Monomeric sugars can be converted to ethanol directly, while starches and cellulose first must be hy­drolyzed to fermentable sugars either enzymatically or chemically (Bashir and Lee, 1994). Like most biofuels processes, bioethanol production from microalgae be­gins with the concentration of algae. The algae are then further dried and ground to a powder. In the next step of the process, the algae mass is hydrolyzed and Saccharomyces cerevisiae yeast is added to the biomass to begin the fermentation process. The resulting fer­mented mash contains about 11—15% ethanol by volume as well as the nonfermentable solids from algae and yeast cells. Ethanol is then distilled off the mash at ~ 96% strength. Despite widespread knowledge of this fermentation process, the details of the conversion pro­cess of algal celluloses-to-bioethanol are only partially understood. Celluloses comprise a large fraction of algal cells walls. These molecules are tightly packed and enzymatic access is often limited without a pretreatment step (Figure 10.6).

Many authors have reported that it is essential to introduce a pretreatment stage to release and convert the complex carbohydrates entrapped in the cell wall into simple sugars necessary for yeast fermentation. Cel­lulose can be made more accessible by the addition of an acid (Figure 10.7). Arantes and Saddler (2010) have sug­gested a model where prior to hydrolysis of cellulose to

FIGURE 10.7

monomeric units, cellulases must adsorb onto the surface of the insoluble cellulose (Figure 10.8). The action of the cellulases serves to loosen tightly packed fibrous cellulosic networks and create additional access to cellulose chains buried within the fibrils. Then the synergistic action of exo- and endoglucanases cleave accessible molecules to form soluble cello-oligosaccharides, or oligomers of <6 sugar units. These oligosaccharides are quickly hydro­lyzed to primarily cellobiose, or two glucose molecules linked by a b (1/4) bond. Cellobiose hydrolyzation to glucose monomers is usually completed by the extraneous addition of b-glucosidase.

Once glucose monomers have been rendered, bio­ethanol from microalgal biomass can be produced through two distinct pathways: direct dark fermentation or yeast fermentation of saccharified biomass. Whereas direct dark fermentation yields are typically much lower, the yeast fermentation process is a very well — established, relatively high-yield, low-energy-intensive process. Because microalgae can be harvested multiple times a year, some species have been shown to
theoretically yield an order of magnitude more bio­ethanol compared to a land-based crop such as corn (Table 10.3). Further, using microalgae as a raw material is strongly advantageous as algae sugars may be derived from multiple sources—from intracellular starches and from the cellulosic cell wall. Nevertheless, to achieve higher yields, it is still necessary to screen for high starch-producing algal strains coupled with identifying mechanisms and culture conditions for inducing maximal accumulation of intracellular starches.

In comparison to terrestrial feedstocks that contain lignin, certain species of microalgae and cyanobacteria have high potentiality for bioethanol production due to their high productivity rates, high biomass ferment­able carbohydrate content, and lack of lignin. Lignin is a recalcitrant substance (i. e. not easily degraded) present in the cell walls of terrestrial biomass that cannot be con­verted to bioethanol—its processing is a major impedi­ment for bioethanol production (Ragauskas et al.,

2006) . Microalgae’s potential can be highlighted by the fact that 75% of algal complex carbohydrates can be
hydrolyzed into a fermentable hexose monomer, and the fermentation yield of bioethanol is ~ 80% of the theoret­ical optimal value (Huntley and Redalje, 2007). Harun et al. (2009) have shown that the blue-green Chlorococum sp. produces a maximum bioethanol concentration of 3.83 g/l obtained from 10 g/l samples that are preex­tracted for lipids versus those that remain as dried intact cells. This indicates that microalgae can be used for the production of both lipid-based biofuels and ethanol bio­fuels from the same biomass as a means to increase their overall economic value (Jones and Mayfield, 2012). The microalgae Chlorella vulgaris and Porphyridium sp., particularly, have been considered as promising feed­stocks for bioethanol production because they can accu­mulate up to 37% and 54% (dry weight) of starch, respectively. The potential for simple, low-cost methods of bioethanol production from microalgae and cyano­bacteria are real. The next phase of biofuel research should develop improved methodologies to increase intracellular ethanol production efficiencies.