Gradual changes in the photosynthetic apparatus triggered by nitrogen depletion during microalgae cultivation in photobioreactor

Changes of photosynthetic activity during natural depletion of nitrogen in a batch culture of Chlorella vulgaris were studied using chlorophyll a fluorescence-based methods. Complex analysis including JIP-test, multiexponential approximation, and the analysis of differential curves was carried out on the recorded fluorescence transients. Three pivot points were detected in dynamics of the JIP-test parameters during culture growing. We associated these time points with different stages of mineral stress progress. Immediately after nitrogen exhaustion in the cultivation medium, a transient increase in the efficiency of PSII was detected. During this short period, the photosynthetic apparatus acclimated to the stress by reducing the number of PSII reaction centers, simultaneously increasing the effective cross section of the light-harvesting antenna per reaction center. However, prolonged nutrient starvation impaired the structure of the photosynthetic apparatus, deactivating the oxygen-evolving complex and impairing the overall electron transport.