophycidae Round & R.M. Crawford in Round et al. 1990
***** Subclass Cymatosirophycidae Round & R.M. Crawford in Round et al. 1990
***** Subclass Odontellophycidae D.G. Mann in Adl et al. 2019
***** Subclass Chrysanthemodiscophycidae D.G. Mann in Adl et al. 2019
**** Class [[Biddulphiophyceae]] D.G. Mann in Adl et al. 2019
***** Subclass Biddulphiophycidae Round and R.M. Crawford in Round et al. 1990, emend.
***** Biddulphiophyceae incertae sedis (''[[Attheya]]'')
**** Class [[Bacillariophyceae]] Haeckel 1878, emend.
***** Bacillariophyceae incertae sedis (Striatellaceae)
***** Subclass Urneidophycidae Medlin 2016
***** Subclass Fragilariophycidae Round in Round, Crawford & Mann 1990, emend.
***** Subclass Bacillariophycidae D.G. Mann in Round, Crawford & Mann 1990, emend.

See [[taxonomy of diatoms]] for more details.