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2 Overview of Algal Biofuel Supply Chain A ssessing the sustainability of algal biofuels requires an understanding of the indi vidual components that make up potential supply chains. This chapter focuses on the basic processes of algal biofuel production from the biology and traits of the organisms, to methods for cultivation, and to processing into liquid fuels. It discusses algal strains and the attributes of those strains critical for biofuel production, the photo auto trophic methods for algae cultivation through open-pond and closed photobioreactor systems, the processes for collection and dewatering if necessary, and the processing of algal lipid, biomass, or secreted products into fuels. It provides the basic descriptions of the supply chain components used in later chapters and summarizes some critical process improve ments that could enhance the overall sustainability of algal biofuels. 2.1 ALGAL FEEDSTOCKS The organisms considered as potential feedstock for algal biofuel production belong to a vast and diverse assemblage of aquatic organisms that carry out oxygen-evolving pho- tosynthesis and lack the stems, roots, leaves, and embryos of plants (Leliaert et al., 2011). The category includes eukaryotic species that are related to the plant lineage and can be further categorized as macroalgae that are large structured species (for example, kelps) or microalgae that are microscopic species (for example, Nannochloropsis spp.). In the context of biofuel, the term "microalgae" also includes cyanobacteria, a diverse prokaryotic lineage whose ancestor gave rise to the plant chloroplast (Keeling, 2010). More than 40,000 species of microalgae have been described, and they collectively cover a comprehensive spectrum of habitats and tolerances of ranges of pH, salinity, and temperature (Van den Hoek et al., 1995; Falkowski and Raven, 1997; Paerl, 2000). McKenzie (2011) estimated that prokaryotic and eukaryotic microalgae are responsible for more than 40 percent of net primary produc- tivity on Earth. Algae can be a more appealing biofuel feedstock than land plants because of their faster biomass doubling cycle, their more accessible forms of stored carbon than 27
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28 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS the lignocelluloses used for cellulosic biofuels, and their ability to thrive on water sources and on land sites that are unsuitable for terrestrial farming. Microalgae contain diverse pigments and metabolites that are desirable as nutritional supplements and colorants. Examples of such products include astaxanthin, an antioxidant derived from the alga Haematococcus, and a high-protein powder derived from cyanobac- terial species of Spirulina (Arthrospira) (Gershwin, 2008; Guedes et al., 2011). Commercial- scale algal ponds that grow these and other microalgae have operated for more than a decade (Del Campo et al., 2007). However, the scale of deployment for algae cultivation for fuel is expected to be much larger than the scale of algae cultivation for nutraceuticals or other specialty products currently available in the market. Generating biofuels from algae requires exploiting and expanding the demonstrated commercial-scale growth of algal biomass, and harvesting the relatively accessible carbon stored therein. Carbon is stored within algal cells in various forms, and these molecules can be accessed by different technologies. Both eukaryotic and prokaryotic algal cells are rich sources of polar lipids that are associated with membranes; in some cases, the photo- synthetic thylakoid membranes are extensive. Carbon is such a crucial element for algae that it is typical for them to store surplus carbon when cellular division is restricted by some factor other than carbon availability--this situation is termed unbalanced growth. In many eukaryotic microalgae, photosynthetic carbon fixation continues under unbalanced conditions. Under extended periods of environmental stress, the excess fixed carbon is stored in the form of neutral lipids called triacylglycerols (TAGs). TAGs are hydrocarbon chains terminated in a carboxylic acid group. The three carboxyl groups are bound to glycerol through an ester linkage. Biofuels containing hydrocarbon chains longer than six carbons are particularly valued because of their high heats of combustion, volatility, and compatibility with existing engines. As discussed later in this chapter, extracted TAGs can be converted to biodiesel using a number of technologies, including transesterification and hydrotreating. Even algal species that do not store large amounts of TAGs can be converted to biofuels through various chemical conversion technologies. For example, species that store polysaccharides can be fermented to yield ethanol, and other biomass processing technologies, such as gasification, pyrolysis, and hydrothermal liquefaction, have shown great utility for the conversion of whole biomass into biofuels. The incipient algal biofuel industry is emerging and evolving from its early founda- tions in algae cultivation for fish feedstuff and for human nutraceuticals. Early technology development of processing algae to fuels emphasized the conversion