Lab-scale investigation of the potential of online chlorophyll a PAM fluorescence in monitoring solar photobioreactors

In solar microalgae cultures, cells experience various dynamics either as a direct result of the diurnal cycle and changes in weather conditions (e.g. light, temperature) or an indirect result of varying environmental parameters (e.g. changes in dissolved oxygen or nutrient concentration with photosynthesis and culture growth). These variations can affect the efficiency of photosynthesis and consequently cell growth. One solution for monitoring solar cultures is online measurement of chlorophyll a fluorescence, a non-invasive method whose interest has already been proven for measuring parameters related to the state of the photosynthetic apparatus, such as the quantum yield of photosynthesis or non-photochemical quenching (NPQ). For this study, the use of PAM chlorophyll a fluorescence was extended by introducing radiative transfer models to enable the effect of light attenuation in the culture volume to be taken into account. The investigation was carried out on a fully controlled lab-scale PBR equipped with a PAM chlorophyll a fluorescence on its optical surface, and replicating various conditions of growing Chlorella vulgaris exposed to solar cycles. By measuring the dynamic responses to environmental changes, PAM fluorescence was found to have potential for detecting stress in the culture (e.g. nutrient limitation, over-accumulation of oxygen) and for online estimation of valuable data such as chlorophyll a content (related to the light attenuation conditions in the culture volume) and growth rate (e.g. biomass productivity).