Third generation bioethanol from invasive macroalgae Sargassum muticum using autohydrolysis pretreatment as first step of a biorefinery

Abstract

Sargassum muticum, an invasive macroalgae in Europe, was employed as material for third generation bioethanol production. As a first step, autohydrolysis was chosen as an eco-friendly pretreatment, seeking for a high enzymatic susceptibility of the solid phase and high content of hexoses as glucose, galactose and mannose, in both liquid and solid phases, which can be subsequently transformed in ethanol via fermentation. Besides, the search of a minimum consumption of energy in the pretreatment is also a key challenge in bioethanol production. At optimum conditions of autohydrolysis pretreatment, more than 90% of the glucan was recovered in the solid phase (while the other 10% appeared as glucooligosaccharides and glucose in the liquid phase). In the enzymatic hydrolysis carried out with the solid phases, glucan to glucose conversions of 94 and 89% were obtained, with the solid mixed with water and the whole slurry, respectively. Moreover, the whole slurry experiments, where all hexoses present in the raw material (glucose, galactose and mannose) from the solid and the liquid phases are fermented, allows to reach maximum ethanol yields of 80% (14.10 g of ethanol/L) referred to the theoretical yield, in a short time.

Recent trends on seaweed fractionation for liquid biofuels production

“…As seen, the most common microorganism was the yeast S. cerevisiae. Recently, an environmentally friendly pretreatment, autohydrolysis, was used for ethanol production from invasive macroalgae Sargassum muticum by SSF, achieving 14 g/L (del Río et al, 2019). Similar ethanol concentration was also reported from acid pretreated Gelidium amanssi by SHF (Kim et al, 2015).…”

“…Seasonal harvesting is proposed as a control strategy as no methods for prevention and removal of invasive seaweeds have been successful. In this way, one of the most interesting ways of valorization of invasive seaweed is the production of biofuels, turning a problem into a benefit, i.e., finding a double environmental and economic advantage: the removal of a hazardous material and the economical revalorization of the marine biomass, which is actually useless and without value (De Ramon N'Yeurt and Iese, 2015;del Río et al, 2019;Milledge et al, 2016). On the other hand, the increasing interest and spreading of other seaweed-based systems, such as wastewaters treatment (to decrease nitrogen and phosphorus or heavy metals content) and nutrient bioextraction from urbanized estuarine waters (Rocha et al, 2019) or multitrophic aquaculture systems, IMTA (Alexander and Hughes, 2017), both onshore or offshore, can also provide a new, more sustainable and significant source of seaweeds.…”

“…Its main advantages include the possibility of operating using only water, lessening the corrosion of equipment; the simplicity of operational procedures; and improved enzymatic digestibility of cellulose (Hu et al, 2011;Ruiz et al, 2013). This pretreatment is one of the most used for the processing of seaweeds for ethanol production and bioactive extraction (del Río et al, 2019;Pérez-Larrán et al, 2019).…”

“…The highest ethanol concentration (13 g/L) from acid treated Chondrus crispus hydrolysate was obtained using S. cerevisiae YPS128, showing the ability to consume the majority of available monosaccharides present in the hydrolysate. The use and engineer of robust S. cerevisiae strains selected for their tolerance to hydrolysate's inhibitors (Costa et al, 2017;del Río et al, 2019;Pereira et al, 2011;Romaní et al, 2015) is also recommended for effective bioethanol production.…”

“…Finally, solid residue after bleaching (60°C for 8 h) was processed by alkali treatment (0.5 M NaOH at 60°C for 12 h) to obtain cellulose that was subsequently hydrolyzed by cellulases enzymes to obtain glucose and fermented to ethanol by the yeast S. cerevisiae. Other integrated valorization alternative consisted on a simple and environmentally friendly process of autohydrolysis (at milder conditions 150°C) for the fractionation of invasive algae (Sargassum) to produce oligosaccharides in the liquid phase and the simultaneous saccharification and fermentation of the solid residue after treatment to produce ethanol (del Río et al, 2019). Under these conditions, 15.22 g/L of oligomers (composed mainly by fucooligosaccharides), 90% of cellulose recovery and 11.32 g/L of ethanol were achieved.…”