of neutral lipids (TAGs) to biodiesel. Choices of algal feedstocks have been expanding to address the goals of fuel production rather than nutritional content and to exploit new technologies for pro- cessing biomass that extend beyond those that focus on TAGs. Ideal attributes for algal feedstock for fuels include rapid and dense growth; efficient use of nutrients, light, and carbon dioxide (CO2) under a range of temperatures; resistance to pests and predators; ac- cumulation of desirable macromolecules that can be processed into fuels; ease of harvest; and the absence of undesirable by-products. Commercial and research interest in the United States has focused on microalgae, and these species are emphasized in this report. Microalgae have been reported to reach short-term maximum productivities of 50-60 g dry weight per square meter (m2) per day in CO2-enriched open ponds in Hawaii and California (Sheehan et al., 1998). These and other data on productivity from laboratory-scale experiments have promoted the reputation of microalgae as prime candidates for providing cheap biomass feedstocks for food, feedstuff, or energy. Some authors have extrapolated values of maximal biomass productivity and combined them with maximal oil content to predict oil yields of 100 tonnes per hectare
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OVERVIEW OF ALGAL BIOFUEL SUPPLY CHAIN 29 (ha) per year. Such reports have spurred investment in intensive research on algal biofuel production. However, such high productivity projections have yet to be obtained in large- scale, long-term experiments. Serious barriers remain for reproducing optimal growth and productivity conditions at a commercial scale. They include maintaining the stability of the culture and delivering the required nutrients and other resources in an efficient manner at such scales. Current yields from large-scale operations range from 40-60 tonnes dry weight of algal biomass production per ha per year, and conservative projections anticipate up to 100 tonnes dry weight of biomass, or 30 tonnes of biodiesel per ha per year in subtropical or tropical, sunny climates (Scott et al., 2010). Estimated yields from a variety of cultivation systems are discussed later in the chapter. 2.1.1 Strain Diversity The choice of strains for biomass production depends on the desired product and technology to be used for fuel production, the source, and the type of cultivation facility (open versus closed). Initial efforts using outdoor ponds focused on production of biodiesel by the transesterification of TAGs to produce fatty-acid methyl esters (FAME).1 Therefore, strains that accumulate TAGs were selected. Five groups of microalgae were classified as high priority for biofuel production by the U.S. Aquatic Species Program (Sheehan et al., 1998): diatoms (Bacillariophyceae), green algae (Chlorophyceae), golden-brown algae (Chrysophyceae), prymnesiophytes or haptophytes (including Prymnesiophyceae), and eustigmatophytes (Eustigmatophyceae). Many strains and genera of eukaryotic microalgae are potential high-oil producers for large-scale culture (Sheehan et al., 1998; Rodolfi et al., 2009). These include species of Tetraselmis, Dunaliella, Chlorococcum, Scenedesmus, and Chlo- rella, and particularly Neochloris oleoabundans and Botryococcus braunii from Chlorophyta; the genera of Amphora, Amphiprora, Cylindrotheca, and Navicula, and the species of Nitzschia dissipata, Phaeodactylum tricornutum, and Chaetoceros muelleri from Bacillariophyta; the spe- cies of Nannochloropsis ocalata and N. salina from Eustigmatophyceae; and the genera of Isochrysis and Pavlova from Haptophyta. Improvements of technologies that convert total biomass to yield drop-in fuels--such as those being pursued by companies such as Inventure (Inventure, 2012), Xtrudx (Xtrudx Technologies, 2012), and Solvent Rescue Limited (Solvent Rescue Limited, 2012) and aca- demic institutions such as Old Dominion University (Hatcher, 2011)--are changing the scope of organisms that are being considered for biofuel production. All categories of algae are rich in polar lipids that can be recovered by such processes, and they have cellulose or other polysaccharide cell walls composed of sugars. Cyanobacteria store excess carbon as glycogen rather than TAGs, and cyanobacteria and macroalgae accumulate quantities of other complex polysaccharides. These and other macromolecules are all potential carbon sources for producing drop-in fuels if appropriate processing technologies are available. In addition, algal carbohydrate potentially can be a feedstock for fermentative fuel production processes that are based on heterotrophic organisms, such as those used by LS9, Inc. (LS9 Inc., 2011) and Solazyme (Solazyme, 2012). Cyanobacteria are used directly for ethanol pro- duction by Algenol (Chance et al., 2011a; Algenol Biofuels, 2012). As of 2012, a number of marine macroalgal species are being considered for biofuel production in India. An example 1As Chapter 3 discusses, algal triacylglycerols are reacted with methanol to form fatty-acid methyl esters (FAME). Due to its higher viscosity compared to conventional liquid transportation fuels, FAME cannot be used as a drop-in fuel, but can be blended with conventional diesel.
