Possibilities of Optical Monitoring of Phosphorus Starvation in Suspensions of Microalga Chlorella vulgaris IPPAS C-1 (Chlorophyceae)

Studies of the impact of inorganic phosphorus (P_i), an important nutrient, on the growth and physiological parameters of single-celled algae are important for investigations of the dynamics of phytoplankton abundance and productivity in natural ecosystems as well as in industrial systems for the cultivation of microalgae. Difficulties in carrying out such studies are associated with the complex kinetics of P_i uptake by cells and the ability of microalgae to store phosphorus in their cells. This situation necessitates efficient methods for express monitoring of microalgal cultures, such as the methods based on the registration of optical properties of cells, in particular absorption and scattering of light and fluorescence of chlorophyll contained in the cells. Here, the results of monitoring the cultures of the chlorophyte Chlorella vulgaris IPPAS C-1 starving for phosphorus are described. It was found that both optical (light absorption in the bands of the key pigments—chlorophylls and carotenoids) and luminescent (variable fluorescence of chlorophyll) parameters closely reflect the culture condition. Registration of optical properties required correction for the contribution of light scattering to the overall extinction of light by microalgal cell suspensions. At the same time, the light scattering signal is an accurate measure of the total number of suspended particles in the suspension. However, it is difficult to monitor cultures containing a significant amount of light-scattering particles lacking photosynthetic pigments (such as heterotrophic bacteria). For such cultures, the use of variable fluorescence- based parameter Fv/Fm reflecting the maximum photochemical efficiency of the photosystem II is advisable.