~~ J. Ph)'c./' 28, 723-729 (1992)

MINI REVIEW

THE SPECIES CONCEPT IN PHYTOPLA NKTON ECOLOGY'

A. IVIichelle Wood and Tanya Leatham

Department of Biology. University of Oregon, Eugene, Oregon 97403

Phytoplankton ecology relies heavily on the use of taxon-insensitive indices like chlorophyll a. con­centrat ion, "e incubations, and light-da rk boules for measurement of phytoplankton abundance and productivity. Even so, phytoplankton ecologists gen­erall y recognize that the species composition or structure of a phytoplankton community has pro­nounced effects on ecosystem structure and func­tion. Numerous studies claim to document interspe­cific differences in important physiological parameters like nutrient utilization and storage ca­pacity (Dortch 1982), dark nitrogen uptake (Paasche et al. 1984), fatty acid composition (Thompson et al. 1990), sensitivity to ultraviolet irradiance (Ka­rentz et al. 199 1), or an iron requ irement for growth (Sunda el al. 1991). However, despite the fact that these are among the finest and most rigorous studies in comparative algal physiology, for the reasons ex­plained below, none ojlhelll aclua.II), delllonsirale s/Jecies­level differences in the traits oj interest.

It has been more than a decade since Gallagher 'S (1980, 1982) and Brand's (198 1, 1982) unequivocal demonstrations of tremendous genetic diversity in natural populations of important phytoplankton species. It is over 40 years since Braarud (195 1) first discussed the ex istence of different ecotypes or races of phytoplankton and almost 20 years since Doyle (1975) d iscussed the importance of this problem in oceanography. Amazingly, as exemplified by the pa­pers cited above, it is sti ll standard practice to char­acterize the phenotype ofa microa lgal species, how­ever defined, using on ly a single clonal isolate. The problem with this approach is that it auempts to identify significant interspecies differences without estimating the magn itude of within-species varia­tion. This is a problem in basic experimental design, not a problem that arises because no menclatural "species" of phytoplankton are not always true bi­ological species [sensu Mayr (1969), that is, com­posed of a set of potentially interbreeding geno­types]. Without an estim ate of intra-group variation , it is impossible to determine whether or not th e differences observed between nominal phytoplank­lOn groups ("species" or otherwise) are any greater than might be observed in a random sample of dif­ferent members of the same group.

Table I summarizes much of th e literature on interclonal variat ion in ecologicall y important traits

I Dedicated lO R. R. L. Guillard, who continues to inspire and promote phytoplankton research e ven in his "retirement."

723

among phytoplankton species or species complexes. It is clear that th ere is tremendous diversity among morphological types that would be identified as the same "species" by the practicing ecologist. Over a wide range of temperatures, Hulburt and Gui llard (1968) found more than a 25% difference in the growth rate of the two different clones of Skelelonpma troJiicu III. When two clones of Bellerachea sJiinifera were exam ined, one required thiamine and one did not (Hargraves and Guillard 1974). When two clones of Fragilaria pinnala were examined, one required BI2 and the other did not (Hargraves and Guillard 1974). When the si licon uptake kinetics and growth rate of two strains of Asterionella formosa were com­pared, a nearly two-fold differe nce in both doubling time and half-saturation constant was observed be­tween the two strains (Kilham 1975). The depend-. ability of interclonal variation evidenced from the data in Table I hints at extraordinary diversity in natural populations, particularly since significant in­terclonal variation is observed when o nly a few strains are compared. It could be argued that many of the studies that involve on ly a few clones exaggerate the overa ll diversity within taxa because they deliber­ately utili ze clones from distinctly different environ­ments. Examples are the work by Underh ill (1977) on a temperate and a subtropical clone of BiddulJihia auri/a and the literature from the Guillard labora­tory establishing the ocean ic-neritic boundary as a selective gradient (recently reviewed by Brand 1988/ 1989). Contraril y, it could be argued that these stud­ies all underestimate genetic diversity within taxa because they rely on genotypes that can be cultured and maintained in the laboratory. Regardless, it is remarkable that for nearly every physiological char­acter examined, significant interclonal variability is found essentiall y every time that strains from the same putative taxon are compared.

