The influence of exogenous organic carbon assimilation and photoperiod on the carbon and lipid metabolism of Chlamydomonas reinhardtii

Microalgae are a promising platform for the production of renewable fuels and oleochemicals. Despite significant research efforts to understand the mechanisms of algal lipid accumulation, the influence of commercially relevant growth conditions on the lipid metabolism is poorly understood. To characterise the impact of differing organic carbon availabilities and photoperiod on the response of the model alga Chlamydomonas reinhardtii to nitrogen stress, the expression of key genes involved in the central carbon metabolism were monitored over a time-course of nitrogen deprivation. In addition, the growth, PSII integrity, chlorophyll content, triacylglycerol (TAG) content, starch content, and fatty acid composition were characterised. Results indicate that both organic carbon availability and photoperiod regulate the lipid accumulation response of C. reinhardtii. Under mixotrophic conditions, organic carbon uptake is favoured over photosynthesis, transcript abundance of lipid synthesis genes rapidly increase and acetate is funnelled to TAG synthesis. In contrast, autotrophic cultures lacking organic carbon experienced a slower rate of photosynthetic degradation and funnelled the majority of sequestered carbon to starch synthesis. Dark periods induced catabolism of both starch and TAG in autotrophic cultures but TAG alone in mixotrophic cultures. Furthermore, diurnal light enhanced starch synthesis under mixotrophic conditions. Finally, transcript analysis indicated that PGD1, important for the routing of oleic acid to TAG, was reliant on organic carbon availability, resulting in reduced C18:1 fatty acid accumulation in autotrophic cultures.