Microalgal biomass as an alternative source of sugars for the production of bioethanol

SANZ SMACHETTI M.; SANCHEZ RIZZA L.; CORONEL C.; DO NASCIMENTO M; CURATTI L.

Principles and Applications of Fermentation Technology

Wiley-Scrivener Publishing LLC

Año: 2018; p. 352 - 378

Second-generation biofuels present clear advantages over first-generation ones, mostly related to the availability, low cost and non-competition with food production of lignocellulose as a feedstock and its reduced environmental impact (Searchinger et al., 2008). However, they face hard-to-overcome disadvantages due to the composition and structure of the biomass, requiring quite intensive mechanical and physicochemical pretreatments, and expensive saccharification enzymes for its conversion into the desired biofuel (Kumar et al., 2009). Lignocellulose pretreatments frequently result in the generation of fermentation inhibitors such as weak acids, furans and phenolic compounds formed or released during hydrolysis. Although some alternatives for detoxification have been shown (such as additional treatments with alkali, sulfite or enzymes, pre-fermentation by a fungus, removal of non-volatile compounds, extraction with ether or ethyl acetate, and improved fermentation technology), implementing them increases production costs (Palmqvist et al., 2000). Furthermore, it has been reported that cellulases impact the most in the total cost of production of second-generation biofuels. They also represent one of the most uncertain parameters in techno-economic analyses mostly due to assumptions on future prices and the heterogeneity in the way results are presented in the literature, making it difficult to cross-compare studies (Olofsson et al., 2017). In view of these difficulties, researchers have envisioned changing both its composition and/or structure by genetically modifying the lignocellulose synthetic pathway in plants. The modification of lignin content in plants has been attempted before for other reasons such as to increase digestibility of feed production and to decrease the need of bleaching in the paper industry. Thus, although some details of the metabolic pathway for lignin biosynthesis are still not completely understood (Sticklen, 2008), some very promising genetic modifications have already been demonstrated. For example, downregulation of cinnamyl alcohol dehydrogenase in poplar resulted in improved lignin solubility in an alkaline medium, decreasing the need for pretreatment before saccharification (Pilate et al., 2002). Also, downregulation of 4-coumarate CoA ligase in the lignin biosynthesis pathway in aspen resulted in a 45% decrease in lignin content and a concomitant 15% increase in cellulose content (Hu et al., 1999) and when coniferaldehyde 5-hydroxylase was additionally downregulated, the lignin content was further reduced to 52% while cellulose increased by 30% (Li et al., 2003). More recently, a proof-of-principle study conducted in alfalfa, in which six different genes for the lignin biosynthetic pathway were downregulated showed reduction or elimination of need for chemical pretreatment in the production of fermentable sugars (Chen, 2007). Although modification of lignocellulosic biomass composition and/or structure by genetic engineering is very promising, further research towards plant structural integrity and defense against pathogens and insects should also be addressed to continue improving lignocellulosic biomass from genetically modified crop plants (Sticklen, 2008).Other genetic modification approaches had also been pursued to improve yield, pretreatment and saccharification: i) increasing the overall biomass productivity by modifying plant growth regulators and other factors such as carbon allocation; efficiency of uptake and use of nutrients, among others; ii) increasing cell-wall polysaccharide content by modifying the expression of genes that are involved in both cellulose and hemicellulose biosynthesis; iii) expressing microbial hydrolases in specific cellular compartments of the plants. The latter is very appealing approach considering the possibility that hydrolytic enzymes could be produce on-site and at a very low cost by the same crop plant. The apoplast accumulation of heterologous hydrolases is often a selected target. However, the expression of thermophilic enzymes would be preferred to avoid premature degradation of the plant cell walls before lignification at cultivation temperatures (Sticklen, 2008).Thus, despite the last years improvements in the technology for converting lignocellulosic biomass into biofules, the structural nature of this feedstock still represent a remarkable challenge. It is presumed that expanding the search for alternative feedstock for biofuels by looking into natural biodiversity would be a reasonable approach. Among non-conventional crops, aquatic photosynthetic species such as macro and microalgae and cyanobacteria, arise as an alternative source of low-cost sugars for biofuels and other applications.