The magnitude of these differences is not trivial. Data for several ultra phytoplankton taxa show that accl imated growth rates of genetically different strains, identified by morphological criteria as mem­bers of the same species and isolated from the same water mass , differ by factors of between 1.2 and 1.6 (Brand 1981). Average growth rates observed in th is study wereon the order of 1.5-2.2 days, which means that, within a few months of continuous directional selection, an initially very rare genotype would be­come the dominant strain in asexually reproducing populations (Fig. I). When the direction of selection changes every few genera lions, as may often happen

\.

724 A . MICm : LLE WOOD A N D TA NYA LEATHAM

TARLE I . Ill /ne/oual variation ill phJsiological Gild biochf'l1I;cal trails of ph),lo/Jlallk toll sPt'cit's (I lid sPf'cit's comp/,.w s. )' "" )'1'5; N = 110.

C har.lCler Varia tion among Spedc5 No.ofdollc:1 dones Re ference

Sali nity-depende nt gro \vth rale ProroCt'lltrlllll mica"s 2 Y Braarud 195 I Phat'odacl),/ulII (r;COrtlll tulII Bohlin 5 Y Hay ward 1968 DitJI1I11I brightwt'llii (West) Grunow 2 Y Brand 1984 Shelp/OllnT/a costalulII (Gre",) Cleve 2 Y Brand 1984 Emiii(l//a h!lx/I)'; (Lo hm .) Hay & Mohle r 2 Y Brand 1984

T emperature-depende nt gro wth ralC

Asln ;o1lt'lla formosa Hass. 2 Y Runner 1937 Na uiwla jJrllicu losa (Brcb. ex Klitz) Hilsc 10 y, Le win 1955 Skrirtollrma t rop iCIWI Cleve 2 Y Hulburt and Cuillard 1968 P. triconlll i um 5 Y Ha ywa rd 1968 Thalla SS;Qsira pS('IuloII(IIIa/oC('Qllica complex 6 Y Goldman and Carpenter 1974 Thallassiosira pSf Ile/olla/IU/OCl'anica complex 14 Y Bra nd et al. 198 1 E. IlIlxlt')'i 44 Y Bra nd 198 1 E. Iwxle;·j 73 Y Brand 1982 Grph),rocapsa oCl'allica Kamptner 19 Y Bra nd 1982 Proroct'llirum mica /I s Ehrenb. 28 Y Brand 1985 C/OS/t'r;um rhrrllbrrgii Meneghin i ex Ralfs 33 Y Kasai and Ichimura 1990

Toxicity GOII)'alllax lamarellsisb Lebour 3 Y Alam et al. 1979 G. III mart I/sis var. excavalab Braarud 4 Y Schmid t and Loeblich 1979 G. Ilimlirt'llSis va r. lamarrllsisb Braa rud 5 Y Schmidt and Loeblich 1979 G. Ilimart'llSisb Lebour 35 Y Maranda et al. 1985 PrologOlI),auiax lamarem is/ca lliliriiab complex 24 Y Ccmbe lla et a1. 1986 Gambt'rie/iseus loxieus Adachi & Fukuyo 17 Y Bo mber et a l. 1989

Vitamin requi rements and hete ro trophic capability AmlJhora coffaeifo rmis Ag. 10 Y Lewin and Lewin 1960 NiltSchia frll slulum (Klitz) Grun. 6 Y Lewin and Le win 1960 T. PSPlle/o llalia / oceall ica complex 4 Y G uillard 1968 P. Irico nlllllllll 5 N' Hayward 1968 Brl/prochra spillifera Hargraves & Guillard 2 Y Hargra ves and G uillard 1974 Brllrrochra pol),morphll Hargraves & Guil1ard 5 Y Hargra ves and Guillard 1974 Fragililirill pinllll /ll Ehr. 2 Y Hargraves and Guill ard 1974

