Relating nitrogen concentration and light intensity to the growth and lipid accumulation of Dunaliella viridis in a photobioreactor

The marine microalgal species Dunaliella viridis is a promising source of biofuels because of its ability to accumulate neutral lipids without freshwater resources for growth. Less energy for biofuel production from D. viridis may also be required because of the possible extraction of lipids through osmotic lysis. However, there is a lack of experimental data on the growth of D. viridis that can be used in mathematical models for process design. In this study, experiments were performed in a 3.2 L photobioreactor with both pH and temperature tightly controlled at 7.6 and 25 °C, respectively. Levels of neutral lipids, biomass, carbohydrate, chlorophyll, and cell counts of D. viridis were evaluated for ten days at various initial light intensities (100, 300, 400, and 600 μmol photons m−2 s−1) and nitrogen levels (9 and 60 g m−3 as nitrogen). The maximum lipid production observed was 26.9 g m−3 with 3.15 g m−3 day−1 average lipid productivity at an initial nitrogen concentration of 9 g m−3 and initial light intensity of 300 μmol photons m−2 s−1. The data show that D. viridis accumulates cellular carbohydrates as a primary carbon and energy storage product and then converts the cellular carbohydrates into cellular lipids as a secondary storage product when nitrogen is depleted. The on-line light data revealed linear relationships between real-time light intensity and biomass concentration (r = 0.96, p < 0.01) and between real-time light intensity and chlorophyll a concentration (r = − 0.89, p < 0.01), suggesting the possible use of real-time light data as a method for in situ quantification of biomass concentration and chlorophyll a concentration.