Abstract
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.
Highlights
Ethanol, lactate, and sugars are conventional electron donors (EDs) that enable production of medium-chain carboxylates (MCC) through microbial chain elongation (CE) in anaerobic fermentation (Wu et al, 2019a)
Primer sequences were removed from adapter-clipped reads using Cutadapt (Martin, 2011) and further sequence data analysis was done through the DADA2 workflow, using the amplicon sequence variant (ASV) approach as described by Callahan et al (2016)
At pH 5.5, H2 addition resulted in the accumulation of on average 3712 mg L−1 (62 mM) more acetate and 610 mg L−1 (6.9 mM) more butyrate in comparison to H2-free controls
Summary
Lactate, and sugars are conventional electron donors (EDs) that enable production of medium-chain carboxylates (MCC) through microbial chain elongation (CE) in anaerobic fermentation (Wu et al, 2019a). When ED concentration in the substrate does not suffice, many lab-scale studies opted in for supplementing chemicalgrade lactate, ethanol or sugars during anaerobic fermentation to achieve high caproate productivities (Grootscholten et al, 2014; Roghair et al, 2018b). Yeast extract, commonly used as nutrient source and microbial growth enhancer (Grootscholten et al, 2013), should be considered as a possible source of EDs since it is able to sustain some carboxylate production by itself (Richter et al, 2013; Chen et al, 2016; San-Valero et al, 2020). If MCC production through anaerobic fermentation is meant to become a more competitive and sustainable biorefinery process, it should not depend on costly ED supplementation (Richter et al, 2013; Chen et al, 2017)
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