Figure 4

Proposed generalized metabolic constellations in Emiliania huxleyi under N-replete and N-limited conditions: Under N-replete conditions (top), photosynthetic light reactions produce ATP and NADPH, which are consumed in the photosynthetic Calvin cycle. After export from the plastid, the produced carbohydrates are submitted to glycolysis to produce pyruvate (Pyr), which is transported to mitochondria and burned in the tricarboxylic acid (TCA-) cycle. The NADH created in cytoplasm and mitochondrion is fed into the quinone-pool (Q) of the mitochondrial respiratory chain. Lipogenesis that relies on the shuttling of citrate and malate as well as NADPH production by the pentose-phosphate pathway (PPP) works normally. Plastidary N-assimilation via the GS-GOGAT cycle provides amino acids for protein synthesis. Under N-limited conditions (bottom), photosynthesis, i.e., carbon and energy input is minimized, so that the flux through glycolysis and the TCA cycle is reduced. Cells strongly induce the citrate-shuttle that exports acetyl-CoA into the cytoplasm and increase the relative intensity of lipid turnover (synthesis and oxidation), which provides required sinks for excess reduction equivalents. To facilitate electron input into Q under these adverse flux conditions, cells make use of a malate:quinone oxidoreductase (MQO). Mitochondrial amino acid oxidation is increased, while enzymes of the ornithine-urea-cycle (OUC) and connected reactions are throttled, putatively leading to accumulation of OUC intermediates as well as polyamines and proline. The oxidation of proline to pyrroline-5-carboxylate (P5C) by proline oxidase (ProO) also appears to be a major entryway for reductants into Q when N is scarce, and a gateway to feed stored N into the metabolism. Especially the diploid stage induces machinery to scavenge external (in)organic nitrogenous compounds (blue shade).