Growth bottlenecks of microalga Dunaliella tertiolecta in response to an up-shift in light intensity

Microalgae are a potential source of valuable products including drugs, chemicals, food supplements, biofuels, animal feed and fertilizers. In outdoor culture, biomass productivity is dependent on how well the microalgae respond to fluctuations in light during the day. Dunaliella tertiolecta is a halotolerant, photoautotrophic microalga. Our objective is to identify rate-limiting steps in growth as D. tertiolecta acclimates to changing light intensity. We studied the physiological, metabolic and transcriptomic changes of D. tertiolecta in a low dilution rate chemostat culture to determine rate-limiting processes in cell growth in response to changes in light intensity. Upon a 10-fold increase in light intensity from 40 to 400 mu mol photons m(-2) s(-1) (HL), the photosynthetic efficiency and maximum photosynthetic rate (PR) per cell, or per chlorophyll, did not change significantly during lag (2h) and exponential growth (8h) phases, which suggests that the photosynthetic system was not the growth rate limiting step in response to higher light intensity. The expression of most genes in 2h and 8h samples was similar. Some genes involved in synthesis of cellular structures, protein processing and the cell cycle might be bottlenecks in the HL response, because they were expressed only at 8h. It might not be practical to increase biomass by genetic modification of D. tertiolecta because of the numerous pathways and genes that are potential bottlenecks. Instead, future research can be directed to improving carbon sequestration by channelling it to storage compounds or excretion as extracellular glycerol.