EukrRef Workshop (Vancouver, 2015)

1. Towards a Natural Classification of Diatoms (Bacillariophyceae)
J.P. Kociolek1, E.C. Theriot2, D.M. Williams3 & M.P. Ashworth2
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, frustuleCorethron 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
LiveIsthmia minima—SEM, frustule
Striatella unipunctata
Live
Perideraion elongatum—Live
Perissonoё cruciata—SEM, valve
Plagiogramma sp.
SEM, valve
Microtabella interrupta—LiveStaurosira 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, valvesNitzschia sp.—SEM, valve
Plagiogrammopsis sp.—SEM, frustule