Regulation of acidogenic metabolism towards enhanced short chain fatty acid biosynthesis from waste: metagenomic profiling

Abstract

Short chain carboxylic (volatile fatty) acid (VFA) production in mixed microbiomes is majorly limited by the prevalence of methanogenic bacteria and the availability of substrate from waste to the biocatalyst during the fermentation process. To enhance the VFA production from food waste, the present study evaluates a strategy for selective enrichment of the biocatalyst by exposing it to acid-shock, followed by operation under alkaline conditions (pH 10). A comprehensive system based analysis was carried out during the bio-based platform chemical synthesis from waste, in conjugation with microbial profiling and bio-electrochemical analysis. After the selective enrichment of the biocatalyst, enhanced VFA synthesis was conducted with pretreated biocatalyst (PT; 11.1 g L⁻¹) and compared with untreated parent biocatalyst (UT; 6.1 g L^−​1). In both systems biohydrogen was the co-product. Variations in the VFA profiles were documented with respect to the biocatalyst used, which influenced the degree of acidification (DOA – PT: 37% and UT: 11%). A high fraction of acetic acid (6.9 g L⁻¹) was observed, followed by butyric acid (2.6 g L⁻¹) and propionic acid (1.3 g L⁻¹) in PT operation, contrary to the control system (acetic acid, 3.9 g L⁻¹, butyric acid, 1.6 g L⁻¹; propionic acid, 0.9 g L⁻¹). Specifically, the PT system showed the biosynthesis of iso-valeric acid: 0.15 g L⁻¹ (C5) and caproic acid: 1.9 g L⁻¹ (C6), which indicates the possibility for chain-elongation through the selective enrichment of the microbial community. The PT system showed E_peak at −0.415 V on the cyclic voltammogram, which corresponds to the involvement of the redox couple, H⁺/H₂, correlating with the enhanced acidogenic process, unlike UT. Tailoring of the parent inoculum (pretreatment) resulted in the enrichment and enhancement of the capabilities of the biocatalyst in secreting the redox mediators, which were not detected in the UT system. Acidogenic firmicutes (spore formers) and fatty acid producing bacteroides were enriched in the PT system along with saccharolytic and proteolytic bacteria (Bacillus cellulosilyticus (alkalophile), Soehngenia saccharolytica, etc.). The presence of Clostridium autoethanogenum and Propionibacterium freudenreichii in the PT system supports effective utilization of complex carbohydrates, facilitating acidification.