Guzman, J. J. L. | Search bioRxiv

bioRxiv2022

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

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 d-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.

bioengineering↗

bioRxiv2020

Suppressing peatland methane production by electron snorkeling through pyrogenic carbon

Northern peatlands are experiencing more frequent fire events as a result of changing climate conditions. Forest fires naturally result in a direct and negative climate impact by emitting large amounts of carbon into the atmosphere. Recent studies show that this extensive emission may shift the soil carbon regime from a sink to a source. However, the fires also convert parts of the burnt biomass into pyrogenic carbon. Here, we show an indirect, but positive, climate impact induced by fire-derived pyrogenic carbon. We found that the accumulation of pyrogenic carbon reduced post-fire methane production from peatland soils by 13-24%. The conductive, capacitive, and redox-cycling electron transfer mechanisms enabled pyrogenic carbon to function as an electron snorkel, which redirected soil electron fluxes to facilitate alternative microbial respiration and reduced the rate of methane production by 50%. Given the fact that methane has a 34-fold greater warming potential than carbon dioxide, we estimate that global greenhouse gas emissions are reduced by 35 Tg CO2e annually through the electron snorkeling of pyrogenic carbon in peatlands. Our results highlight an important, but overlooked, function of pyrogenic carbon in neutralizing forest fire emissions and call for its consideration in the global carbon budget estimation.

microbiology↗