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30 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS is the red algal species Kappaphycus alvarezii, a species cultivated for its high carrageenan2 content (Russell, 1983; Rodgers and Cox, 1999; Woo et al., 2000). Species of Spirulina have properties suitable for aquaculture, and they are grown at relatively large scales for sale as a nutritional supplement (Earthrise Nutritional, 2009a). Still, the spectrum of cyanobacteria that could be suitable for fuel production is largely unexplored. P rokaryotic algal species provide additional diversity in light harvesting, tolerance of growth habitat and pH, and facility of genetic modification.3 Moreover, some cyanobacterial species are diazotrophs; that is, they are able to fix atmospheric nitrogen (N). Although no current commercial operations rely on a nitrogen-fixing strain, several filamentous strains that have good light- harvesting properties and for which genetic methods are well developed are diazotrophic (Heidorn et al., 2011; Ruffing, 2011). The use of these strains as a biofuel feedstock or as a ni- trogen provider for non-fixing strains (to reduce nutrient input) has received little attention. Clear differences exist in carbon storage forms (important as fuel feedstock), dominant pigments (important for solar energy capture), and accessory pigments such as carotenoids (which can be valuable commercial products) among different algal divisions (Table 2-1). Furthermore, their pigmentation and composition are affected by growth conditions and environmental stress. Emphasizing individual strains that are intended for monoculture discounts potential advantages that could be associated with mixed cultures. A recent study showed increased lipid production in algal cultures as a function of species diversity in mixed cultures under nutrient-limiting growth conditions (Stockenreiter et al., 2012). However, this effect has been demonstrated only at the laboratory scale or in low-density natural algal populations, and requires confirmation for extended periods of time and at relevant volumes. More- over, lipid production of mixed algal culture could be different under the nutrient-replete conditions of ponds designed for maximal growth. Mixed cultures might facilitate cross- protection, diversity of products through product conversion, flocculation and harvesting improvements, and efficient use of light in the water column (Stomp et al., 2007). However, mixed cultures increase the heterogeneity of the potential product, which could affect the quality of yield and the ability to optimize the diverse characteristics of the mixture for a single product. The potential to enhance the supply chain of algal biofuel through growth of mixed cultures merits additional research to determine the effects on desirable product yield and biomass accumulation (see section Cultivation in this chapter). Because data are not available for large-scale, mixed-species systems, this report introduces the concept of mixed culture systems but focuses primarily on monoculture systems. Among the biggest challenges for strain selection is the difficulty of translating desir- able strain properties from the laboratory to the field. A desirable strain would have robust growth in open ponds under natural weather and cultivation conditions, and would retain attributes that are selected and measured in the controlled conditions of the laboratory. However, the ability to grow well and compete when exposed to environmental conditions is difficult to predict. Few strains are already proven to be robust in outdoor mass cultiva- tion, and years of investment in time and process went into their commercial development. 2A gelatinous substance extracted from red algae and widely used as a stabilizing or thickening agent in indus- trial, pharmaceutical, and food products. 3Within the text of this report, the committee will distinguish whether it is discussing "genetic modification" or "genetic engineering" specifically. The committee considers genetic modification to be a general term and includes in its definition any organism whose genetic material has been altered through an array of approaches, including traditional cross breeding, mutagenesis, and genetic engineering. Genetic engineering is a modern technique that enables the introduction of a foreign gene or genes into the genome of an organism through recombinant DNA methods in an attempt to introduce a new trait into that organism.
