Enhancing the productivity of caproic acid in open culture chain elongation: A comparative study of biofilm systems

Growing global energy consumption and climate challenges have emphasized the need for biotechnology-based methods to synthesize valuable chemicals. Biological chain elongation (CE) shows great potential for decarbonization by producing valuable biochemicals - specifically medium-chain fatty acids (MCFAs) - from waste streams containing simple short-chain chemical building blocks like acetic acid and ethanol. However, one of the key parameters that impacts the commercial viability of CE, hence its integration in sustainable chemical manufacturing, is the volumetric productivity. In this study, we compared two biofilm systems using commercially available carriers (respectively AnoxK™ Z-200 and K5) with a planktonic system to examine how biofilms enhance the conversion of acetate and ethanol to caproic acid (a medium chain carboxylic acid). The results show that the Z-200 and K5 systems achieved productivity up to 3.46 ± 0.08 g caproate/L/d and 8.1 ± 0.8 g caproate/L/d, respectively, outperforming the planktonic system at 3.02 ± 0.12 g caproate/L/d. Cycle studies further proved the superior performance of the biofilm systems, as shown by short lag-time and fast reaction kinetics. We validated biofilm formation in the CE process through microscopic visualization using scanning electron microscopy (SEM), confocal laser scan microscopy (CLSM), biomass quantification, and analysis of extracellular polymeric substances (EPS). Analysis of the microbial community through 16S rRNA gene sequencing revealed that the biofilm systems were enriched by functional microbes (including Clostridium sensu stricto 12, Bacteroides, Lachnoclostridium, Caproiciproducens, and Proteiniphilum) previously associated with chain elongation microbiomes. The superior performance in the biofilm systems likely stems from improved biomass concentration, enriched functional microbes, and increased EPS production favouring retention of functional taxa. Overall, this work demonstrates how microbial biofilms can improve productivity of MCFA in CE systems, potentially expanding CE applications and improve decarbonization potential.