Resolution of codominant phytoplankton species in a eutrophic lake using synchrotron-based Fourier transform infrared spectroscopy

Synchrotron-based Fourier-transform infrared (FTIR) microspectroscopy was used to distinguish micropopulations of the codominant algae Microcystis aeruginosa (Cyanophyceae) and Ceratium hirundinella (Dinophyceae) in mixed phytoplankton samples taken from the water column of a stratified eutrophic lake (Rostherne Mere, UK). FTIR spectra of the two algae showed a closely similar sequence of 10 bands over the wave-number range 4000–900 cm 1. These were assigned to a range of vibrationally active chemical groups using published band assignments and on the basis of correlation and factor analysis. In both algae, intracellular concentrations of macromolecular components (determined as band intensity) varied considerably within the same population, indicating substantial intraspecific heterogeneity. Interspecific differences were separately analysed in relation to discrete bands and by multivariate analysis of the entire spectral region 1750–900 cm 1. In terms of discrete bands, comparison of individual intensities (normalised to amide 1) demonstrated significant (99% probability level) differences in relation to six bands between the two algal species. Key interspecific differences were also noted in relation to the positions of bands 2, 10 (carbohydrate) and 7 (protein) and in the 3-D plots derived by principal component analysis (PCA) of the sequence of band intensities. PCA of entire spectral regions showed clear resolution of species in the PCA plot, with indication of separation on the basis of protein (region 1700–1500 cm 1) and carbohydrate (region 1150–900 cm 1) composition in the loading plot. Hierarchical cluster analysis (Ward algorithm) of entire spectral regions also showed clear discrimination of the two species within the resulting dendrogram.