Nitrogen metabo lism T. Pst'lldolilina/oceallica complex 3 Y Carpenter and Guill ard 19 71 F. pillnalll 2 Y Carpenter a nd C uill ard 19 71 Biddlliphill lIurila (Lyngb.) Breb. & Codey 2 Y Underhill 1977

Sil icon metaboli sm E. huxlt)· j 2 Y Eppley e l a I. 1969 A·formosa 2 Y Kilham 1975 T. pst'udOllUlili / oCl'lIlIica complex 2 Y Nelson et a l. 1976

Zinc-dependent growth ra te S. coslalmn 2 Y J e nsen et al. 1974

pH-dependent growth rate S)'1wra pelrrslm ij complex 4 Y Weect a l. 199 1

Sensiti vity to po llutants T. PSwdolllllla /ocea nica complex 3 Y Fisher ct a1. 1973 F· I}imlllili 2 Y Fisher el a1. 1973 T. Im u(/ollllllll/OCfllllicli complex II Y Murphy and Bclastock 1980 S. cosla/llIIl 6 Y Mu rphy and BclasLOck 1980

Serologica l affin ity E. hu:d r),i 6

, Shapiro Cl al. 1989 Micromolllls pusilla (Butcher) Manton & Parke 4 N' Shapiro et a l. 1989 E. hu:clt)'i 2 Y' van Bleijswijk et al. 199 1

Chemical composition; carbon part itioning P. triconllllll1n 2 Y T erry el al. 1983 Chlit loarllS mut lleri Lemmerman 10 Y J ohansen e t a\. 1990

Luminescence Gon)'lIlllllX txcavlllll (Braarud) Balech 10 Y Schmidt el al. 1978

-- ------------------~-

PI-IYTOI'LA NKTO N SPECI ES CONCEPT 725

TABLE 1. Continued.

Characu:r SI)Ccit.-s No. of clonc~

Variation among clones Kefc:n:nce

PhOloadapli vc parameters S. cos/atulII

Diel periodicityr P. triCOn/UIIlIll

a Also seen in colony form and osmotic IOlerancc. b Now AIt'xalldr;um spp. (BaJech 1985). ( No heterotrophic capability observed in an y clone.

3 Y Ga llaghe r el al. 1984

2 N Terry e l tt l. 1983

d Antisera to whole cells of one of the test d ones: variation in strength of ant igenic reac tion o bserved among test clones. ~ Antisera to coccolith polysaccharide. r In growth rate . chlorophyll synthesis. and biochemic il synthesis.

to planktonic populations, considerable genetic di­versity can be maintained very easily. For species that rarely reproduce sexuall y, selection during asexua l generations wi ll increase hidden genetic variability, and occasiona l recombination is required to reestabish th e greatest possible heritable phe­notypic variatio n (Lynch and Gabriel 1983).

Gallagher's work on population genetic change in Skelelollellla coslalwlI (Gallagher 1980, 1982) remains the premier demonstration that changing environ­mental conditions are corre lated with changes in th e genetic composition of a phytoplankton population. Recently, however, there have been additional ef­fons to use protein e lectrophoresis to eva luate ge­netic diversity within populations . Electrophoretic studies of toxic dinoAagellates show local popula­tions to be much more genetica lly distinct from one another on the west coast than on the east coast of North America (Cembella and Taylor 1986, Hay­home et al. 1989) .

