Phospholipid-derived fatty acids as chemotaxonomic markers for ...

118Mar Ecol Prog Ser 324: 113–125, 2006Pigment composition of algal culturesThe pigment composition of the species for whichthe PLFA composition is described above was measured,but is not discussed in detail here. The resultsagreed well with general data on the pigment compositionof microalgae. Remarkable was that our strain ofPhaeocystis globosa contained only a minute amountof 19’-hexanoyloxyfucoxanthin, a pigment which isoften used as a biomarker for this species.PLFA and pigment in the Scheldt EstuaryThe variations in PLFA and pigment concentrationand composition are shown for July 2003 in Fig. 2.The pigment data are averages of 2 samples perstation, PLFA data averages of 8 samples perstation. A large decline in biomass from freshwater tohigh-salinity water was apparent from the concentrationsof both chlorophyll a (chl a) and total PLFA(Fig. 2a). In addition to this variation in biomass, theFig. 2. Pigments and PLFA concentrations in the Scheldt Estuary in July 2003. (a) Chlorophyll a (chl a) and total PLFA (µg l –1 );(b,c) pigment concentrations relative to chl a; (d–f) PLFA contribution as percentage of total PLFA

Dijkman & Kromkamp: PLFA as phytoplankton biomarker123marine species. With this in mind, we selected specieseither isolated from the Scheldt estuary or laboratorystrains of species relevant to the estuary.The culture conditions were chosen to approximateenvironmental conditions in the estuary. The cultivationtemperature of 15°C was in the middle of the temperaturerange of the estuary, which was 11, 20 and14°C in April, July and October respectively. Similarly,the relatively low irradiance of 40 µmol m –2 s –1 wasconsidered appropriate for this very turbid estuary.The fatty acid composition of algae is known to vary inresponse to changes in environmental parameters suchas temperature (Zhu et al. 1997), irradiance (Brown etal. 1996) and growth phase (Brown et al. 1996, Zhu etal. 1997). However, most literature data concern totalfatty acids and information on PLFA is scarce. A 6-foldincrease in irradiance from 26 to 170 µmol m –2 s –1 led tochanges of maximally 3% in the contribution of individualPLFA to total PLFA in Thalassiosira pseudonana(Dijkman, unpubl. data). Zhu et al. (1997) reported differencesof maximally 6% in PLFA composition forIsochrysis galbana grown at either 15 or 30°C. Thevariations between species are in the same order ofmagnitude (Table 2) and we did not try to simulate theeffects of these changes on the estimated group abundance.Especially in a net heterotrophic estuary such as theScheldt (Heip et al. 1995), bacteria make an importantcontribution to the PLFA composition. Obtaining aproper estimation of the PLFA composition of theScheldt estuary posed a problem. In turbid estuariessuch as the Scheldt, a major fraction of bacteria is associatedwith particles. In addition, many of the freelivingbacteria are also trapped by the filters, whosepores are rapidly clotted when water with a high particle-loadis filtered (Selje & Simon 2003). Thus, eventhough many bacteria are small enough to passthrough the pores of the glass fiber filters, a large fractionof the bacteria will be retained on the filters. Thisprevents separation of bacteria from eukaryotes bysize-fractionation, which might otherwise be used toobtain the PLFA composition of the bacterial community.The PLFA composition of bacteria that we used inthe CHEMTAX matrix (Table 3) is an approximationand, as discussed in the ‘Results’, this has added uncertaintyto our results. Fortunately, this had little effecton the relative phytoplankton composition.Zooplankton were ignored in our analysis, althoughsome zooplankton were seen in the samples. It wasassumed that their abundance was so low that theirimportance was minor compared to phytoplankton andbacteria. This assumption is supported by the goodagreement between pigment and PLFA data. A possibilitywould have been to remove zooplankton by prefilteringthe samples over a zooplankton filter. Wechose not to do this since many of the algal speciesformed large colonies similar in size to the zooplanktonor were attached to larger particles and would thusalso have been removed by a zooplankton filter.The phytoplankton composition calculated frompigment and PLFA data (Fig. 3) is in good agreementwith variations in phytoplankton composition along anestuarine salinity gradient in general and the Scheldtestuary in particular. Lemaire et al. (2002) measuredthe pigment composition in 9 European estuaries, 1 ofwhich was the Scheldt estuary, and these authorsreported that Bacillariophyceae were dominant in allestuaries. Other groups present were Chlorophyta,Cryptophycea and Dinophyceae, which agrees wellwith our results. A spring bloom of Phaeocystis sp. is areoccurring phenomenon in many temperate marinewaters (Schoemann et al. 2005) and is usually observedin spring in the Scheldt estuary at salinities higher than18 PSU.As the result of tidal forcing and the morphology ofthe estuary, the turbidity maximum of the Scheldt estuaryis around 2 to 8 PSU. This high turbidity results inan unfavorable light climate for phytoplankton. Bacterialgrowth and relative abundance are usually high inthis area (Heip et al. 1995). Despite the uncertainty inthe fractional abundance of bacteria, the maximumabundance at intermediate salinity was always reproducibleand independent of the ratio file used.It should be remembered that the results in Fig. 3show the relative group abundance. Due to the largedecrease in biomass from low to high salinity (seeFig. 2a), the absolute concentration of bacteria as wellas that of most phytoplankton groups actually decreasedfrom the freshwater site to the mouth ofthe estuary.Some degree of uncertainty is inherent in comparisonsof cell counts, pigment data and PLFA data, witherrors being associated with all methods. Uncertaintyin estimations of group abundance using CHEMTAX iscommonly assumed to be in the range of 25% for individualgroups (e.g Lewitus et al. 2005); this agrees wellwith the variation of maximally 26% in bacterial abundancethat occurs when a different PLFA input ratio forthe bacteria is used. We found the PLFA analysis to bemore robust than the pigment analysis, probably dueto the larger number of markers that can be includedwhen PLFA are used. A different grouping of the fielddata or including a different number of biomarkers hadremarkably little effect on the outcome, inducing variationsof only a few percent. Part of the differencebetween pigment-derived and PLFA-derived groupabundance was the result of normalizing the pigmentsto chl a and PLFA to 16:0. Both chl a and 16:0 per unitbiomass vary between species and in response tochanges in environmental parameters.

