The term algae can refer to microalgae, cyanobacteria (the so-called "blue-green algae"), and macroalgae (or seaweed). As a first approximation, the composition of algal biomass is similar to that of conventional plant biomass, with both containing primarily lipids, carbo­hydrates, and protein. However, unlike conventional plant crops, algae lack the structural component lignin. This can be viewed as advantageous in the separation of more valuable carbohydrates from less valuable lignin, which is often complicated and resource intensive. Also, algae are commonly cultured under dilute conditions, and whereas this results in the need for extensive dewatering, it also allows for growth conditions to be tweaked to meet market demands in real time (Foley et al., 2011).

Unlike plants that contain predominantly cellulose Ip (monoclinic crystalline form), algal cells contain cellulose Ia (triclinic crystalline form) (Hayashi et al., 1997; Atalla and Van der Hart, 1984). The latter form contains weaker hydrogen bonding resulting from spatial ar­rangement of individual cellulose chains with respect to one another. Carbohydrate profiles of algae and terrestrial plants also differ significantly. Both groups contain hemicelluloses— heterogeneous polysaccharide composed of pentoses, mainly xylose, that can be utilized for fermentative bioethanol production. In addition, algae contain various contents of other heteropolysaccharides that are largely species-dependent. Red seaweeds, for example, are mainly composed of polymers of modified galactose: carrageenan and agar. The major cell wall component of red algae K. alvarezii is j-carrageenan (Khambhaty et al., 2012; Meinita et al., 2012), a linear, sulphated polysaccharide composed of galactose that cannot be directly metabolized to ethanol. Another rhodophyte, Gelidium amansi, is predominantly composed of agar (Kim et al., 2011), a polysaccharide composed of D — and L-galactose derivatives. Brown algae of Laminaria sp. (Adams et al., 2009; Horn et al., 2000; Kim et al., 2011), on the other hand, are rich in mannitol and contain large quantities of laminaran, a polysaccharide composed of 1,3 linked and 1,6 linked glucopyranose units terminated with D-mannitol. These sugars and sugar alcohols could be an additional pool of carbohydrates when combined with an appro­priate conversion scheme. Besides these heteropolysaccharides, both micro — and macroalgae store their reserves as starch. The highest contents of starch were reported for microalgae C. reinhardtii UTEX90 (Choi et al., 2010; Nguyen et al., 2009) and reached as much as 35-45% of dry cellular weight (Daroch et al., 2012).

In addition to fungible biofuels, a variety of biofuels and products can be generated using algae precursors. There are several aspects of algal biofuel production that have com­bined to capture the interest of researchers and entrepreneurs around the world: (1) high per-acre productivity, (2) nonfood-based feedstock resources, (3) use of nonproductive, nonarable land, (4) utilization of a wide variety of water sources (fresh, brackish, saline, marine, produced, and waste water), (5) production of both biofuels and valuable coprod­ucts, and (6) potential recycling of CO2 and other nutrient waste streams (Varfolomeev and Wasserman, 2011).