Optimization of energy production and central carbon metabolism in a non-respiring eukaryote

Most eukaryotes respire oxygen, using it to generate biomass and energy. Yet, a few organisms lost the capacity to respire. Understanding how they manage biomass and energy production may illuminate the critical points at which respiration feeds into central carbon metabolism and explain possible routes to its optimization. Here we use two related fission yeasts, Schizosaccharomyces pombe and Schizosaccharomyces japonicus , as a comparative model system. We show that although S. japonicus does not respire oxygen, unlike S. pombe , it is capable of efficient NADH oxidation, amino acid synthesis and ATP generation. We probe possible optimization strategies using stable isotope tracing metabolomics, mass isotopologue distribution analysis, genetics, and physiological experiments. S. japonicus appears to have optimized cytosolic NADH oxidation via glycerol-3-phosphate synthesis. It runs a fully bifurcated TCA ‘cycle’, supporting higher amino acid production. Finally, it uses the pentose phosphate pathway both to support faster biomass generation and as a shunt to optimize glycolytic flux, thus producing more ATP than the respiro-fermenting S. pombe . By comparing two related organisms with vastly different metabolic strategies, our work highlights the versatility and plasticity of central carbon metabolism in eukaryotes, illuminating critical adaptations supporting the preferential use of glycolysis over oxidative phosphorylation. ### Competing Interest Statement The authors have declared no competing interest.