Abstract
Medium-chain carboxylates (MCC) derived from biomass biorefining are attractive biochemicals to uncouple the production of a wide array of products from the use of non-renewable sources. Biological conversion of biomass-derived lactate during secondary fermentation can be steered to produce a variety of MCC through chain elongation. We explored the effects of zero-valent iron nanoparticles (nZVI) and lactate enantiomers on substrate consumption, product formation and microbiome composition in batch lactate-based chain elongation. In abiotic tests, nZVI supported chemical hydrolysis of lactate oligomers present in concentrated lactic acid. In fermentation experiments, nZVI created favorable conditions for either chain-elongating or propionate-producing microbiomes in a dose-dependent manner. Improved lactate conversion rates and n-caproate production were promoted at 0.5–2 g nZVI⋅L–1 while propionate formation became relevant at ≥ 3.5 g nZVI⋅L–1. Even-chain carboxylates (n-butyrate) were produced when using enantiopure and racemic lactate with lactate conversion rates increased in nZVI presence (1 g⋅L–1). Consumption of hydrogen and carbon dioxide was observed late in the incubations and correlated with acetate formation or substrate conversion to elongated products in the presence of nZVI. Lactate racemization was observed during chain elongation while isomerization to D-lactate was detected during propionate formation. Clostridium luticellarii, Caproiciproducens, and Ruminococcaceae related species were associated with n-valerate and n-caproate production while propionate was likely produced through the acrylate pathway by Clostridium novyi. The enrichment of different potential n-butyrate producers (Clostridium tyrobutyricum, Lachnospiraceae, Oscillibacter, Sedimentibacter) was affected by nZVI presence and concentrations. Possible theories and mechanisms underlying the effects of nZVI on substrate conversion and microbiome composition are discussed. An outlook is provided to integrate (bio)electrochemical systems to recycle (n)ZVI and provide an alternative reducing power agent as durable control method.
Highlights
Production of biochemicals from renewables is of outmost importance to reduce anthropogenic impact on the environment
Substituting nano zero-valent iron (nZVI) with hydrogen at 0.45 atm or 1.2 atm in experiment III did not result in n-caproate formation
At 5 g nZVI·L−1 propionate formation instead of n-caproate was favored. n-butyrate formation was primarily observed in the first 2 days. pH increased to a value of 7.5 (Figure 2C) and propionate started being produced from the leftover lactate to reach 6.5 ± 1.2 g·L−1 of propionate with no clear acetate consumption thereafter
Summary
Production of biochemicals from renewables is of outmost importance to reduce anthropogenic impact on the environment. Short-chain carboxylates (SCC, up to 5 carbons) and methane are commonly observed in biological conversion of organics during anaerobic fermentation whereas medium-chain carboxylates (MCC, 6–12 carbons) are produced by specialized chain-elongating bacteria in the presence of reduced compounds in the so-called chain elongation process (Angenent et al, 2016). Pyruvate is further converted to acetyl-CoA and carbon dioxide (CO2) through pyruvate:ferredoxin oxidoreductase (PFOR) (Liu et al, 2020). This two-step lactate oxidation to acetate (Eq 2) yields electrons and carbon for the reverse-β-oxidation (RBO) pathway (Liu et al, 2020). In the RBO pathway, acetate is elongated with two carbons from acetyl-CoA to even-chain carboxylates such as n-butyrate (nC4) and n-caproate (nC6). Propionate can be produced from lactate (Eq 9) by organisms such as Megasphaera elsdenii (Hino and Kuroda, 1993) or propionic acid bacteria (PAB) (Seeliger et al, 2002; GonzalezGarcia et al, 2017)
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