Mixed culture chain elongation for consumption of acetate and ethanol in anaerobic fermentation: The impact of salt type, dosage and acclimation

Abstract

Microbial chain elongation is a newly developed carboxylate platform-based bioprocess, which often encounters high salinity stress due to saline feedstock and pH adjustment. In this study, we systematically investigated the effects of salt types (Na+, K+, and NH4+), dosage, and salinity acclimation on microbial chain elongation, and identified the microbial community by high throughput 16S rRNA gene sequencing. The results showed that a high level of Na+ and NH4+ (12.5 g/L of cations) exerted seriously inhibitory effects without chain elongating activity, while K+ had the slightest inhibition only with a little longer lag phase and lower products yield. The chain elongating products yields and the selectivity of caproate decreased with the increasing Na+ concentration, and 8.6 g/L of Na+ was found to be the threshold value for un-acclimated inoculum used for chain elongation. The acclimation to high saline conditions greatly promoted the consumption of acetate and ethanol with a shorter lag phase, and recovered a robust elongating activity for butyrate production. Furthermore, the high throughput 16S rRNA gene sequencing analysis results indicated that six genera, such as Clostridium IV and Clostridium sensu stricto, closely relating chain elongation process were depressed by high salinity, and the salinity acclimation helped to enrich the functional microbes. These findings could provide useful information for engineering microbial chain elongation process under saline conditions.