Molecular understanding of Eubacterium limosum chemostat methanol metabolism

Abstract

Methanol is a promising renewable energy carrier that can be used as a favourable substrate for biotechnology, due to its high energy efficiency conversion and ease of integration within existing infrastructure. Some acetogenic bacteria have the native ability to utilise methanol, along with other C₁ substrates, such as CO₂ and formate, to produce valuable chemicals. Continuous cultures favour economically viable bioprocesses. However, the performance of acetogens has not been investigated at the molecular level when grown on methanol. Here we present steady-state chemostat quantification of the metabolism of Eubacterium limosum, finding maximum methanol uptake rates up to 640 ± 22 mmol/gDCW/d, with significant fluxes to butyrate. To better understand the metabolism of acetogens under methanol growth conditions, we sampled chemostats for proteomics and metabolomics. Changes in protein expression and intracellular metabolomics highlighted key aspects of methanol metabolism and bottleneck conditions preventing the formation of the more valuable product, butanol. Interestingly, a small amount of formate in methylotrophic metabolism triggered a cellular state known in other acetogens to correlate with solventogenesis. Unfortunately, this was prevented by post-translation effects, including an oxidised NAD pool. There remains uncertainty around ferredoxin balance at the methylene-tetrahydrofolate reductase (Mthfr) and at the Rnf level.