Optimizing PHB Bioplastic Accumulation in Sinorhizobium meliloti grown on CO2-derived Formate and Bicarbonate

Abstract

Reducing global carbon expenditure requires dedication to innovation and problem-solving at every level, including a rapid divestment from fossil-based fuels and materials. Fossil fuel-based feedstocks can be replaced with carbon-sequestering alternatives that are microbially converted into value-added products, and renewable energy can be used to power these processes. In this work, we aimed to develop an electro-biohybrid system for the conversion of CO2 to the bioplastic polyhydroxybutyrate (PHB). This system involves the electrochemical reduction of CO2 to formate, which would serve as a feedstock for a PHB-producing microbe. For the microbe, we chose S. meliloti due to its natural ability to grow on a wide range of carbon substrates including formate, its genetic tractability, and its ability to naturally synthesize valuable bioproducts such as PHB. When carbon availability exceeds the supply of critical nutrients, S. meliloti accumulates PHB to store surplus reductive power and carbon. The bacterium then degrades the polymer in times of starvation or physical stress in order to achieve long-term survival under difficult conditions. Our tests revealed that although S. meliloti can use formate as a carbon source, formate concentrations above 20 mM are toxic and inhibit the growth of S. meliloti. There was no significant difference in the growth of S. meliloti when provided 20 mM of commercially available formate compared to electrochemically generated formate. This confirmed that the electrochemical process did not generate toxic by-products that would inhibit growth. In an effort to increase PHB yield, we used a S. meliloti mutant lacking the PHB depolymerase gene. We observed the mutant accumulating more PHB than the wild type in balanced growth conditions. Overall, this work contributes to the development of a fossil fuel-independent chemical industry that can sequester waste CO2 into value-added products while mitigating climate change and closing the carbon loop.