Aromatic acid metabolism in Methylobacterium extorquens reveals interplay between methylotrophic and heterotrophic pathways

Efforts towards microbial conversion of lignin to value-added products face many challenges because lignin’s methoxylated aromatic monomers release toxic C ₁ byproducts such as formaldehyde. The ability to grow on methoxylated aromatic acids (e.g., vanillic acid) has recently been identified in certain clades of methylotrophs, bacteria characterized by their unique ability to tolerate and metabolize high concentrations of formaldehyde. Here, we use a phyllosphere methylotroph isolate, Methylobacterium extorquens SLI 505, as a model to identify the fate of formaldehyde during methylotrophic growth on vanillic acids. M. extorquens SLI 505 displays concentration-dependent growth phenotypes on vanillic acid without concomitant formaldehyde accumulation. We conclude that M. extorquens SLI 505 overcomes potential metabolic bottlenecks from simultaneous assimilation of multicarbon and C ₁ intermediates by allocating formaldehyde towards dissimilation and assimilating the ring carbons of vanillic acid heterotrophically. We correlate this strategy with maximization of bioenergetic yields and demonstrate that formaldehyde dissimilation for energy generation rather than formaldehyde detoxification is advantageous for growth on aromatic acids. M. extorquens SLI 505 also exhibits catabolite repression during growth on methanol and low concentrations of vanillic acid, but no diauxie during growth on methanol and high concentrations of vanillic acid. Results from this study outline metabolic strategies employed by M. extorquens SLI 505 for growth on a complex single substrate that generates both C ₁ and multicarbon intermediates and emphasizes the robustness of M. extorquens for biotechnological applications for lignin valorization.