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OVERVIEW OF ALGAL BIOFUEL SUPPLY CHAIN 31 TABLE 2-1 Characteristics of Photoautotrophic Algaea Dominant Photosynthetic Accessory Pigments Principal Energy Division Pigment(s) (Carotenoids) Storage Compound % Proteinb % Lipidb Cyanoprokaryota Phycobilins, Zeaxanthin, beta-carotene, Glycogen, other 10-70 1-20 (blue-green algae) Chlorophyll a myxoxanthin, echinenone, polysaccharides, canthaxanthin polyhydroxyalka- noates, Bacillariophyceae Chlorophyll a, Fucoxanthin, Lipid 5-35 5-55 Chlorophyll c beta-carotene, diadinoxanthin, diatoxanthin Haptophyceae Chlorophyll a, Beta-carotene Chrysolaminaran 5-30 5-55 Chlorophyll c Chlorophyceae Chlorophyll a, Lutein, Starch 5-30 5->50 Chlorophyll b beta-carotene, violaxanthin, neoxanthin Haptophyceae Chlorophyll a, Fucoxanthin Starch 5-35 5-50 Chlorophyll c Raphidophyceae Chlorophyll a Diatoxanthin Lipid 5-35 5-55 Rhodophyceae Phycobilins, Starch 5-15 5-15 Chlorophyll a Phaeophyceae Chlorophyll a Fucoxanthin Starch 5-15 5-15 Chrysophyta Chlorophyll Beta-carotene, fucoxanthin Lipids (oil) Leucosin 20-30 30-40 a and c Eustigmatophyta Chlorophyll a vialaxanthin, beta-carotene Lipids (oil) 10-30 40-65 a Table shows wide ranges in the percentage of lipids and proteins, reflecting that these and other parameters are dramatically affected by growth conditions. b Percentages are given as a percent of dry weight. Successful mass cultivation of new strains likewise will require intensive work to com- mercialize, whether those strains are native, genetically modified, or bred for improved attributes. 2.1.2 Desirable Strain Properties Regardless of the technology or strain, the goal is to maximize the quantity of a final product per unit time, area, or water volume. Further, the desire is to maximize the product output per unit input of energy, nutrients, and other resources. Biomass and lipid accumu- lation per unit time are two measures of productivity (see Rodolfi et al., 2009 for example). Many other criteria are important for selecting algal strains for commercial biofuel produc- tion, including variables that alter cost in the supply chain that are important for economic viability (for example, AQUAFUEL, 2009). Ideally, the criteria for strain selection are mea- surable. Among important selection criteria are: · Photosynthetic efficiency. The most objective measure to compare productivity of algae with land crops is photosynthetic efficiency. Photosynthetic efficiency is defined as the percent of available light (energy) that is converted into biomass energy. However, this definition might not be the most relevant for a given supply chain, depending on how the biomass will be processed and what the final prod- ucts and coproducts will be (Box 2-1).
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32 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS BOX 2-1 Relevance of Photosynthetic Efficiency to Biofuel Production The amount of biofuel produced per unit of land area is a key parameter in the evaluation of any biofuel production process. Photosynthetic efficiency, a measure of how efficiently light energy is converted to chemical energy, is one of the key determinants of overall biomass yield. The measure relevant to biofuel production is the amount of energy contained in biomass expressed as a ratio of the solar energy sup- plied (Blankenship et al., 2011). The calculation is performed for a typical area integrated over a year or a growing season. When done this way, values of up to 3 percent have been reported for microalgae (Wijffels and Barbosa, 2010). Some authors choose to calculate photosynthetic efficiency based on only the percentage of photosynthetically active radiation (PAR) present (Ort et al., 2011), or even only the PAR absorbed (Janssen et al., 2001). These calculations lead to considerably higher values and lead to some confusion around the potential for biofuel production from algae. Further complicating this particular discussion is determination of the heat of combustion, or the heat- ing value,a to be used. For measures of total photosynthetic efficiency, the heat of combustion is generally taken to be the higher heating value of the dried biomass (Jenkins et al., 1998). The critical feature for this discussion is not the exact efficiency, but rather that the value is far below what should be theoretically possible (Robertson et al., 2011). Indeed, many have lamented that photo- synthesis uses one of the "slowest metabolic enzymes in the contemporary biosphere" (Parikh et al., 2006; p.113). Considerable improvement in photosynthesis might be realized by any number of techniques of modern biology. Improvements in photosynthesis would lead directly to more prolific production of bio- fuels, which would consequently reduce the land, water, nutrient, and energy inputs required. Improve- ments to photosynthesis would directly improve the sustainability of algal biofuels. a "The higher heating value (also known as gross calorific value or gross energy) of a fuel is defined as the amount of heat released by a specified quantity (initially at 25°C) once it is combusted and the products have returned to a temperature of 25°C, which takes into account the latent heat of vaporization of water in the combustion products. The lower heat- ing value (also known as net calorific value) of a fuel is defined as the amount of heat released by combusting a specified quantity (initially at 25°C) and returning the temperature of the combustion products to 150°C, which assumes the latent heat of vaporization of water in the reaction products is not recovered" (DOE-EERE, 2012a). · Quantity of final products. This category includes the total amount of biomass, its composition, and the products to be refined, extracted, or excreted from the biomass: · Total caloric value of the biomass (for combustion or a total biomass processing technology). · Percent lipids and lipid composition (for biodiesel). · Percent starch and carbohydrate composition (for subsequent fermentation and to identify higher value by-products such as agar). · Percent protein and protein composition (soluble and insoluble protein for food and feedstuff). · Total secretion of desirable products.4 · Presence of high-value coproducts. 