Biomass fermentation technologies offer alternative methods to produce platform chemicals that currently originate from fossil sources. This research showed that an enriched microbiome was capable to produce isobutyrate (i-C 4 ) from acetate via methanol-based chain elongation. A long-term continuous reactor experiment showed that the selectivity for i-C 4 and/or n-butyrate (n-C 4 ) could be reversibly adjusted by changing the reactor pH. A reactor pH of 6.75 led to formation of (carbon per total carbon of products) 0.78 n-C 4 and 0.024 i-C 4 , whereas a reactor pH of 5.2 led to a selectivity of 0.24 n-C 4 and 0.65 i-C 4 . This shift in product spectrum was also represented by a shift in microbial composition. The results suggest that a Eubacterium genus is responsible for the formation of n-C 4 , whereas a Clostridium luticellarii strain is responsible for the formation of a mixture of i-C 4 and n-C 4 . The formation of n-C 4 and i-C 4 at a low pH was observed to be coupled according to the thermodynamics of isomerization. At a reactor pH of 5.5 and 5.2, the product ratio of i-C 4 :n-C 4 approached 0.69 i-C 4 :0.31 n-C 4 , which is the theoretical ratio that would be achieved when determined by the equilibrium of isomerization. Various batch experiments at pH 5.5 and 5.2 confirmed that addition of either n-C 4 or i-C 4 at the start of the batch would directly lead to the formation of the other butyrate component. Moreover, batch experiments performed at pH 6.5 produced mainly n-C 4 and led to the development of a completely different microbiome. The imposed pH is a strong selection pressure that can facilitate changes in product selectivities for n-C 4 and i-C 4 during methanol-based chain elongation of acetate.

No abstract