Abstract
Chain elongation reactor microbiomes produce valuable medium-chain carboxylates (MCC) from non-sterile residual substrates where lactate is a relevant intermediate. Gas supply has been shown to impact chain elongation performance. In the present study, the effect of nitrogen gas (N2) supply on lactate metabolism, conversion rates, biomass growth, and microbiome composition was evaluated in a lactate-fed upflow anaerobic reactor with continuous or intermittent N2 gas supply. Successful MCC production was achieved with continuous N2 gas supply at low superficial gas velocities (SGV) of 0.22 m∙h−1. Supplying N2 at high SGV (>2 m∙h−1) either continuously (2.2 m∙h−1) or intermittently (3.6 m∙h−1) disrupted chain elongation, resulting in production of short-chain carboxylates (SCC), i.e., acetate, propionate, and n-butyrate. Caproiciproducens-dominated chain-elongating microbiomes enriched at low SGV were washed out at high SGV where Clostridium tyrobutyricum-dominated microbiomes thrived, by displaying higher lactate consumption rates. Suspended growth seemed to be dominant regardless of SGV and gas supply regime applied with no measurable sludge bed formed. The highest MCC production from lactate of 10 g COD∙L−1∙d−1 with electron selectivities of 72 ± 5%was obtained without N2 gas supply at a hydraulic retention time (HRT) of 1 day. The addition of 5 g∙L−1 of propionate did not inhibit chain elongation, but rather boosted lactate conversion rates towards MCC with n-heptylate reaching 1.8 g COD∙L−1∙d−1. N2 gas supply can be used for mixing purposes and to steer lactate metabolism to MCC or SCC production.
Highlights
Chain elongation reactor microbiomes produce valuable medium-chain carboxylates (MCC) from non-sterile residual substrates
The present study aimed to evaluate the effects of N2 gas supply on lactate metabolism and conversion rates during continuous lactate-based chain elongation
By the end of phase 1, about 40% of the fed lactate was converted (1.2 ± 0.04 e− eq·L−1 ·d−1 ; 8.9 ± 0.3 g·L−1 ·d−1 ) to mainly n-caproate (Figure S3). n-caproate was produced at concentrations of 5.4 ± 0.2 g·L−1 with productivities and selectivities of 0.7 ± 0.4 e− eq·L−1 ·d−1 (2.6 ± 0.1 g·L−1 ·d−1 ) and 60 ± 6%, respectively (Figure 2). n-heptylate and n-caprylate were produced at selectivities of 5 ± 2% and 2 ± 1%, respectively
Summary
Chain elongation reactor microbiomes produce valuable medium-chain carboxylates (MCC) from non-sterile residual substrates. MCC are saturated monocarboxylic acids with 6 to 12 carbon atoms that find applications in lubricants, bioplastics, antimicrobials, feed additives, and biofuels production [1]. MCC can be used in a second bioprocess to accumulate medium-chain length polyhydroxyalkanoates in bacteria such as Pseudomonas putida [2]. Chain-elongating microbiomes utilize energy-rich substrates, e.g., ethanol, lactate, and glucose, as electron donors to elongate short-chain carboxylates (SCC, 1–5 carbon units) to MCC through a series of biochemical condensation and reduction reactions in the reverse-β-oxidation (RBO) pathway [1]. Electron donors are oxidized to provide electrons (e.g., NADH) and acetyl-CoA for the RBO pathway where two acetylCoA are elongated to even-chain carboxylates, e.g., n-butyrate (nC4), n-caproate (nC6), or n-caprylate (nC8). Lactate is an interesting electron donor that can be obtained from residual biomass materials [3,4,5] and has been successfully converted to n-caproate [4,6,7,8]
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