Biorefinery for the co-production of protein, hydrochar and additional co-products from a green seaweed Ulva sp. with subcritical water hydrolysis

Marine seaweeds are promising biomass feedstock for the co-production of food, energy and chemicals in a biorefinery. In this study, subcritical water hydrolysis (SWH) was applied to the biomass of green seaweed Ulva sp., fast-growing cosmopolitan seaweed. The SWH was done with seawater at 180 degrees C and 10.5 bar during 40 min with 8% w/w solid load. This treatment resulted in 211 +/- 7 mg of hydrochar g(-1) dry weight (DW) of Ulva sp. with higher heating value (HHV) double that of the initial biomass. The liquid fraction content per gram of Ulva DW included 5.2 +/- 1.15 mg of 5-HMF; 24.1 +/- 2.84 mg total monosaccharides (composed of 14.3 +/- 1.78 mg glucose, 5.1 +/- 0.41 mg rhamnose, 2.3 +/- 0.41 mg fructose, 1 +/- 0.06 mg xylose, 0.9 +/- 0.08 mg galactose and 0.6 +/- 0.11 mg glucuronic acid); 58 +/- 11.78 mg protein (corresponding to 84.9 +/- 13.2% of the total protein); and free amino acids (3.64 +/- 0.07 mg leucine, 2.08 +/- 0.13 mg arginine, 1.54 +/- 0.01 mg isoleucine and 1.06 +/- 0.03 mg alanine). Two-step fermentation optimization was done with Saccharomyces cerevisiae and Escherichia coli in Ulva hydrolysate following the SWH, with ethanol yield of 4.67 +/- 0.76 mg g(-1) DW Ulva. Based on these process results, some economics and sustainability indicators were estimated for large-scale macroalgae-based biorefinery. The required offshore areas for Ulva cultivation to satisfy the entire national demand in Israel for plant based protein, char for electricity production (20% blend co-firing with coal), or ethanol for transportation (10% blend in gasoline), are 0.8%, 3.1% and 34.3%, respectively of the Israeli exclusive economic zone in the Mediterranean sea. The total revenue expected for the co-products derived from Ulva sp. via SWH varies between $0.26 and $1.06 kg(-1) DW.