A study on microalgae hydrolysis and ethanol production by a recombinant Streptomyces lividans TK24

Abstract

The production of bioethanol from microalgal biomass was studied. The whole process was comprised of two steps. In the first step, harvested Nannochloropsis oceanica cell mass was hydrolyzed by using an acid. In the second step, ethanol was produced by a recombinant Streptomyces lividans TK24 from the hydrolysate. In the preliminary study, the acid hydrolysate was found to contain 7 different sugars of glucose, galactose, xylose, rhamnose, mannose, ribose, and fucose with negligible amount of toxic compounds that inhibit microbial activity. Among three acids of HNO3, HCl, and H2SO4 tested for the hydrolysis of microalgal biomass, HNO3 was found to be most effective and thus selected for the subsequent experimental work. To optimize the hydrolysis conditions, effects of acid concentration and reaction temperature on hydrolysis were investigated. The highest sugar yield of 78 % was obtained at 90 min when the acid concentration and the reaction temperature were 2N and 90oC, respectively. A S. lividans TK24 strain transformed with the pyruvate decarboxylase (pdc) and alcohol dehydrogenase Ⅱ (adhⅡ) genes was used for ethanol fermentation. It could metabolize various carbon sources including above-mentioned sugars contained in the microalgal hydrolysate except for fucose. When a simulated acid hydrolysate containing in g/L glucose 10, galactose 8, xylose 3, rhamnose 1, mannose 5 and ribose 3 (30 g/L in total), 5.1 g/L of ethanol was produced.