Increased anode respiration enhances utilization of short-chain fatty acid and lipid wet-extraction from Scenedesmus acutus biomass in electro-selective fermentation

Abstract

Electro-selective fermentation (ESF) can improve lipid extraction from Scenedesmus acutus biomass using green solvents like hexane and 1:1 hexane: isopropanol, but accumulation of short-chain fatty acids (SCFAs) wastes useable electrons. To eliminate the electron loss, we designed a flat-plate microbial electrolytic cell (MEC) to have a large anode area for a biofilm of anode respiring bacteria (ARB) that oxidize the SCFAs and generate current that produces H2 gas at the cathode. In 9-day batch tests of ESF in the flat-plate MEC, 18% of the total electrons in the feeding biomass were scavenged by the ARB biofilm and converted to current, leaving minimal concentrations of SCFAs. Extraction with two solvents -- 1:1 hexane:isopropanol and 100% hexane – was evaluated at the end of the 9-day experiments. 30% of the total lipids became hexane-extractable with ESF, compared to <1% in the feeding biomass. Furthermore, hexane had 100% selectivity towards saturated long-chain fatty acids (LCFAs), which are superior for biofuel production. Thus, ESF improved the quantity and quality of extractable FAME-based biofuel from S. acutus, while directing electron equivalents in SCFAs to current and H2 generation at the cathode.