4Some companies, such as Joule and Algenol, have taken a dramatically different approach, relying not on ac- cumulation of biomass, but on the secretion of desirable products from stable algal cultures (Robertson, D.E., S.A. Jacobson, F. Morgan, D. Berry, G.M. Church, and N.B. Afeyan. 2011. A new dawn for industrial photosynthesis. Photosynthesis Research 107(3):269-277). In this paradigm that uses photobioreactors, the criteria for strain selec- tion are different from those used for open ponds. Planktonic unicellular species that would be difficult to protect from grazers and to harvest from ponds, are desirable within bioreactors. Well-developed genetic model organ- isms that are amenable to genetic engineering (such as Synechocystis sp. strain PCC 6803, Synechococcus sp. strain PCC 7002, and Synechococcus elongatus PCC 7942; and the unicellular green alga Chlamydomonas reinhartii) can be used in the controlled environment of photobioreactors.
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OVERVIEW OF ALGAL BIOFUEL SUPPLY CHAIN 33 · Nutrient and other resource requirements. These include the quantity of nutri- ents, such as CO2, nitrogen, and phosphorus; the type and quality of the water supply; and siting requirements. Strains could be selected because of their nutrient- use efficiency. Strains also might be selected because of their ability to flourish in brackish or wastewater, which would reduce the demand on freshwater supplies, and in the climatic conditions of a particular site. · Robustness. This term describes the overall stability of the crop, which depends on resistance to extremes of climate and environmental variables (for example, com- petitors, pathogens and predators, salinity and dissolved solutes, temperature, and pH). Tolerances to these variables vary widely within the diverse spectrum of mi- croalgae. The ability to thrive in water with various salts, metals, and other solutes could become increasingly important as competition for freshwater use among dif- ferent sectors increases. Resistance to high pH allows growth in alkaline conditions that favor a monoculture crop over sensitive predators and pathogens. Filamen- tous species or species with large cell size tend to be more resistant to grazers than unicellular species with small cell size (Tillmann, 2004). Tolerance to a broad range of temperatures could be important if the algae are cultivated in regions with high daily or seasonal fluctuation in temperature. To maintain year-round production, it might be desirable to rotate strains that have different temperature tolerance pro- files. The wide spectrum of sites that are under consideration for production ponds will require organisms with different light, water quality, and climatic tolerances. Robustness might be assessed by scoring the strain success under a wide range of potentially relevant conditions such as in Evens and Niedz (2011). · Harvestability. Harvesting cost and energy consumption can vary dramatically among different algal strains (Uduman et al., 2010). Contributing factors include the sedimentation rate and the capability for induced bioflocculation5 or auto-floc- culation. Filamentous strains that can be seined, species with positive buoyancy, or species that settle out of the water column quickly once agitation ceases might not require centrifugation, and they can be harvested easily. Growing mat-forming algae or algal films could facilitate harvesting (Tang et al., 1997), but to the commit- tee's knowledge, such approaches have not been scaled up. Strategies that rely on harvesting secreted products rather than biomass simplify the harvesting step, but such strategies require photobioreactors for algae cultivation to prevent contami- nation by microorganisms that would consume the product. · Processability and extractability. This parameter includes factors that influence the ease of extracting algal oil or processing algal biomass to fuels, for example, cell volume, thickness and toughness of the cell wall, the presence of tough fibers (for example, cellulose and silica) or cell walls, and the moisture content (Brennan and Owende, 2010). A measure for processability and extractability could be the energy input per gram of dry weight necessary for fractionation and full recovery of all biomass components. · Added value of coproducts. The algal biomass could be used to produce coprod- ucts that have an intrinsic added value, such as carotenoids, phycobilins, docosa- hexaenoic acid, or eicosapentaenoic acid (Pal et al., 2011). Coproducts can offset some of the costs of the biofuel product. A specification of the compounds and their expected added value per gram of dry biomass needs to be indicated. However, the market value of coproducts could decrease under an excessive-supply and low- demand condition. 5Bioflocculation is the clumping together of microorganisms through biological interactions.
