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A. Catarina Guedes, Helena M. Amaro ’ ,
Isabel Sousa-Pinto1’3 , F. Xavier Malcata1,4

1CIIMAR/CIMAR — Interdisciplinary Centre of Marine and Environmental Research,
University of Porto, Porto, Portugal
2ICBAS — Institute of Biomedical Sciences Abel Salazar, Porto, Portugal
^Department of Biology, Faculty of Sciences, University of Porto, Porto, Portugal
^Department of Chemical Engineering, University of Porto, Porto, Portugal

10.1 INTRODUCTION

Over the past 50 years, the world population more than doubled. This fact, coupled with an extension of life expectancies and rising standards of living, has led to a dramatic increase in primary energy consumption, chiefly from fossil sources (Jones and Mayfield, 2012).

A suitable alternative is to produce biofuel from photosynthetic organisms, that is, higher plants, algae, and cyanobacteria, which can use sunlight and carbon dioxide to produce a va­riety of organic molecules, namely carbohydrates, proteins, and lipids. These biomolecules can then be used to generate biomass rich in fuel-like metabolites that can then be extracted (Yang, Guo et al., 2011; Jones and Mayfield, 2012). However, the problem remains: What to do with the spent biomass? In particular, third-generation biofuels based on micro — and macroalgae offer an excellent possibility to displace fossil fuels; it is even believed that ances­tors of marine microorganisms were responsible for the formation of petroleum in the first place (Goh and Lee, 2010).

Macroalgae (or seaweeds) are multicellular organisms that take many forms and sizes. They are classified into three broad groups based on their pigmentation: brown algae (Phaeophyceae), red algae (Rhodophyta), and green algae (Chlorophyta). In contrast, microalgae are microscopic organisms, which, beyond Rhodophyta and Chlorophyta, may belong to another three specific groups of unicellular organisms: blue-green algae (Cyanobateria), diatoms (Bacillariophyta), and dinoflagellates (Dinophyceae). These species are commonly referred to as phytoplankton (Garson, 1993; Samarakoon and Jeon, 2012).

Despite looking similar to land plants, microalgae miss the lignin cross-linking in their cellulose structures because their growth in aquatic environments does not require strong supports (John, Anisha et al., 2011). On the other hand, macroalgae contain significant amounts of sugars (at least 50%) suitable for fermentation (Wi, Kim et al., 2009). In certain marine algae (e. g., red algae), the carbohydrate content is strongly influenced by the presence of agar, a polymer of galactose and galactopyranose. Recent research has attempted to develop methods of saccharification to release galactose from agar and to release glucose from cellulose so as to increase fermentation yields in terms of bioethanol (Jones and Mayfield, 2012). Other studies have shown that red algae such as Gelidium amansii and brown algae such as Saccharina japonica are both potential sources of biohydrogen via anaerobic fermentation (Jones and Mayfield, 2012). Unfortunately, harmful algal blooms in lakes, ponds, and oceans may result in drastic effects on those ecosystems, so removal of those algae for biogas production is welcome (Du, Li et al., 2011).

Microalgae are ubiquitous microorganisms that are characterized by a remarkable metabolic plasticity; they may indeed be cultivated in brackish and wastewaters that provide suitable nutrients (e. g., NH4, NO3, and PO4-) at the expense of only sunlight and atmospheric carbon dioxide (CO2). On the other hand, metabolic engineering has been taken advantage of to produce molecular hydrogen or to improve the lipid content as storage products (Amaro et al., 2011).

Overall, economic analyses have consistently indicated that algal-based biofuel feasibility hinges on the possibility of production coproducts with a market value from the spent biomass (Stephens, Ross et al., 2010). A wide range of fine chemicals may indeed be extracted from said biomass, depending on the species at stake (Raja, Hemaiswarya et al., 2008); these hold added value sufficiently high to contribute to the economic feasibility of biofuel manu­facture. Such bioproducts include sugars for production of bioethanol and biomethane, both via fermentation of biomass; intermediate value products, e. g., proteins for animal feedstock; and high-value products such as active principles bearing antimicrobial, antioxidant, antitumoral, and anti-inflammatory features for pharmaceutical purposes. Finally, biomass may be pyrolyzed to produce sequestered carbon in the form of biochar, which holds value as a soil enhancer (Kruse and Hankamer, 2010). A general overview of applications of spent biomass is given in Figure 10.1.

When discussing the upgrade of spent biomass, one should take into account the process that originated it or the target metabolite from which the biofuel is obtained. For example, if the objective is to produce biohydrogen, the spent biomass consists of essentially intact cells, whereas when accumulated lipids are required of biodiesel, the spent biomass takes the form of oilcake. Compounds such as carbohydrates, hydrocarbons, and the biomass itself may still be transformed into secondary biofuels such as ethanol, oil, biochar, and syngas, as shown in Figure 10.2. On the other hand, the spent biomass from production of a biofuel may be used to high value-added products via extraction (see Table 10.1). Therefore, this chapter is organized according to two perspectives: spent biomass used for further biofuel production and spent biomass as a source of value-added products, namely as fine chemicals or feed or even in bioremediation