Among freshwater phytoplankton, there have been two extensive e lectrophoretic studies of ge­netic polymorphism in natural populations. Soudek and Robinson (1983) examined electrophoretic vari­ability among strains of Asierionella!orlllosa from 32 different lakes or waterways. Whereas their study clearly shows a high degree of variation among all the clones, the authors also suggest that the low genetic variabili(y observed among isolates from the same site reAects genetic homogeneity in local pop­ulations. However, since most si tes were sampled only once, and since the median number of clones isolated from each site was less than three, Galla­gher's work suggests that the sampling program was inadequate to evaluate within-site genetic variabil­ity. Coesel and Menken (1988) examined genetic variabili ty within and among 17 populations of the desmids Closterium ehrenbergii and C. moniliferulII . A total of 278 clones were isolated from 17 popula­tions in Holland and England, with a median o f more than ten clones per site. This more statistically rig­orous study showed very little genetic variation (but some) within populations. The Dutch and English C. ehrenbergii strains were almost as distinct from

each other as the English C. ehrellbergii and the Dutch C. moniliJerum strains.

T he development of oligonucleotide probes and random amplified polymorphic DNA (RAPD) assays for use in phytoplankton population genetics (cf. Medlin 1991 , Deragon and Landry 1992, McCourt and Helfgott 1992, companion minireview by Man­hart and McCourt 1992) promises to make much more detailed studies of population genetic dynam­ics in natural populations possible. Of particular in­terest will be comparisons of spatial and tempora l scales of genetic homogeneity in systems that are reiatively closed to immigration (e.g. lakes, tempo­rary ponds) and systems that are relatively open (e.g. oceans, estuaries, streams).

For phytoplankton ecologists, interest in genetic diversity within putative species is imponant pri­mari ly because of the possible importance of pop­ulat ion genetic change as an adaptive response to

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Flc. I . Time for initially rare. but eco logica lly adapted. ge­no type to become abundant whe n the more abundant genotypes grow 10% (0) to 400% (6) more slO\ ... ly than the adapted geno­type. Conceptua lly. this cou ld be compared to clona l succession in a changi ng environment or to the takeover ora population by a strain with a new ravorable mutation. Model assumes asexua l reproduction (rrom Wood 1989).

726 A. MICH EL LE WOOD AN D T ANYA LEATHAM

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a;

'" ~ 1.0 >

'" v = 0.9 e 0-~

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0.6 21 22 23 24 2S 26 27

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~ '" ~ 1.0 .~ U = 0.9 e 0-~ 0.8

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FIG. 2. Genotype x environment interaction (G x E) among clones of ProrocmlrWlllllicans Ehrenb. isolated from George's Bank (lOp) and the Gulf of M aine (boLlom). Each line conneCtS the growth ra te or a given clone at 220 C \\'ith the growth rale of the same clone at 260 C. Different lines represent different clones. Signi ficant differences in the slope of the lines ind icate signi ficant G x E, thal is, sign ificant genetica ll y determined differences in the effect of the increase in temperature on different clones. G x E is more pronounced in the clones from the Gulf of Maine than in the clones fro m George's Bank. Data from Brand ( 1985).

changing environme nts (cf. Wood 1988, Lynch et al. 199 1). A character likely to be under selection during the current period of apparent globa l change is the "reaction no rm" or respo nse fun ctio n char­acteristic of each genotype (cf. Parejko and Dodson 199 1. Gomulkiewicz and Kirkpatrick 1992). Differ­ences in the response of two genotypes to the same e nvironmental change can be identified statistically as genotype x environment (G x E) interaction us­ing analysis of variance techniques (Gallagher and Alberte 1985, Bell 1990) or graphica lly (Fig. 2) as shown fo r the dino flagellate Prorocenlrwl/. micans. In this las t instance, there is li ttle G x E interaction. T he di ffere nce in growth rate at 26° C and 22° C is about the same for all strains from George's Bank , e"ven tho ugh there are differences in the absolute

growth rate. Among the Gul f of Maine isolates. three strains are affected much less by increased temper­ature than th e o th er strains. T hese were amo ng the slowest-g rowing strains at 220 C, and, because of their different reaction norm , the different ial in growth rate amo ng the clones is increased at the higher temperature. If these data were represen­tat ive of the populations in situ, theory would predict that an increase in temperature would cause the re lative abundance of these genotypes to decrease dramatica lly after several generations.

Where Do We Co Jrom Here?

