Jatropha (Jatropha curcas) is an oilseed species that has generated the most excitement in recent years in terms of its potential as a feedstock for biodiesel production. It is a multipurpose bush or low-growing tree, native to tropical America that can be used as a hedge, to reclaim land and as a commercial crop (Carriquiry et al., 2010; Azam et al., 2005; Openshaw, 2000). Jatropha is now grown in many tropical and subtropical regions within Asia and Africa. The oil derived from Jatropha has been shown to yield a biodiesel that meets European and US quality standards (Pandey et al., 2012; Akbar et al., 2009; Azam et al., 2005). Jatropha is known as a diesel fuel plant; the seed can yield a substantial quan­tity of oil that can be converted to biodiesel without prior refining (Carriquiry et al., 2010; Becker and Makkar, 2009). This plant is currently underutilized but could help in meeting the challenges of global bio­fuel demand (37 billion gallon) by 2016. Jatropha can be grown in semiarid conditions and/or marginal soils without large investment inputs (Jongschaap et al.,

2007) . While nonedible and toxic to humans and some animals (toxic substances include toxalbumin curcin, phorbol, saponins, trypsin inhibitor and a toxic lectin; Rakshit et al., 2013; Pimentel et al., 2012; Carels, 2009), its oil can be burnt directly or processed into biodiesel, which makes it an especially attractive biofuel crop in remote rural areas (Akbar et al., 2009; Jongschaap et al., 2007). The interest in Jatropha has been fueled by very optimistic claims of a concurrent capability to producing high oil yields and recovering wasteland (Achten et al., 2008). However, to date, critical questions remain regarding its ability to be economically viable

when grown in poor environmental conditions. Attain­ment of consistently high yields has only been achieved with relatively high levels of nutrient inputs and on good soils (International Energy Agency (IEA) Bio­energy, 2008). Nonetheless, the possibility of cultivating energy crops such as J. curcas L. has the potential to enable some smallholder farmers, producers and pro­cessors to improve their economic and social conditions, and support rural development. In addition to growing on degraded and marginal lands, this crop has special appeal, in that it grows under drought conditions and animals do not graze on it (Pandey et al., 2012; Carri — quiry et al., 2010).

Another important biodiesel feedstock are microalgae, which comprise a diverse group of aquatic photosyn­thetic microorganisms that grow rapidly and have the capability to yield large quantities of lipids adequate for biodiesel production (Ahmad et al., 2011; Amaro et al., 2011; Singh et al., 2011; Carriquiry et al., 2010; Mata et al., 2010; Li et al., 2008; Chisti, 2007; World Watch Insti­tute (WWI), 2007). Algae were initially investigated as a potential source of fuel during the gas scare of the 1970s (Li et al., 2008). The National Renewable Energy Labora­tory (NREL) started its algae feedstock studies in the late 1970s, but its research program was discontinued in 1996. Recent renewed interest has led the NREL to restart its research into the bioenergy/biodiesel potential of algae (Donovan and Stowe, 2009). The potential for algae to provide biomass for biodiesel production is now widely accepted. Furthermore, algae are recognized among the most efficient raw material for this purpose, and some studies (Carriquiry et al., 2010; Chisti, 2007) assert microalgae represent the "only source of biodiesel that has the potential to completely displace fossil diesel". One of the main advantages is the ability of microalgae to produce large amounts of biomass per unit of land. In addition, microalgae can be grown in saline water, coastal seawater, freshwater and on nonarable land, hence reducing the competition for land with conventional agri­culture (Khan et al., 2009), and creating economic oppor­tunities in arid or salinity affected regions (Carriquiry et al., 2010; Schenk et al., 2008)

Cultivation of microalgae, which is considered one of the major bottlenecks to commercial development, is being done mainly on open ponds, on closed bioreac­tors, and in hybrid systems (Brennan and Owende, 2010; Mata et al., 2010; Ugwu et al., 2008). While conven­tional open ponds are old systems for biomass produc­tion and account for the majority of microalgae cultivated today, closed bioreactors that achieve higher biomass productivity are being developed (Khan et al., 2009; Schenk et al., 2008). Open ponds are often perceived to be less expensive than bioreactors, as they require less capital and are cheaper to operate (Carriquiry et al., 2010; Khan et al., 2009). However, open ponds are more susceptible to contamination from unwanted species (Schenk et al., 2008), suffer from high water losses due to evaporation and reduced process control and reproducibility. Algal biomass production systems can be adapted to various levels of operational and technological skills; some microalgae yield chemically useful fatty acid profiles and an unsa­ponifiable fraction, which supports biodiesel production with high oxidation stability (Natrah et al., 2007; Minowa et al., 1995; Dote et al., 1994; Milne et al., 1990). In a biorefinery context, the lipid profiles of micro­algae can also provide a valuable source of omega-3 fatty acids, such as docosahexaenoic acid and eicosapen — taenoic acid (Yen et al., 2013; Doughman et al., 2007). Some important microalgal species are listed in Table

