Towards a Natural Classification of Diatoms (Bacillariophyceae) J.P. Kociolek 1 , E.C. Theriot 2 , D.M. Williams 3 & M.P. Ashworth 2 1 University of Colorado Museum of Natural History, Boulder, CO, 80309, USA 2 Department of Integrative Biology, University of Texas at Austin, Austin, TX, 78712, USA 3 Department of Life Sciences, Natural History Museum, London, UK The diatoms are chlorophyll-c containing, heterokont algae which occur in marine and freshwater habits as significant contributors to both the plankton and benthos. Diatoms are hyperdiverse, with estimates for the number of diatom species range from 10,000 to over 100,000. Diatoms possess a unique, multi-part silica shell—the frustule. The siliceous nature of this frustule makes diatoms important contributors to global geochemical cycling of silica in the oceans and also provide a rich fossil history for diatom diversification. The simplest distinction in valve morphology is in the symmetry of the valve—seen in the diagrams to the right—between the radially-symmetrical “centric” cells (a), and the bilaterally -symmetrical “pennate” cells (b). The utility of valve symmetry and other characters observable by light microscopy (spines, pore fields, plastid morphology) served as the classical basis for diatom identification and classification, but have been interpreted in such a way that the proposed high-level classification schemes named groups which were non-monophyletic—”unnatural.” The introduction of DNA characters to the diatom phylogeny has re-ignited the search for a classification scheme which accurately reflects the evolution of the diatoms. Early attempts at a DNA-supported high- level classification scheme have also ended up with non-monophyletic named groups. We are proposing a new classification of the diatoms, using consistently-resolved, DNA-supported, as-yet unnamed monophyletic groups (clades) as the basis for higher classification of the diatoms which will better reflect their diversity and evolutionary history. Diatoms: the basics R1 R2 R3 P1 P2 P3 A1 A2 RAPHE Bolidomonas ML tree using nuclear SSU rRNA, chloroplast rbcL, psbC, psbA, psaA, psaB, atpB from 208 taxa (Theriot et al. 2015) Medlin & Kaczmarska 2004: based on nuclear-encoded rRNA sequence data (SSU) Coscinodiscophyceae Mediophyceae Bacillariophyceae Round, Crawford & Mann 1990: based on light and electron microscopy Coscinodiscophyceae Fragilariophyceae Bacillariophyceae Karsten 1928: based on light microscopy Centrales Pennales R1 clade R2 clade R3 clade Diatom Classification Schemes from Round, Crawford & Mann 1990 from Round, Crawford & Mann 1990 “centric” morphology “pennate” morphology valve valve girdle Representative genera: Leptocylindrus, Tenuicylindrus, Corethon Representative genera: Ellerbeckia, Proboscia, Melosira, Aulacoseira, Paralia, Endictya, Stephanopyxis, Podosira Representative genera: Rhizosolenia, Guinardia, Coscinodiscus, Actinocyclus, Actinoptychus, Aulacodiscus P1 clade Representative genera: Thalassiosira, Cyclotella, Triceratium, Odontella, Biddulphia, Attheya, Lithodesmium, Ditylum, Eunotogramma P2 clade Representative genera: Cerataulina, Eucampia, Hemiaulus, Chaetoceros, Bacteriastrum, Acanthoceros, Urosolenia P3 clade Representative genera: Trigonium, Lampriscus, Stictocyclus, Isthmia, Climacosphenia, Chrysanthemodiscus, Toxarium, Ardissonea, A1 clade A2 clade RAPHE clade Representative genera: Striatella, Asterionellopsis, Bleakeleya, Delphineis, Rhaphoneis, Dimeregramma, Plagiogramma Representative genera: Fragilaria, Synedra, Hyalosynedra, Staurosira, Opephora, Diatoma, Cyclophora, Licmophora, Tabularia, Asterionella, Grammatophora, Thalassionema Representative genera: Eunotia, Nitzschia, Pseudo-nitzschia, Achnanthes, Gyrosigma, Navicula, Neidium, Caloneis, Pinnularia, Fallacia, Stauroneis, Amphora, Entomoneis, Surirella, Phaeodactylum, Gomphonema, Cymbella, Cocconeis, Mastogloia, Diploneis Stephanopyxis turris—Live Podosira baldjickiana—Live Rhaphoneis amphiceros—Live Lithodesmium undulatum—Live Astrosyne radiata—Live Thalassionema sp.—LM Ditylum brightwelli—SEM, frustule Corethron hystrix—Live Cerataulina pelagica—Live Biddulphia tridens—Live Attheya septentrionalis SEM, frustule Aulacodiscus oreganus—LM, valve Trigonium formosum—Live Leptocylindrus danicus SEM, frustule Leptocylindrus danicus—Live Corethron sp.—SEM, valve Proboscia sp.—Live Rhizosolenia imbricata—Live Guinardia striata—Live Aulacoseira baicalensis LM, valve Endictya oceanica—SEM, valve Paralia sp.—SEM, valve Actinocyclus octonarius—Live Coscinodiscus cf granii—LM, valve Trieres (Odontella) sinensis Live Chaetoceros cf didymus—Live Eucampia zodiacus—SEM, valve Urosolenia eriensis—SEM, frustule Cyclotella nana—SEM, valve Hemiaulus hauckii—SEM, valves Climacosphenia elongata—SEM, valves Chrysanthemodiscus floriatus Live Isthmia minima—SEM, frustule Striatella unipunctata Live Perideraion elongatum—Live Perissonoё cruciata—SEM, valve Plagiogramma sp. SEM, valve Microtabella interrupta—Live Staurosira construens—SEM, valve Opephora sp,—SEM, valve Licmophora abbreviata LM, valve Hyalosynedra sp.—LM, valve Grammatophora cf macilenta—LM, frustule Phaeodactylum tricornutum—SEM, valve Surirella sp.—Live Mastogloia sp.—Live Didymosphenia geminata—LM, valve image by T. Nakov Diploneis budyana—SEM, valve Meuniera membranacea—SEM, valve Climaconeis sp.—Live Navicula sp.—LM, valve Nitzschia sp.—SEM, valve Neidium sp.—LM, valve Nitzschia sp.—SEM, valves Nitzschia sp.—SEM, valve Plagiogrammopsis sp.—SEM, frustule