Chlorella vulgaris acclimated cultivation under flashing light: An in-depth investigation under iso-actinic conditions

Abstract

Chlorella vulgaris was cultivated under continuous and flashing light. Special care was taken in designing a culture device ensuring iso-actinic conditions, and a protocol ensuring acclimation. Continuous light cultures were carried out from 25 to 800 μmolPhotonPAR/m2/s. They served as control. Flashing light (square shape, duty cycle of 0.5) was applied with four frequencies: 0.1, 1, 10, and 100 Hz. The monitored outcomes were the culture growth rate, cell pigment content (chlorophylls, lutein, violaxanthin, and zeaxanthin), and photosynthetic apparatus (Fv/Fm and light curve). Acclimated cultures under continuous light showed photolimitation and photosaturation phases, without sign of photoinhibition, even under 800 μmolPhotonPAR/m2/s. Frequencies of 1 Hz and above induced little to no difference between cells acclimated under flashing light and their control (same amount of continuous light). Indeed, cells exhibited the same growth rate, similar absolute pigment composition, and light curve characteristics. They only differ by a lower chlorophyll b to chlorophyll a ratio at high intensity, suggesting an acclimation strategy favoring an increased number of photosystems instead of an increased light-harvesting capability. Conversely, cells cultivated at 0.1 Hz showed a lower growth rate and an incapacity to adapt efficiently to high incident illumination. From a biotechnological perspective, these results support the idea that high frequency flashing light, in the tested conditions, does not bring benefits with respect to the same average amount of continuous light. Consequently, it may ease the design of large scale photobioreactors, where light/dark cycles are inevitable.