2.7 with their corresponding oil content. The physical and fuel properties of biodiesel from algal oil are compa­rable, in general, to those of fuel diesel (Amin, 2009; Rana and Spada, 2007; Miao and Wu, 2006).

The use of microalgae could be a suitable alternative in the future, if improved high-rate production systems are available at scale, because these algae are one of the most efficient biological producers of oils on the planet and are a versatile biomass source (Demirbas, 2011; Mata et al., 2010; Macedo, 2007; Campbell, 1997). In fact, microalgae with a lower oil content (~ 30% of the dry biomass) could yield 58,700 L oil/hectare per year or 51,927 kg biodiesel/hectare per year. In comparison, Jatropha (J. curcas L.), with an oil content of 28% (dry weight), can yield 741 L oil/hectare or a biodiesel pro­ductivity of 656 kg biodiesel/hectare per year (Mata et al., 2010). On average, the biodiesel production yield from microalgae can be 10—20 times higher than the yield obtained from oleaginous seeds and/or vegetable oils (Mata et al., 2010; Gouveia and Oliveira, 2009; Chisti, 2007; Tickell, 2000). Therefore, in the future microalgae may become one of the Earth’s most important renew­able fuel feedstocks for an number of reasons: their higher photosynthetic efficiency, biomass productivities, faster growth rates (in comparison with terrestrial plants), higher CO2 fixation and O2 production rates, and ability to grow in liquid medium, in variable climates and in ponds on nonarable land including marginal areas unsuitable for agricultural purposes (e. g. desert and seashore lands). Microalgae can also grow in nonpotable water or even in systems to combine waste treatment and biomass production (Zeng et al., 2012). They also use far less water than traditional crops and do not displace food crops; their production is not seasonal and biomass can be harvested daily (Chisti, 2007, 2008; Spolaore et al., 2006; Campbell, 1997).

CONCLUSIONS

In summary, achieving the feedstock yields to meet bioenergy requirements will generally require lignocel — lulosic crops rather than food crops. Pretreatment is likely to be required, and could be conducted close to the site of harvesting, as the pretreated biomass would be reduced in bulk, and thus cheaper to transport. The ideal pretreatment should be low cost, yield mini­mum levels of inhibitory compounds, result in a minimum loss of the main polysaccharides and enable maximum recovery of different fractions from the biomass. Pretreated biomass is also more amenable to downstream enzymatic bioconversion. There are major challenges ahead to reduce bioenergy production costs, many of which can provide significant opportu­nities for fundamental research and innovation in science and engineering. Bioenergy production, espe­cially from second — and third — generation feedstocks, can yield many socioeconomic benefits. Selection of the appropriate feedstocks in combination with positive sustainable agronomic and resource management ap­proaches will reduce global dependency on fossil fuels. However, well-integrated and well-conceived strategies are required so that bioenergy can maintain the environ­ment, support biodiversity, conserve water resources, lead to a reduction in emissions and enable rural development. Lignocellulosic biomass has several advantages over conventional sugar — and starch-based raw materials and has been projected to be one of the main sources of bioenergy and biofuels in the near future. With the application of existing technologies and future advances, biomass to bioenergy can provide a significant positive alternative in the energy and biofuel sector.

Acknowledgments

The authors are grateful for research funding from Enterprise Ireland and the Industrial Development Authority, through the Technology Centre for Biorefining and Bioenergy (TCBB), as part of the Compe­tence Centre program under the National Development Plan 2007—2013. The support of Mr B. Bonsall, Technology Leader (TCBB), and Prof. V. O’Flaherty, Chair of Microbiology, School of Natural Sci­ences, & Deputy Director of the Ryan Institute for Environmental, Marine and Energy Research at NUI Galway, Ireland, is gratefully acknowledged.