• acclimation;
  • biodiversity;
  • cell composition;
  • FTIR spectrometry;
  • hierarchical cluster analysis;
  • homeostasis;
  • phytoplankton;
  • species identification

Fourier transform infrared (FTIR) spectrometry was used to study the spectral features of 12 eukaryotic and two prokaryotic species of microalgae. The algae were cultured in liquid media containing either NO3 or NH4+ as the sole N-source; for the NH4+ treatment, the algae were subjected to short-term (24 h) or long-term (1 month) incubations; for the hypersaline species, cells were also grown in the presence of 2 M NaCl. Over 500 spectra, acquired from at least three distinct cultures for each species, in each growth regime, were subjected to hierarchical cluster analysis (HCA) and were successfully separated according to their taxonomy, showing that the overall spectra were characteristic of each species and that this technique could be fruitfully employed to separate microalgal species living in a similar condition (as would be the case for a natural assemblage). In addition, in most cases, it was possible to differentiate between algae subjected to different growth treatments although belonging to the same species. We also demonstrated that it is possible to accurately identify species and determine the nutritional status of their environment of origin (e.g., N-source), provided that suitable FTIR spectral libraries are available. This study aims to provide the basis for the development of rapid, easy, and inexpensive methods for the evaluation of biodiversity in natural phytoplankton samples and to monitor the water quality of natural environments.