Photosynthetic characterization of two Nannochloropsis species and its relevance to outdoor cultivation

Despite the increased interest in exploring the potential of algal biomass production for food stock and renewable energy, very little work has been done in developing reliable screening protocols to enable the identification of species that are best suited to mass cultivation outdoors. Nannochloropsis is an algal genus identified as a potential source of lipids due to its ability to accumulate large quantities of these compounds, especially under nutrient-limiting conditions. The objective of the current work was to use two species of this genus, Nannochloropsis oceanica and N. oculata , as model organisms to develop a protocol that will allow the evaluation of their capacity to yield high biomass productivity under outdoor conditions. Growing the alga under different light intensities and measuring growth rate as well as a range of photosynthetic parameters based on light response curves and variable fluorescence highlighted significant differences between the two species. Our data show that N. oceanica cells have a better capacity to respond to higher light intensities, as reflected by growth measurements, photosynthetic electron transport rates, and oxygen evolution as well as their response to the very high photon flux densities expected in outdoor culture. On the other hand, N. oculata showed a higher tolerance to oxidative stress as reflected in its resistance to the reactive oxygen species generating compounds Rose Bengal (RB) and methyl viologen (MV). Based on the above evidence, we suggest that N. oceanica may perform better than N. oculata when grown under high light conditions typically found outdoors in summer, while N. oculata may perform better than N. oceanica under oxidative stress conditions usually found in outdoor cultures exposed to a combination of high light and low temperature commonly occurring in winter time.