Molecular heterogeneity in Aphanizomenon flos-aquae and Anabaena flos-aquae (Cyanophyta): A synchrotron-based Fourier-transform infrared study of lake micropopulations

Synchrotron-based Fourier-transform infrared (FTIR) microspectroscopy was used to characterize micropopulations of the cyanobacteria Anabaena flos-aquae and Aphanizomenon flos-aquae in mixed phytoplankton samples taken from the epilimnion and hypolimnion of a stratified eutrophic lake (Rostherne Mere, UK). All FTIR spectra (normalized to amide I) showed a closely similar sequence of 10 bands over the wavenumber range 4,000-900 cm -1 and were assigned to a range of vibrationally active chemical groups. Interspecific differences between Anabaena and Aphanizomenon were revealed using correlation and multivariate analysis (principal component analysis, PCA) of the set of individual band intensities. Differences between species (separately for epilimnion and hypolimnion) were also analysed by comparing band intensity means, and by performing PCA over a broad spectral region (1,750-900 cm -1 ). Comparison of intensity means showed that spectra obtained from epilimnetic algae had significant differences in bands 5-7 and in bands 4, 9 and 10. However, in the spectra obtained from the hypolimnion algae, a different pattern was observed, with significant differences between the species restricted to bands 7-10. PCA of the spectral region 1,750-900 cm -1 also demonstrated interspecific differences in relation to carbohydrate and protein concentrations, with Anabaena displaying higher carbohydrate levels in both the epilimnion and hypolimnion. Intraspecific comparisons of band intensity means for epilimnion and hypolimnion subpopulations demonstrated clear differences in Aphanizomenon , where significant differences were observed between epilimnion (higher carbohydrate concentration compared with protein) and hypolimnion (lower carbohydrate concentration compared with protein) subpopulations. Anabaena however showed only minor differences. PCA comparison of the spectral region 1,750-900 cm -1 , followed by data visualization using scores plots and loading plots, also demonstrated quantitative differences. Scores plots resolved the epilimnion and hypolimnion subpopulations in Aphanizomenon (but not Anabaena ), with the loading plot showing large differences in carbohydrates, supporting the analysis using discrete peak intensities. © 2006 British Phycological Society.