Assessment of growth phases of the diatom Ditylum brightwellii by FT-IR and Raman spectroscopy

Most applications of monitoring microalgae necessitate fast deployable and sensitive methods to assess cellular physiology, preferably at the scale of single cells. Vibrational spectroscopies, namely Fourier transform infrared (FT-IR) and Raman spectroscopy, have been proven to be valuable tools since they are label-free, nondestructive, and fast applicable. Infrared spectra contain spectral contributions of macromolecules, such as proteins, nucleic acids, carbohydrates, biosilica and lipids. A major concern of this method is the requirement to dry the sample, which might severely affect cellular physiology. Raman spectroscopy, on the other hand, can be applied in aqueous environments, and provides complementary information to FT-IR spectroscopy, because Raman spectra of microalgae are dominated by the photosynthetic pigments, e.g. chlorophyll a and carotenoids. In our current study we applied both spectroscopy methods to study chemical variations within individual cells of the diatom Ditylum brightwellii during the course of growth phases. Culture growth was monitored via cell counting and related spectral changes were examined employing partial least square (PLS) regression and linear discriminant analysis (LDA) classification. FT-IR models revealed a pronounced decrease of protein and carbohydrate content, a concurrent increase in cellular lipid level and constant biosilica content during culture growth. Interestingly, the highest nucleic acid content appeared in the stationary phase. Raman based models could easily identify cells in the exponential phase indicating highest chlorophyll a amount in that particular stage. Further cell proliferation was accompanied with distinct variances in the carotenoid pool. Comparison of model performances showed that Raman based models were more stable and more accurate than respective FT-IR based models. This leads us to conclude that Raman spectroscopy is a promising method for examining microalgal cell physiology.