When diatoms are stressed for inorganic nitrogen they remodel their intermediate metabolism and redirect carbon towards lipid biosynthesis. However, this response comes at a significant cost reflected in decreased photosynthetic energy conversion efficiency and growth. Here we explore a molecular genetics approach to restrict the assimilation of inorganic nitrogen by knocking down nitrate reductase (NR). The transformant strain, NR21, exhibited about 50% lower expression and activity of the enzyme but simultaneously accumulated over 40% more fatty acids. However, in contrast to nitrogen-stressed wild-type (WT) cells, which grow at about 20% of the rate of nitrogen-replete cells, growth of NR21 was only reduced by about 30%. Biophysical analyses revealed that the photosynthetic energy conversion efficiency of photosystem II was unaffected in NR21; nevertheless, the plastoquinone pool was reduced by 50% at the optimal growth irradiance while in the WT it was over 90% oxidized. Further analyses reveal a 12-fold increase in the glutamate/glutamine ratio and an increase NADPH and malonyl-CoA pool size. Transcriptomic analyses indicate that the knock down resulted in changes in the expression of genes for lipid biosynthesis, as well as the expression of specific transcription factors. Based on these observations, we hypothesize that the allocation of carbon and reductants in diatoms is controlled by a feedback mechanism between intermediate metabolites, the redox state of the plastid and the expression and binding of transcription factors related to stress responses. Significance Statement Carbon and nitrogen metabolism are tightly coupled and the fluxes of these two elements into macromolecules is largely pre-determined. Here we show that photosynthetic fixed carbon can be re-routed towards lipid accumulation in the diatom Phaeodactylum tricornutum with relatively low bioenergetic cost, and propose that lipid accumulation is controlled by a feedback mechanism between intermediate metabolites, the redox state of the plastid, and the expression and binding of stress response-related transcription factors.
All Science Journal Classification (ASJC) codes
- Plant Science
- Cell Biology
- Phaeodactylum tricornutum
- nitrate reductase