Hopefully, the preceding remarks provide con­vincing evidence for the importance of changing the status quo. T he data do not sUPPQrt the continued assum ption that " clonal di ffe rences in marine phy­toplankton in general are believed to be rare" (J en­sen et al. 1974: 155). We suggest that several im­portant steps be taken.

Exercise more caution when interpreting the ecological significance oj dala obtained Jrom si.ngle strains oj par­ticular taxa. Studies involving many species, but with only one strain per species, can be used effectively to compare groups defined at higher taxonomic lev­els, assumin g data from replicates are ava ilable to evaluate within-strain variatio n. T his is the ap­proach used by Keller (1989) to show that significant dimethyl sulfide production is confin ed to only a few classes of ph ytoplankton and by Brand (199 1) to show that oceanic phytoplankto n have lower iron requirements for growth than coastal phytoplank­ton .

Begi'n to consider the strain designation an essential part oj the nOlllenclature Jo,· phytoplankton that aTe in wlillre. When discussing the resul ts obtained with o ne strain of a putati ve species, the strain designa­tio n, isolation information, and source of cultured material should always be provided in the text. If the genus is clear ly identifiable, but the species-level identification has not been confirmed by someone familiar with published descriptions of type mate­rial, the strain designatio n ca n be substi tuted fo r the specific epithet in both the text and tit le ofthe paper. If the strain has been fi rmly identified to species, care should be taken in the tex t and tiLie to avoid using the species name alone (with out strain desig­natio n) when discussing traits or properties that have only been studied in a particular strain . T hese are all common practices in microbiology and a matter of required style in some microbiological journals. Since this is becomin g mo re common in the phy­toplankton literature (Kana and Glibert 1987, Gar­cia-Pichel and Castenholz 1990, 199 1), we suggest that this concept of identi fica t ion be extended to eukar yotic phytoplankton and adopted more gen­erally in our discipline . If morphologically similar or identica l phytoplankton can be geneticall y and ph ysiologically very di ffe rent, we are likely to have

PHYTOPLANKTON SPEC I ES CONCE PT 727

more realistic insights into the life of a phytoplank­ter if we begin to think of each clonal genotype as a very distinct lineage, with differences from other members of its "species" possibly as great as from members of another "species" and , for some char­acters, greater simi larity to members of another "species" than of its own.

Identify the ecological significance of morphological traits used to dislinguish a.mong species. Morphologica l vari­ation is well known within putative phytoplankton taxa and is often caused by environmental factors. At some point, when genetically determined mor­phological variation among specimens is great enough. putative species are split into multiple spe­cies groups (cf. Theriot 1987, 1992, Rines and Har­graves 1990, Medlin 199 1, Young and Westbroek 1991). Much of this analys is seems irrelevent to many phytoplankton ecologists, although seemingly mi­nor morphological features can play important func­tional roles (Medl in et al. 1986) and, therefore, af­fect an individual 's fitness. Despite recent advances in the study of phytoplankton sexual reproduction (Mann 1984, 1987, 1988, Blackburn and Tyler 1987, Destombe and Cern bella 1990, Faust 1992), ecolo­gists wi ll probably be relying on morphologica l cri­teria to describe community structure for some time to come. For this reason , it would be useful to know when morphological criteria used to distinguish phy­toplankton species consistently inAuence Aux pro­cesses like sinking and grazi ng, optical properties like scattering coefficient, or other ecologically im­portant variables. For example, permanent attach­ments can be formed between adjacent cells in chains of Skeletonellla cosla.lum (Grev.) Cleve but not among cells in chains of the nearly morphologically iden­tical S. pseudocosta.tum Medlin (Med lin 199 1). The kind of studies we suggest would determine whether or not this difference affects the likelihood of chain disintegration in the two species. Does grazing on Skeletonema produce m OTe intact survivors in popu­lations of S. pseudocoslalUln and more dissolved or­ganic carbon from broken cells in S. costatwn? If so, correct identification of Skelelonema. species in nat­ural populations could provide anci llary information about the kinds of grazing and detrital pathways operating in the ecosystem.