124Mar Ecol Prog Ser 324: 113–125, 2006Despite these uncertainties, the agreement betweenpigment and PLFA data was generally good. Comparedto pigments, PLFA are less specific. Phospholipidsare important membrane lipids in all cells and are notrestricted to photosynthetic taxa. This can be an advantage,since information on non-photosynthetic taxacan also be derived (as in our data for the bacteria), butcan also be a complication since few PLFA are uniqueto a single group. Our data on the Scheldt estuaryshow that PLFA can be a useful complement to pigmentdata. This was most obvious for the bloom ofPhaeocystis sp., which was easily overlooked using thephotosynthetic pigments, but showed up clearly in thePLFA analysis. It should be remembered that thespecificity of a PLFA and selection of a PLFA as biomarkerfor a specific group will also depend on thetaxa contributing to the PLFA composition of the sample.The results of the present study give us confidencethat PLFA can be used in future applications such asthe calculation of group-specific growth rates.Acknowledgements. We thank E. Boschker, J. Peene, P. vanRijswijk, M. Houtekamer, J. Nieuwenhuize, U. Wollenzien,J. J. Middelburg and the crew of the RV ‘Luctor’ for their helpand assistance during the measurements and M. Doeleman forthe 18S molecular sequencing. We also thank 3 anonymous reviewerswhose remarks helped considerably to improve thismanuscript. This work is part of a Flemish-Dutch cooperationfor Sea Research (VLANEZO) and is financed by the DutchOrganization of Scientific Research (NWO 832.11.002 and832.11.007) and the Fund for Scientific Research-Flanders(FWO). 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