Regulation of starch, lipids and amino acids upon nitrogen sensing in Chlamydomonas reinhardtii

Abstract

Chlamydomonas reinhardtii has recently emerged as a viable alternative source of fossil fuel. However, the metabolic flow of carbon in C. reinhardtii towards making the carbon reserve is not yet understood. In addressing this issue, we have monitored the assimilation of 13C-labeled acetate by the wild-type (cw15) and the starch deficient (sta6) strains of C. reinhardtii using 13C NMR. The dynamics of starch and lipid reserves was studied under nitrogen starvation and supplement conditions. The starch was found to accumulate and mobilize faster than triacylglycerol (TAG) through the formation of CO2aq and bicarbonate. Addition of exogenous acetate significantly slowed down mobilization of both labeled starch and TAG reserves, suggesting that cells preferred incoming acetate to internal reserve as a source of carbon. In parallel, we undertook the analyses of cellular free amino acid pool changes using LC/MS to investigate how specific amino acid changes contribute towards “nitrogen sparing effect” during nitrogen starvation. Bulk of remainder amino acids do not exhibit any discernible changes in their steady state pool sizes, suggesting the possibility that carbon flow leading to starch, followed by lipid is homeostatically balanced during nitrogen starvation such that the amino acid carbon levels in the cells stay largely regulated. Taken all together, this study thus describes an excellent cellular system for probing the nitrogen starvation, nitrogen uptake and the consequent changes in carbon flow in relation to both starch/lipid accumulation and their re-mobilization and the changes in free amino acid pools in C. reinhardtii cells.