Acclimation dynamics of Chlorella vulgaris to sudden light change

Abstract

Chlorella vulgaris photoacclimation was monitored over eight instantaneous light intensity changes. The intensities ranged between $35\mathrm{\mu mol\; Photo}{n}_{\mathrm{PAR}}.m^{-2}.s^{-1}$ and $600\mathrm{\mu molPhoto}{n}_{\mathrm{PAR}}.m^{-2}.s^{-1}$. Cultures were grown in ultra-thin flat panel photobioreactors under continuous light and maintained in low cell density to ensure homogeneous light availability. Photoacclimation was evaluated through spectral quantification of pigments and fluorometric assays. The former gave access to a proxy of chlorophyll and carotenoid content, the latter to the Photosystem-II cross-section (${\sigma }_{\mathrm{PSII}}$) and qualification of the photosynthetic machinery (via $\mathrm{OJIP}$ assays). Both the acclimated steady-state values of pigment content and the dynamic of their evolutions after sudden light intensity change were monitored. The characteristic times of the transitions were estimated based on a first-order assumption. Results consistently showed that antenna size adjustment of Chlorella vulgaris was primarily dictated by the light availability, both regarding the acclimated steady-state values and the acclimation dynamics. An energetic limitation was highlighted by the acclimation dynamics at low light. The characteristic time of transition was estimated to be $16.6\pm 2.17h$ for the transition to the lowest light intensity ($35\mathrm{\mu mol\; Photo}{n}_{\mathrm{PAR}}.m^{-2}.s^{-1}$) and $3.55\pm 1.01h$ for intensities higher than the maximal intensity of photolimitation ($120\mathrm{\mu mol\; Photo}{n}_{\mathrm{PAR}}.m^{-2}.s^{-1}$). No hysteresis effect was observed as light intensities were shifted once and reverted to their original values. These results extend the literature regarding photoacclimation dynamics of antenna size and photosynthetic apparatus. They are well-suited to calibrate photoacclimation models and can provide valuable insight into the strategies to implement for culture scale-up, fed-batch, and semi-continuous processes.