Chlorella vulgaris cultivation under super high light intensity: An application of the flashing light effect

Abstract

Chlorella vulgaris was grown under super high illumination (7000 μmolPhotonPAR/m2/s). Continuous and flashing light patterns were applied to investigate potential gain originating from the flashing light effect (square pattern, duty cycle of 0.5). Three control configurations were tested, all under continuous light: 200 μmolPhotonPAR/m2/s (conventional cultivation conditions), 4000 μmolPhotonPAR/m2/s (about the same average amount of energy), and 7000 μmolPhotonPAR/m2/s (maximum incident light intensity, cells did not grow). The experiments were conducted in ultra-thin flat-panel photobioreactors to ensure isoactinic growth conditions. After acclimation, the monitored outcomes were: growth rate, macronutrient composition (proteins, carbohydrates, lipids), pigment content, and transient fluorescence (OJIP assays). The results showed that Chlorella vulgaris can grow at a similar rate under 200 μmolPhotonPAR/m2/s of continuous light and under 7000 μmolPhotonPAR/m2/s of flashing light for frequencies between 0.1 and 100 Hz. Acclimation mechanisms did not alter cell macronutrient composition, yet, chlorophyll contents decreased while carotenoids increased (allegedly linked to an increased expression of the VAZ cycle). OJIP tests also revealed potential up-regulation of the water-water cycle (featuring PTOX enzyme), which would allow faster repletion of the PQ pool, delaying photosynthetic apparatus saturation. Above 100 Hz (1000 Hz in this study), cells exhibited the same growth rate as under the equivalent amount of continuous light. It suggests that light alternation is too fast for the cells to perceive it. On the contrary, too low frequencies (0.01 Hz) showed lower performances than under the same average intensity. In this condition, during the light phase, cells are exposed to harmful conditions (continuous 7000 μmolPhotonPAR/m2/s), and the too-long dark phase only allows the cells some rest but does not bring any benefits. In terms of applicability, these results pave the way for the use of super high light (either artificial or by sunlight concentration) to overcome the energy limitation burdening photoautotrophic microalgae cultivation.