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

Richard T Smith,D James Gilmour

doi:10.1016/j.algal.2018.01.020

Abstract

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.

Highlights

Nitrogen deprivation elicits a stress response in microalgae resulting in a rapid and global metabolic shift which, typically, leads to the accumulation of carbon storage molecules such as the biofuel feedstock triacylglycerol (TAG) [1]
This study focused on 8 key genes involved in carbon input into TAG synthesis and the surrounding carbon metabolism which have been highlighted by previous transcriptomic studies as being responsive to N deprivation and lipid accumulation (Fig. 1) [1,10,12]
On the sixth day of N deprivation chlorophyll content had been reduced by 91% and 88% for M-24-NF and M-12-NF respectively (p < 0.001); the effect of the daily dark period on this degradation appears to be negligible for mixotrophic cultures (p > 0.05)

Summary

Introduction

Nitrogen deprivation elicits a stress response in microalgae resulting in a rapid and global metabolic shift which, typically, leads to the accumulation of carbon storage molecules such as the biofuel feedstock triacylglycerol (TAG) [1]. Recent metabolic studies have focused almost exclusively on C. reinhardtii under idealised laboratory conditions of both mixotrophy and continuous light [1,6,7,10,11,12,13,14,15] Under these conditions, upon N deprivation, genes for nitrogen transporters are immediately upregulated, quickly (< 1 h) followed by a metabolic shift to a N ‘reuse and recycle’ phase [1]. Upon N deprivation, genes for nitrogen transporters are immediately upregulated, quickly (< 1 h) followed by a metabolic shift to a N ‘reuse and recycle’ phase [1] This phase results in a cessation of growth, chlorosis, and the degradation of high abundance N-rich molecules, such as ribosomes ( plastid ribosomal proteins), RuBisCo, and other abundant photosynthesis related proteins [1,14,16,17]. Cells begin to upregulate starch and TAG synthesis pathways leading to a redirection of carbon away from protein synthesis towards energy dense carbon storage molecule accumulation [12]

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Conclusion