From formic acid to single-cell protein: genome-scale revealing the metabolic network of Paracoccus communis MA5.

With the increase in population growth and environmental pollution, the daily protein supply is facing great challenges. Single-cell protein (SCP) produced by microorganism fermentation is a good alternative for substituting plant- and animal-derived proteins. In this study, Paracoccus communis MA5 isolated from soil previously demonstrated an excellent ability to synthesize SCP directly from sodium formate. To investigate the central metabolic network of formic acid assimilation and protein synthesis, genome-scale analyses were performed. Genomic analysis showed that complete tetrahydrofolate cycle-, serine cycle-, glycolytic pathway-, tricarboxylic acid (TCA) cycle- and nitrogen metabolism-relevant genes were annotated in the genome. These pathways play key roles in the conversion of formic acid into proteins. Transcriptional analysis showed that sodium formate stress could stimulate the metabolic pathway in response to environmental stress, but weaken the sulfur metabolic pathway to inhibit amino acid synthesis, resulting in a decrease in protein content (30% vs 44%). However, under culture conditions with ammonium sulfate, metabolic pathways associated with protein synthesis were accelerated, causing an increase in protein content (53% vs 44%); while the tetrahydrofolate cycle associated with formic acid assimilation was inhibited, causing a 62.5% decrease in growth rate (OD: 0.21 vs 0.56). These results provide evidence of protein synthesis from sodium formate in strain MA5 at the gene level and lay a theoretical foundation for the optimization of fermentation systems using formic acid as a carbon source.