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34 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS · Local origin of strains. Using locally selected strains could ease management and improve sustainability (RSB, 2011). Some governments have sought to restrict the importation of nonnative species, for example, the 81st Texas Legislature House Bill 3391 (2009). However, the cosmopolitan nature and wind-borne movements of algae make it unlikely that legislation can reasonably define species as native or nonnative. Regardless of legislation, local strains might have unique adaptations to the local climate, water, and possible parasites that imported or laboratory-grown strains might not have. · Non-toxic. The selection of non-toxic algal strains will increase social acceptability and reduce the potential impacts related to occupational exposures and accidental releases. 2.1.3 Strain Development and Engineering Modern agriculture has advanced primarily on the development of improved germ- plasm, and algae cultivation will likely advance using similar approaches. As with tradi- tional agriculture, advances in breeding, mutagenesis, and genetic engineering are likely to play roles in algal germplasm enhancement. Domestication of algae potentially could change their phenotype dramatically because the desired characteristics for production are different from those that have evolved in the selective pressures of the wild and because hy- pereutrophic aquaculture conditions will support genotypes that would not be fit in natural environments. Breeding and engineering will enable the stacking of desirable traits within a single species or mixture of species. The definition of desirable traits, product type desired, choice of production organism, and specification of growth and harvesting methods will influence the needs for further development on a case-by-case basis. The understanding of genetics, physiology, and metabolism at present is uneven across the spectrum of genera and species of algae that might have desirable features for algal biofuel production. Major hurdles include the need to develop genetic technologies for new species that have not been domesticated previously and that have desirable char- acteristics for large-scale cultivation. The application of genomic approaches could ac- celerate the analysis of new strains by addressing changes in gene expression for a given organism under various conditions and identifying conserved and nonconserved genes among organisms. Those approaches facilitate the identification of candidate genes that might be relevant for particular pathways of interest (Flaherty et al., 2011; Karpowicz et al., 2011; Lopez et al., 2011; Weckwerth, 2011). Cryogenic storage methods, such as those used at the Culture Collection of Algae at the University of Texas (UTEX, 2012), also may prove important to maintaining germplasm stocks and to replenishing pond inocula with a desired genotype after genetic drift of the crop population. Cultured algae, particularly cultures held for more than 10 years in selective media, have been shown to have reduced growth and production of unexpected secondary metabolites (Martins et al., 2004). A factor that might be overlooked in efforts to genetically engineer metabolic pathways in algae is that both eukaryotic and prokaryotic strains possess circadian clocks that time the peaks of daily rhythmic changes in physiological and metabolic functions (Suzuki, 2001; Ditty et al., 2003; Matsuo and Ishiura, 2010; O-Neill et al., 2011). The mechanisms and the physi- ological and metabolic consequences of circadian rhythms are insufficiently understood in these organisms. At present, few eukaryotic algal species are readily amenable to breeding or genetic engineering. Published transformation methods are well developed for Chlamydomonas reinhardtii and Phaeodactylum tricornutum. Solazyme appears to rely on genetically engineered
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OVERVIEW OF ALGAL BIOFUEL SUPPLY CHAIN 35 Chlorella species for heterotrophic fermentation of algal oils. About 30 strains of eukaryotic microalgae have been transformed using biolistic bombardment, vigorous mixing with glass beads, electroporation, or deoxyribonucleic acid (DNA) transfer from Agrobacterium tumefaciens. Strains that have been transformed include representatives of green, red, and brown algae; diatoms; euglenoids; and dinoflagellates (Radakovits et al., 2010). However, in many cases the reported transformation is only transient (Radakovits et al., 2010), and these reports have not led to routine adoption and application for most of those strains. Nevertheless, the transformations demonstrate that developing genetic systems for diverse species is possible with focused effort. Targeted gene inactivation by homologous recombination has been a long-standing challenge for manipulation of Chlamydomonas and other algal nuclear genes. However, Kilian et al. (2011) made progress in this area when they reported successful knockouts of Nannochloropsis sp. nuclear genes encoding nitrate reductase and nitrite reductase. Various genes have been