Near-neutral pH increased n-caprylate production in a microbiome with product inhibition of methanogenesis

[Display omitted] •The pH was varied for an ethanol-based chain-elongation microbiome with extraction.•Mildly-acidic pH (5–6) led to n-caproate (C6) production.•Near-neutral and neutral pH (6.75 and 7.0) led to n-caprylate (C8) production.•A maximum n-caprylate production rate of 75.6 ± 0.6 mmol C L-1 d-1 was obtained.•Near-neutral and neutral pH was linked to more diverse microbiomes. The pH is a critical parameter in chain-elongating bioreactors, affecting: (1) the concentration of inhibitory undissociated carboxylic acids, which in turn affects the efficiency of product extraction; (2) the thermodynamics; and (3) the kinetics. Here, we examined the effect of five different pH levels (5.5 to 7.0) on n-caprylate (C8) production using an anaerobic sequencing batch reactor (ASBR) with continuous membrane-based liquid–liquid extraction (pertraction). We found that the product spectrum was directed by pH: mildly acidic pH (5–6) led to n-caproate (C6) production, while near-neutral and neutral pH (6.75–7) favored n-caprylate production. In particular, the pH of 6.75 led to the maximum values of volumetric n-caprylate production rate (75.6 ± 0.6 mmol C L-1 d-1; 0.06 g L-1 h-1) and n-caprylate concentration in the fermentation broth (420 mM C; 7.57 g L-1). Given that methane production remained low at near-neutral and neutral pH, we theorized that the high concentration of undissociated n-caprylic acid (5.71 mM C) inhibited methanogenesis. We then demonstrated such an inhibitory effect at neutral pH in: (1) microcosm experiments; and (2) the continuous bioreactor by adding methanogenic sludge. Furthermore, 16S rRNA gene sequencing analysis revealed that near-neutral and neutral pH led to more diverse microbial communities than at mildly-acidic pH. For the first time, we report predominant n-caprylate production in a microbiome at near-neutral and neutral pH conditions where methanogenesis was controlled by the inhibitory effects of undissociated n-caprylic acid. At the same time, extraction of this species occurred even at near-neutral and neutral pH.