Phenol promoted caproate production via two-stage batch anaerobic fermentation of organic substance with ethanol as electron donor for chain elongation

The conversion of organic wastes/wastewater into medium chain fatty acids (MCFAs) such as caproate has attracted much attention, while the effects of toxic compounds on the process have rarely been studied. The present study investigated the effects of phenol (0-1.5 g/L), which is a toxicant and present in various organic wastes, on the caproate production in the chain elongation (CE) process with ethanol as electron donor via two-stage batch anaerobic fermentation of glucose. The results showed phenol ≤ 1 g/L did not affect short chain fatty acids (SCFAs) production, while 1 g/L phenol increased caproate production by 59.9% in the following CE process. The higher selectivity of caproate and higher consumption of ethanol contributed to the higher caproate production at 1 g/L phenol. It was also shown 1 g/L phenol had more positive effect on CE of butyrate than acetate. 1.5 g/L phenol inhibited both SCFAs production and CE processes. 16S rRNA genes analysis showed phenol had slight effect on the microbial communities for SCFAs production, while it obviously changed the dominant microbes in CE process. For CE process, metagenomic analysis was further conducted and phenol mainly affected fatty acid biosynthesis (FAB) pathway, but not reverse β-oxidization (RBO) pathway. 1 g/L phenol increased the abundances of genes in FAB pathway, which could be related with the higher caproate production. Genome reconstruction identified the dominant microbial species in CE process, which were changed with different concentrations of phenol. Most of the dominant species were new microbial species potentially involved in CE. The syntrophic cooperation between Petrimonas mucosa FDU058 and Methanofollis sp. FDU007 might play important role in increased caproate production at 1 g/L phenol, and their adaption to phenol could be due to the presence of genes relating with active efflux system and refolding of proteins.