suppressed successfully in Chlamydomonas by interfering ribonucleic acid (RNAs) (Cerutti et al., 2011). High-throughput methods to introduce interfering RNAs could provide an effective way for gene inactivation in diverse strains that do not exhibit homologous recombination of transgenic DNA. Another challenge for nuclear modification is that gene expression is often silenced when heterologous genes are inserted randomly into the Chlamydomonas reinhardtii nuclear genome (Fuhrmann et al., 1999). Manipulation of the chloroplast genome is facile in C. reinhardtii, but not in other algae (Radakovits et al., 2010). A report of stable chloroplast transformation in Porphyridium suggests that chlo- roplast transformation via homologous recombination might be a universally applicable approach (Lapidot et al., 2002). Waaland et al. (2004) reviewed macroalgal species as candi- dates for genomic research and concluded that the red alga Porphyra yezoensis exhibits nu- merous attributes conducive to further analyses. Extensive biochemical and physiological research has been conducted on the macroalgae because of their use in the food industry. Because there is extensive variation in the extent and type of genetic malleability among different algal species, technologies would have to be developed on a case-by-case basis for individual new algal types whose physiological and metabolic properties suggest their potential as production strains. Moreover, it will be highly desirable to develop methods that can be used to more rapidly develop a genetic system de novo in new strains or species as they are discovered. Genetic manipulation is more straightforward among cyanobacteria than eukaryotic algae because prokaryotes are amenable to techniques of bacterial genetics (Figure 2-1); some species are naturally transformable and take up exogenous DNA without specific intervention (Heidorn, 2011; Ruffing, 2011). Figure 2-2 shows some of the biochemical pathways in cyanobacteria that can be engineered to produce different desired products. Methods for gene inactivation via homologous recombination and the stable expression of transgenes, from plasmids or integrated into the chromosome, are well established in at least a dozen diverse species (Ducat et al., 2011; Ruffing, 2011). However, the developed model organisms have been maintained in the laboratory for several decades and are not likely to be suitable for growth under outdoor cultivation conditions. The Spirulina species that grow robustly outdoors have proven recalcitrant to manipulation. Despite some re- ports of transgenic Spirulina (Toyomizu et al., 2001; Kawata et al., 2004), many laboratories have failed to achieve stable transformation of the organism. This failure is likely, at least in part, due to a host of restriction endonucleases that specifically cleave foreign DNA (Zhao et al., 2006). Steps that protect plasmids by methylation while they are in an Escherichia coli host and before they are introduced to the cyanobacterium by conjugation have facilitated genetic technologies for the nitrogen-fixing filamentous strains Anabaena (Nostoc) sp. PCC
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36 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS nucleoid photosynthetic thylakoid membranes carboxysome FIGURE 2-1 Overview of cyanobacterial organization. NOTE: The cartoon diagram in the middle shows the longitudinal section of a representative cyanobacterium (modeled after Synechococcus elongatus). The major features are indicated on the cartoon diagram above and the electron micrograph below. SOURCE: Adapted from Ducat et al. (2011). Micrograph image courtesy of and reprinted with permission from Lou Sherman, Purdue University. 7120, Anabaena sp. ATCC 29413, and Nostoc punctiforme ATTC 29133 (Elhai et al., 1997). Similar approaches are likely to work for other strains that initially resist transformation. A filamentous cyanobacterium isolated from an outdoor pond that has robust growth properties similar to Spirulina species has been found to be easily manipulated by conjugal introduction of transgenes and transposons (Taton et al., 2012). This finding suggests that diverse cyanobacterial model strains that are more relevant for biofuel development than current laboratory strains could be readily developed. Genetic engineering holds the promise of transplanting completely novel pathways from heterologous sources and making products of tailored composition (Figures 2-1 and 2-2; Ruffing, 2011). Some demonstrations from genetically engineered cyanobacteria in- clude the production of 1-butanol, isobutyraldehyde, N-alkanes, free fatty acids, and sugars from transformable species of Synechococcus (PCC 7002 and 7942), Thermosynechococcus (BP- 1), and Synechocystis (PCC 6803) (Atsumi et al., 2009; Niederholtmeyer et al., 2010; Lan and Liao, 2011). Transgenic strains could play an important role in biofuel production, and some companies are making major investments in these technologies (for example, the Exxon Mobil alliance with Synthetic Genomics, Inc.; Marler, 2011; Roessler, 2011) even though
