Fermentation of Organic Residues to Beneficial Chemicals: A Review of Medium-Chain Fatty Acid Production

Stamatopoulou, Panagiota; Malkowski, Juliet; Conrado, Leandro; Brown, Kennedy; Scarborough, Matthew

doi:10.3390/pr8121571

Cited by 49 publications

(30 citation statements)

References 109 publications

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“…Although co-feeding of conventional EDs has been an extensively adopted strategy to increase MCC yields ( Wu et al, 2019a ; Stamatopoulou et al, 2020 ), only a handful of studies tested the effect of co-feeding H 2 and CO 2 together with conventional EDs, short-chain carboxylates (SCC), and/or complex substrates. Among these studies, some evidence has been shown for a net positive effect of H 2 co-feeding ( Steinbusch et al, 2011 ; Arslan et al, 2012 ; Nzeteu et al, 2018 ; Wu et al, 2019b ; González-Tenorio et al, 2020 ; Liu C. et al, 2020 ).…”

Section: Introductionmentioning

confidence: 99%

Hydrogen as a Co-electron Donor for Chain Elongation With Complex Communities

Baleeiro

Kleinsteuber

Sträuber

2021

Front. Bioeng. Biotechnol.

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Electron donor scarcity is seen as one of the major issues limiting economic production of medium-chain carboxylates from waste streams. Previous studies suggest that co-fermentation of hydrogen in microbial communities that realize chain elongation relieves this limitation. To better understand how hydrogen co-feeding can support chain elongation, we enriched three different microbial communities from anaerobic reactors (A, B, and C with ascending levels of diversity) for their ability to produce medium-chain carboxylates from conventional electron donors (lactate or ethanol) or from hydrogen. In the presence of abundant acetate and CO2, the effects of different abiotic parameters (pH values in acidic to neutral range, initial acetate concentration, and presence of chemical methanogenesis inhibitors) were tested along with the enrichment. The presence of hydrogen facilitated production of butyrate by all communities and improved production of i-butyrate and caproate by the two most diverse communities (B and C), accompanied by consumption of acetate, hydrogen, and lactate/ethanol (when available). Under optimal conditions, hydrogen increased the selectivity of conventional electron donors to caproate from 0.23 ± 0.01 mol e–/mol e– to 0.67 ± 0.15 mol e–/mol e– with a peak caproate concentration of 4.0 g L–1. As a trade-off, the best-performing communities also showed hydrogenotrophic methanogenesis activity by Methanobacterium even at high concentrations of undissociated acetic acid of 2.9 g L–1 and at low pH of 4.8. According to 16S rRNA amplicon sequencing, the suspected caproate producers were assigned to the family Anaerovoracaceae (Peptostreptococcales) and the genera Megasphaera (99.8% similarity to M. elsdenii), Caproiciproducens, and Clostridium sensu stricto 12 (97–100% similarity to C. luticellarii). Non-methanogenic hydrogen consumption correlated to the abundance of Clostridium sensu stricto 12 taxa (p < 0.01). If a robust methanogenesis inhibition strategy can be found, hydrogen co-feeding along with conventional electron donors can greatly improve selectivity to caproate in complex communities. The lessons learned can help design continuous hydrogen-aided chain elongation bioprocesses.

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Section: Introductionmentioning

confidence: 99%

Hydrogen as a Co-electron Donor for Chain Elongation With Complex Communities

Baleeiro

Kleinsteuber

Sträuber

2021

Front. Bioeng. Biotechnol.

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“…The fermentation products comprise a mixture of straight-chain fatty acids, resulting from sugars and amino acids fermentation and β-oxidation of fatty acids, usually referred to as short-chain (C2–C4), medium-chain (C6–C10), long-chain (C12–C18), and a group of branched-chain fatty acids [ 2 ]. Recently, particular interest has grown in the biological production of medium chain fatty acids (MCFAs) from renewable resources as a key step to tackle societal dependence on fossil fuels [ 3 , 4 , 5 ]. These chemicals can be used as precursors of liquid biofuels or commercial chemicals [ 6 ].…”