Identify ecologicall), important traits consistenil), cor­related wilh the morphological features used to distinguish among species or sub-species. II is also important to identify ecologically important characters that are uniquely correlated with the morphological char­acters used to distinguish among phytoplankton spe­cies. For example, the 11tultiseries form of Nitzschia pungens is identified by the characteristic number of rows of poroids between the costae on the valve (Hasle 1965). However, a ll strains in the multis ... ies form also make the potent neurotoxin domoic acid, and other forms of Nitzschia pungens apparently do not (Bates et al. 1989, Fryxell et al. 1990). T his is the kind of situation where accurate taxonomy, based

on traditional morphological criteria, provides ex­tremely important information about ecosystem-lflV­eI processes.

A.M.W. tha nks R. R. L. Guillard for his willingness to r ide in a Pinto. We also thank P. Kociolek. E. Therio t. and L. VanValen for useful d iscuss io n, E. Theriot for access to unpubli shed work , T. Kibota for timely assistance. and N. Apelian, S. Brawley. R. Casl.en hol z, R. Lande. L. Shapiro, and an anonymous reviewer for cri tica l review of the manuscript. This work was supponed by DOE contract DE-FGD6-92ER61417.

Qlles/ioll (Castenholz): Genic variation, as measu red by enzyme elec trophoreti c palterns, o ligonucleotide probes, RAPD assays, and other modern methods, can now be used to characterize a population referred to as a "species" t hat was originally defined by phenotypic characte rs. In you r opinions, how does this new genic information on variabi lity affect spec ies limits and spec ies concept, panicularly for these eukaryotic groups of phytoplank­ton that you have been discussing?

AlIswn: There are two main wa ys. First , some of these methods will permit the kind of work Jane Ga ll agher did to be repeated more easily for other species. Her study (Ga llagher 1980, 1982) te ll s us that a relativel y loca l population of one d iatom essenti ally goes through seasonal succession at the clona l level; genetica ll y different clones domi nate the population of SkelelOI/i'lI/a cos/alulI/ in Naraganseu Bay at different times of the year. It wou ld be very va luable to know if this is representative of phytoplankton popu lations genera ll y.

The second way these techniques are influencing species con­cepts is in the way reflected by Medlin's ( 199 1) paper o n S. cosla/IUII

and S. "mu/oros/alum. Here, molecu lar genet ic data is used with traditio nal morphologica l cr iteria to establish a new species. The ribosoma l RNA data presented in th is paper lend considerable credibi lity to her ident ification of a new species. Since the mor­phologica l data come from the examination of relatively few iso­lates, and therefore very few individual specimens from nature, this is a leve l o f credibility that wou ld have been hard to establish from Ihe morphologica l data alone.

QIU'Sl ioll (Manhart and McCourt): Should the typesofphytoplank­ton used by ecologists be given formal nomenclatural status?

Answer: Presumably ecologists do not use generic or specific names that do not have formal nomenclatural status . We're cenainly nOt trying to rewrite the International Code of Nomenclature. What we are really suggesting is that a ll discussions of a species' phenotype take intraspecific ge netic va riation into account. If o nl y one stra in of a species is used in a stud y, it is impossible to extrapolate to the spec ies at large. In particular, it can not be assumed that the physiological or biochem ica l phenotype of one strain is shared by all other strains o r genotypes that share the same morphology. We're also say ing that when allJolle (system­atist, ge neti cist, ecologist, or what have you) uses cultured ma­terial , the strain(s) should be carefull y identified. In something of a more radical departure from standard approaches with eu­karyotic algae. we wou ld also suggest that, for species where there are ce rtain we ll-stud ied stra ins, the forma l species descript ion might be amended to incl ude molecular, genetic, physio logical, and biochemical features of strains t hat conform to the original type description . This information could then be used to reso lve the ta xo nomic pos itio n of morphologica l variants from type.

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728 A. MICHELLE WOOD AND TANYA LEATHAM

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