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OVERVIEW OF ALGAL BIOFUEL SUPPLY CHAIN 37 FIGURE 2-2 Schematic representation of engineered biochemical pathways in cyanobacteria. NOTE: Core metabolism of photosynthetic processes is shown in black text. Branch points used to pro- duce various desired compounds are highlighted in colored boxes. Abbreviations: 3-PGA, 3-phosphoglyc- erate; FNR, ferredoxin NADP+ reductase. SOURCE: Adapted from Ducat et al. (2011). strains have not been used in outdoor systems. The use of engineered strains in outdoor cul- tivation will be regulated according to the type of genetic modifications applied. The U.S. Environmental Protection Agency (EPA) under the Toxic Substances Control Act (TSCA) recognizes microorganisms that carry sequences from another genus as new organisms that require regulatory permitting (EPA, 2011). Under TSCA, organisms that are modified by technologies based solely on rearranging and reinserting endogenous genetic material into strains of interest are not categorized as genetically modified. Thus, self-cloned spe- cies can be used in open ponds without special oversight. Growing genetically modified algae in photobioreactors will follow the same regulatory standards that are common in the fermentation and biotechnology industries. Irrespective of the algal strain cultivated and its end use, some areas of improvement in strain and cultivation are generally desirable. These include: · Modulation of carbon allocation. · Increases in culture density. · Net increase in photosynthetic efficiency. · Algal crop protection. · Other enhancements. 188.8.131.52 Modulation of Carbon Allocation The basic strategies to adapt microalgae to increased oil production for processing to diesel were summarized by Radakovits et al. (2010). A major target of genetic engineering
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66 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS BOX 2-2 Research and Development for Enhancing Algal Biofuel Production Research and development directed at domestication of algae for biofuel production is vitally important. This effort will require improving the functional understanding of the biology, physiology, and ecology of microalgae. This upstream research and development will help inform and guide downstream engineer- ing methods and designs for cultivation and processing systems that will enhance the entire algal biofuel production chain. Thus, concerted, complementary efforts in algal domestication and biofuel production will include: ·Development of strategies to improve carbon fixation rates and yields of algal crops at commercial production-level scale. ·Development of algal strains or multi-species assemblages that achieve high productivity and high volumetric concentrations over a wide range of environmental conditions (including variations in temperature and light levels) and are as easily harvested and processed as possible. ·Evaluation and development of improved crop protection methods. ·Design and development of robust, low-cost, long-lasting production systems for algal strains or multi-species assemblages that demand minimal regulations and control of environmental parameters. ·Development of strains that excrete oil or other fuel precursors, especially immiscible products. ·Development of improved harvest technologies that reduce energy required during collecting and processing. ·Design and development of integrated biological and engineering production strategies that obvi- ate algae harvesting, drying, and oil-extraction processes. ·Design and development of integrated biological and engineering production strategies that con- tinually reuse the algae, water, and nutrients. ·Design and development of systems that can process whole biomass into fuels. SUMMARY FINDING FROM THIS CHAPTER Algal strain development is needed to enhance traits that contribute to increasing fuel production per unit resource use, reducing the environmental effects per unit fuel pro- duced, and enhancing economic viability. Improvements in biomass or product (lipid, alcohol, or hydrocarbons) yield, culture density, nutrient uptake, ease of harvest, and photosynthetic efficiency are some of the improvements that would improve sustain- ability of algal biofuels. REFERENCES 81st Texas Legislature House Bill 3391, Bill introduced by the Texas House of Representatives relating to the con- tinuation and functions of the Parks and Wildlife Department; changing the elements of an offense. § Chapter 952. (Regular Session, June 19, 2009). Acién Fernández, F.G., J.M. Fernández Sevilla, J.A. Sánchez Pérez, E. Molina Grima, and Y. Chisti. 2001. Airlift- driven external-loop tubular photobioreactors for outdoor production of microalgae: Assessment of design and performance. Chemical Engineering Science 56(8):2721-2732. Algae Energy. 2012a. Open cultivation system for growing algae. Available online at http://algae-energy.co.uk/ biofuel_production/cultivation/. Accessed May 15, 2012. ------. 2012b. Photobioreactors. Available online at http://algae-energy.co.uk/biofuel_production/pbrs/. Ac- cessed May 15, 2012.
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