Mixotrophy for carbon-conserving waste upcycling

Modern chemical production, on which human quality of life depends, is unsustainable. Alternative, sustainable manufacturing routes must therefore be developed. Electrochemical and biological processes offer promise for upgrading waste streams, including recalcitrant CO ₂ and plastic-derived wastes. However, these processes are challenged by the inherent heterogeneity and high energy input requirements for upcycling of the chemical endpoints of the “take-make-waste” economy. Cupriavidus necator is emerging as a potentially useful catalyst to valorize such complex feedstocks because of its extreme metabolic flexibility, which allows it to utilize a wide array of substrates, and its ability to use CO ₂ directly via the Calvin-Benson-Bassham cycle. C. necator natively oxidizes hydrogen to power carbon utilization, but its inherent flexibility offers an as-yet unexplored opportunity to couple waste stream oxidation with CO ₂ utilization instead, potentially enabling carbon conservative waste upcycling. Here, we uncover the constraints on carbon conservative chemical transformation using C. necator as a model. We systematically examine the carbon yield and thermodynamic feasibility of mixotrophic scenarios that combine waste-derived carbon sources with hydrogen oxidation to power carbon reassimilation. Then, we evaluate carbon-carbon mixotrophic scenarios, with one carbon source providing electrons in place of hydrogen oxidation. We show that acetate, ethylene glycol, and glycerol all have high potential as inputs to carbon-neutral or carbon-negative mixotrophic fermentative upgrading, with acetate being critical to all scenarios explored here. In contrast, we find that carbon conservation is likely infeasible for most other waste-derived carbon sources. This work provides a roadmap to establishing novel C. necator strains capable of carbon efficient waste upcycling.