Oxygen-dependent redox control enables growth-decoupled biotransformations in Cupriavidus necator

Genetic instability and environmental heterogeneity present persistent challenges to the stable microbial production of reduced, value-added chemicals. This work shows that limiting oxygen availability redirects electrons from respiration toward targeted reductive pathways in Cupriavidus necator, steering growth-decoupled biotransformations - exemplified by the conversion of acetoin to 2,3-butanediol and of glycerol to 1,3-propanediol. In cell suspension assays under a hydrogen-containing atmosphere, anoxic conditions stabilised near-stoichiometric acetoin-to-2,3-butanediol conversion (93% molar efficiency) despite minimal fructose consumption, indicating that the reductive pathway served as the primary sink for reducing equivalents. Under oxic conditions, rapid product re-oxidation lowered the final 2,3-butanediol titre to 2.6 mM. The same principle extended to 1,3-propanediol formation in a glycerol-kinase-deficient strain: anoxic incubation yielded 6.6 mM 1,3-propanediol, whereas aerobic conditions suppressed accumulation despite rapid substrate uptake. Viability assays showed that Cupriavidus necator retained above 84% viable cells under electron-acceptor exclusion for 144 h, declining to about 68% at 312 h and below 30% by 696 h, with cell aggregation from 48 h onward. This defined operational window, together with the lithoautotrophic capability of the organism, distinguishes the approach. By providing a metabolic configuration in which loss of production would be expected to be selectively disfavored, oxygen-controlled redox steering offers a route toward more robust reductive biotransformations. Oxygen availability thus acts as a single, tunable lever determining whether reducing equivalents flow into reduced products or into respiration.