Photosynthetic and growth responses of Nannochloropsis oculata (Eustigmatophyceae) during batch cultures in relation to light intensity

Light is one of the most important factors affecting photosynthesis and growth of microalgal cultures. Photosynthetic responses to light change with the stage of the culture; however, little is known about how changes in the photosynthetic capacity with the duration of batch culture affect growth at the end (final harvest day) of the culture. Accordingly, we studied the photosynthetic performance and growth of Nannochloropsis oculata in the exponential phase (day 4), the linear phase of growth (day 8) and stationary phase (day 15) of batch cultures grown under a range of light intensities from 20 to 200 lmol photons m² s¹. Nannochloropsis oculata showed a high ability to acclimate to changing light conditions; cells almost doubled their cellular Chl a concentration under low light conditions and increased their photosynthetic oxygen evolution capacity, which allowed this alga to maintain a stable photosynthetic efficiency in the three stages of the culture studied. Changes in the Chl a per cell under different light conditions were not paralleled by changes in antenna size, which suggests that N. oculata uses an alternative strategy to acclimate to changing light levels. Finally, increasing photosynthetic capacity in cells growing under 20 lmol photons m² s¹ did not avoid light limitation and decreasing exponential growth rate. However, the lack of differences in final cell density achieved between 20 and 200 lmol photons m² s¹ treatments suggests that bicarbonate limitation started to affect photosynthesis and growth at the higher light treatments, from the linear phase of growth onwards. Understanding of the photosynthetic and growth responses of N. oculata under changing light conditions, where the heterogeneity of light distribution in the cultures substantially affects growth, is crucial to optimisation of the photosynthetic efficiency and growth of microalgal cultures.