Section: Introductionmentioning

confidence: 99%

Direct Conversion of Food Waste Extract into Caproate: Metagenomics Assessment of Chain Elongation Process

et al. 2021

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In a circular economy strategy, waste resources can be used for the biological production of high added-value substances, such as medium chain fatty acids (MCFAs), thus minimising waste and favouring a sustainable process. This study investigates single-stage fermentation processes for the production of MCFAs in a semi-continuous reactor treating the extract of real food waste (FW), without the addition of external electron donors. Two sequential acidogenic fermentation tests were carried out at an organic loading rate (OLR) of 5 and 15 gCOD L−1d−1 with a hydraulic retention time of 4 days and pH controlled at 6 ± 0.2. The highest level of caproate (4.8 g L−1) was observed at OLR of 15 gCOD L−1d−1 with a microbiome mainly composed by lactate-producing Actinomyces, Atopobium, and Olsenella species and caproate-producing Pseudoramibacter. Metagenomic analysis revealed the presence of key enzymes for the production of lactate, such as lactate dehydrogenase and pyruvate ferredoxin oxidoreductase, as well as several enzymes involved in the reverse β-oxidation pathway, thus suggesting the occurrence of a lactate-based chain elongation process.

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“…At first thought, MCC-producing mixed cultures should be protected from O 2 at all costs. So far, almost all isolated MCC producers are strict anaerobes (one exception was described by Stamatopoulou et al (2020)) to which O 2 causes damage via direct and indirect ways. O 2 gives rise to reactive oxygen species (ROS), such as O 2 − and H 2 O 2 , which are intermediates produced during O 2 reduction that severely damage cells if not promptly neutralized (Johnson and Hug, 2019).…”

Section: Introductionmentioning

confidence: 99%

Effect of Oxygen Contamination on Propionate and Caproate Formation in Anaerobic Fermentation

Baleeiro

Ardila

Kleinsteuber

et al. 2021

Front. Bioeng. Biotechnol.

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Mixed microbial cultures have become a preferred choice of biocatalyst for chain elongation systems due to their ability to convert complex substrates into medium-chain carboxylates. However, the complexity of the effects of process parameters on the microbial metabolic networks is a drawback that makes the task of optimizing product selectivity challenging. Here, we studied the effects of small air contaminations on the microbial community dynamics and the product formation in anaerobic bioreactors fed with lactate, acetate and H2/CO2. Two stirred tank reactors and two bubble column reactors were operated with H2/CO2 gas recirculation for 139 and 116 days, respectively, at pH 6.0 and 32°C with a hydraulic retention time of 14 days. One reactor of each type had periods with air contamination (between 97 ± 28 and 474 ± 33 mL O2 L−1 d−1, lasting from 4 to 32 days), while the control reactors were kept anoxic. During air contamination, production of n-caproate and CH4 was strongly inhibited, whereas no clear effect on n-butyrate production was observed. In a period with detectable O2 concentrations that went up to 18%, facultative anaerobes of the genus Rummeliibacillus became predominant and only n-butyrate was produced. However, at low air contamination rates and with O2 below the detection level, Coriobacteriia and Actinobacteria gained a competitive advantage over Clostridia and Methanobacteria, and propionate production rates increased to 0.8–1.8 mmol L−1 d−1 depending on the reactor (control reactors 0.1–0.8 mmol L−1 d−1). Moreover, i-butyrate production was observed, but only when Methanobacteria abundances were low and, consequently, H2 availability was high. After air contamination stopped completely, production of n-caproate and CH4 recovered, with n-caproate production rates of 1.4–1.8 mmol L−1 d−1 (control 0.7–2.1 mmol L−1 d−1). The results underline the importance of keeping strictly anaerobic conditions in fermenters when consistent n-caproate production is the goal. Beyond that, micro-aeration should be further tested as a controllable process parameter to shape the reactor microbiome. When odd-chain carboxylates are desired, further studies can develop strategies for their targeted production by applying micro-aerobic conditions.

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Fermentation of Organic Residues to Beneficial Chemicals: A Review of Medium-Chain Fatty Acid Production

Cited by 49 publications

References 109 publications

Hydrogen as a Co-electron Donor for Chain Elongation With Complex Communities

Hydrogen as a Co-electron Donor for Chain Elongation With Complex Communities

Direct Conversion of Food Waste Extract into Caproate: Metagenomics Assessment of Chain Elongation Process

Effect of Oxygen Contamination on Propionate and Caproate Formation in Anaerobic Fermentation

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