INTERCOLONIAL VARIABILITY IN MACROMOLECULAR COMPOSITION IN P-STARVED AND P-REPLETE SCENEDESMUS POPULATIONS REVEALED BY INFRARED MICROSPECTROSCOPY(1).

Macromolecular variability in microalgal popula-tions subject to different nutrient environments wasinvestigated, using the chlorophyte alga Scenedesmusquadricauda (Turpin) Bre´b. as a model organism.The large size of the four-cell coenobia in the strainused in this study ( 35 lm diameter) convenientlyallowed high quality spectra to be obtained from indi-vidual coenobia using a laboratory-based Fouriertransform infrared (FTIR) microscope with a con-ventional globar source of IR. By drawing sizablesubpopulations of coenobia from two Scenedesmuscultures grown under either nutrient-replete orP-starved conditions, the population variability inmacromolecular composition, and the effects ofnutrient change upon this, could be estimated. Onaverage, P-starved coenobia had higher carbohydrateand lower protein absorbance compared withP-replete coenobia. These parameters varied betweencoenobia with histograms of the ratio of absorbanceof the largest protein and carbohydrate bands beingGaussian distributed. Distributions for the P-repleteand P-starved subpopulations were nonoverlapping,with the difference in mean ratios for the two popula-tions being statistically significant. Greater variancewas observed in the P-starved subpopulation. Inaddition, multivariate models were developed usingthe spectral data, which could accurately predict thenutrient status of an independent individual coenobi-um, based on its FTIR spectrum. Partial least squaresdiscriminant analysis (PLS-DA) was a better predic-tion method compared with soft independent model-ing by class analogy (SIMCA).Key index words: FTIR spectroscopy; macromole-cular composition; nutrient limitation; populationvariability; ScenedesmusAbbreviations: FTIR, Fourier transform infrared;PCA, principal component analysis; PLS-DA,partial least squares discriminant analysis; SIMCA,soft independent modeling by class analogyFTIR spectroscopy has recently emerged as a newtechnique to monitor changes in macromolecularcomponents, such as lipids, proteins, and carbohy-drates, in microalgae (see Heraud et al. 2007 for areview). We have previously demonstrated, by com-parison with conventional compositional assays, thatFTIR spectroscopy can accurately measure relativechanges in macromolecular composition in microal-gae. The rapid, simultaneous measurement fromvery small quantities of algal sample allows frequentmonitoring of macromolecular changes in microal-gal cultures of small volume, hitherto impossibleusing conventional chemical analyses. Macromolecu-lar change in microalgae was shown to be extremelylabile in response to changes in nutrient status(Giordano et al. 2001).We extended this work, by using a high bril-liance synchrotron source of IR light, to the in vivoanalysis of single algal cells (Heraud et al. 2005).Significant changes in macromolecular compositionwere measured in Micrasterias cells that had been