Prediction of the radiation characteristics and the light absorption rate of Chlamydomonas reinhardtii cultivated under a progressive nitrogen starvation and accumulating carbon reserves

This paper aims to develop a new method to predict the radiation characteristics of the green microalgae C. reinhardtii during progressive nitrogen starvation that accounts for the physiological changes induced by nutritional stress. First, the radiation characteristics of C. reinhardtii cultivated under progressive nitrogen starvation were predicted in the photosynthetically active radiation (PAR) region based on Lorenz-Mie theory using effective spectral optical properties of cells accounting for pigment contents, cessation of cell division, cell volume and size, and carbon reserve storage in the form of starch. The predictions of the spectral mass absorption and scattering cross-sections agreed very well with experimental measurements for a wide range of starch mass fractions. Second, the predicted and measured radiation characteristics were used to determine the local rate of photon absorption (LRPA) in the PAR region using the two-flux approximation and simplified two-flux approximation. The LRPA PAR-averaged relative difference between experimental calculation and numerical predictions was less than 9% for all starch mass content considered. In addition, the influence of the mass scattering cross-section on the PAR-averaged LRPA was found to be negligible despite the starch accumulation in microalgae cells. Finally, the numerically-validated tools developed in this study could be used for generating a database of radiation characteristics helpful in order to model growth and carbon reserve accumulation kinetics in microalgae.