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

Area of Science:

• Microbial biotechnology
• Biochemical engineering
• Environmental science

Background:

• Electron donor scarcity limits economic production of medium-chain carboxylates from waste streams.
• Co-fermentation of hydrogen is a potential strategy to overcome this limitation in microbial chain elongation.

Purpose of the Study:

• To investigate how hydrogen co-feeding influences chain elongation and medium-chain carboxylate production in diverse microbial communities.
• To determine optimal abiotic conditions for hydrogen-aided chain elongation and identify key microbial taxa involved.

Main Methods:

• Enrichment of three microbial communities with varying diversity from anaerobic reactors.
• Testing effects of abiotic parameters (pH, acetate concentration, methanogenesis inhibitors) on carboxylate production.
• 16S rRNA amplicon sequencing to identify microbial community composition and correlate taxa with hydrogen consumption.

Main Results:

• Hydrogen co-feeding facilitated butyrate production in all communities and improved i-butyrate and caproate production in more diverse communities.
• Optimal conditions increased caproate selectivity from 0.23 to 0.67 mol e-/mol e-, yielding 4.0 g L-1 caproate.
• Hydrogenotrophic methanogenesis by Methanobacterium occurred despite high acetic acid and low pH, indicating a trade-off.

Conclusions:

• Hydrogen co-feeding significantly enhances caproate production selectivity in complex microbial communities.
• The genera Caproiciproducens and Clostridium sensu stricto 12 were identified as potential caproate producers.
• Developing effective methanogenesis inhibition strategies is crucial for optimizing continuous hydrogen-aided chain elongation bioprocesses.