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

Efforts toward 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 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 acid. M. extorquens SLI 505 displays concentration-dependent growth phenotypes on vanillic acid without concomitant formaldehyde accumulation. We conclude that M. extorquens SLI 505 overcomes metabolic bottlenecks from simultaneous assimilation of multicarbon and C ₁ intermediates by allocating formaldehyde toward 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 diauxic patterns 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.

IMPORTANCE

Lignin, one of the most abundant and renewable carbon sources on Earth, is a promising alternative to non-renewable fossil fuels used to produce petrochemicals. Degradation of lignin releases toxic C ₁ byproducts such as formaldehyde, and thus most microorganisms are not suitable for biorefining lignin. By contrast, Methylobacterium extorquens SLI 505 is capable of growth on high concentrations of aromatic acids without concomitant formaldehyde accumulation. In addition, we show that the growth of M. extorquens SLI 505 on aromatic acids is coupled to the production of the bioplastic, polyhydroxybutyrate. Aromatic acids serve as a model by which to understand how M. extorquens SLI 505 balances methylotrophic and heterotrophic pathways during growth to provide strategies for growth optimization when using complex substrates in both ecological and industrial fermentation applications.