Towards a Natural Classification of Diatoms (Bacillariophyceae)

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

Bolid

om

onas

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


Ellerbeckia, Proboscia, Melosira, Aulacoseira, Paralia,

Endictya, Stephanopyxis, Podosira


Rhizosolenia, Guinardia, Coscinodiscus, Actinocyclus,

Actinoptychus, Aulacodiscus

P1 clade Representative genera:

Thalassiosira, Cyclotella, Triceratium, Odontella, Biddulphia, Attheya,

Lithodesmium, Ditylum, Eunotogramma


Cerataulina, Eucampia, Hemiaulus, Chaetoceros, Bacteriastrum,

Acanthoceros, Urosolenia


Trigonium, Lampriscus, Stictocyclus, Isthmia, Climacosphenia,

Chrysanthemodiscus, Toxarium, Ardissonea,

A1 clade A2 clade

RAPHE clade


Striatella, Asterionellopsis,

Bleakeleya, Delphineis,

Rhaphoneis, Dimeregramma,

Plagiogramma


Fragilaria, Synedra, Hyalosynedra, Staurosira,

Opephora, Diatoma, Cyclophora, Licmophora,

Tabularia, Asterionella, Grammatophora,

Thalassionema


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