Microalgae-based biorefinery – From biofuels to natural products

Accepted Manuscript Microalgae-based biorefinery – From biofuels to natural products Hong-Wei Yen, I-Chen Hu, Chun-Yen Chen, Shih-Hsin Ho, Duu-Jong Lee, JoShu Chang PII: DOI: Reference:

S0960-8524(12)01601-X http://dx.doi.org/10.1016/j.biortech.2012.10.099 BITE 10749

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Bioresource Technology

Please cite this article as: Yen, H-W., Hu, I-C., Chen, C-Y., Ho, S-H., Lee, D-J., Chang, J-S., Microalgae-based biorefinery – From biofuels to natural products, Bioresource Technology (2012), doi: http://dx.doi.org/10.1016/ j.biortech.2012.10.099

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A revised manuscript -035(BITE-D-1264R3) submitted to Bioresource Technology

Microalgae-based biorefinery – From biofuels to natural products

Hong-Wei Yen1, I-Chen Hu2,3, Chun-Yen Chen4, Shih-Hsin Ho5, Duu-Jong Lee6, and Jo-Shu Chang4,5,7*

1

Department of Chemical and Materials Engineering, Tunghai University, Taichung, Taiwan

2

Far East Bio-Tec Co., Ltd, Taipei , Taiwan

3

Far East Microalgae Ind Co., Ltd. Ping-Tung, Taiwan

4

University Center for Bioscience and Biotechnology, National Cheng Kung University, Tainan, Taiwan

5

Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan

6

Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan

7

Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, Taiwan

*

Correspondence:

Prof. Jo-Shu Chang Phone: +886-6-2757575 ext. 62651 Fax: +886-6-2357146 E-mail: [email protected]

BITE-D-12-03564R3 (revised manuscript).doc 1

31/10/2012

Abstract The potential for biodiesel production from microalgal lipids and for CO2 mitigation due to photoautotrophic growth of microalgae have recently been recognized. Microalgae biomass also has other valuable components, including carbohydrates, long chain fatty acids, pigments and proteins. The microalgae-based carbohydrates consist mainly of cellulose and starch without lignin; thus they can be ready carbon source for the fermentation industry. Some microalgae can produce long chain fatty acids (such as DHA and EPA) as valuable health food supplements. In addition, microalgal pigments and proteins have considerable potential for many medical applications. This review article presents comprehensive information on the current state of these commercial applications, as well as the utilization and characteristics of the microalgal components, in addition to the key factors and challenges that should be addressed during the production of these materials, and thus provides a useful report that can aid the development of an efficient microalgae-based biorefinery process.

Keywords: microalgae; carbohydrates; lipids; biorefinery; long-chain fatty acid; pigments

2

1. Introduction It has been estimated that about 90% of our current energy consumption is provided by coal, natural gas and petroleum, with less than 10% coming from renewable energy sources (Chen et al., 2011; Demirbas, 2010). Based on current consumption habits,, it is very likely that there will be no more oil reserves after 2050 (Campbell & Laherrère, 1998; Ho et al., 2011, 2012). Moreover, even if oil reserves remain plentiful, the associated environmental pollution, including the release of CO2, is widely seen as presenting a serious threat to the current global order, particularly through its effects on climate change (Ho et al., 2011b). Fixation of CO2 by photosynthetic organisms on earth has contributed significantly to the global carbon cycle. The CO2 produced from natural or human activities can be consumed by plants and algae, converting it to biomass and other metabolic products through photosynthesis and Calvin cycle. Phototrophic growth of microalgae can transport atmospheric carbon into a cycle in which no additional CO2 is created. Since microalgae-based CO2 fixation is much faster and more efficient than that of terrestrial plants, it has thus been considered to have the potential to serve as a commercially feasible process for mitigation of CO2 emissions (Ho et al. 2011b). Most microalgae are unicellular photosynthetic microorganisms that can fix the dissolved inorganic carbon and CO2 in the gaseous effluents to form chemical energy through photosynthesis. The majority of microalgae have much higher cell growth and CO2 fixation rate (around 10–50 times higher) than terrestrial plants. Moreover, the CO2 fixation is accompanied by production of microalgae biomass, which could be converted to a variety of biofuels, pigments, cosmetic, nutritious food and animal feed, representing additional benefits from the microalgae CO2 fixation process (Ho et al. 2011b) . 3

Microalgal biomass cultivation is regarded as a potential way to overcome our current reliance on fossil fuels for a number reasons, such as the high area yields compared with other crops, high oil contents in some strains, low water consumption rates, and the possibility of producing microalgae on infertile lands. These advantages have attracted several oil companies, such as Exxon, BP, Chevron, Shell and Neste Oil, to invest in this area (Mascarelli, 2009). Moreover, a recent study indicated that the production of microalgal biofuels is relatively close to being economically feasible, given expected developments in both market conditions and production technology (Stephens et al., 2010). In addition to the microalgal lipids that could potentially be converted to practical biodiesel, the biomass of microalgae also contains many other valuable components, including carbohydrates, lipids, polyunsaturated fatty acid, pigments and proteins, all of which are worth developing into refined products for various applications (Lammens et al., 2012). Nevertheless, there are still several problems to be solved during the development of microalgae-based biorefinery technologies. The most challenging problems for the microalgae industry include high installing and operating cost, difficulty in controlling the culture conditions, contamination bacteria or alien algae, unstable light supply and weather. Several strategies have been proposed to cope with those challenges. First, it is of great importance to isolate good microalgae/cyanobacteria strains that are rich in the target products, tolerate high (or low) temperature, and easily become predominant in the culture environment. Attaining good microalgae strains is the key factor leading to stable and sustainable microalgae cultivation, which is extremely important in industrial applications of microalgae. Next, identifying preferable culture conditions for improving target-product production as well as designing efficient and economical feasible microalgae cultivation system (or photobioreactors) are also 4

critical to improve the productivity of microalgal biomass or target products (Chen et al., 2011). In particular accumulation of different components (such as lipids, carbohydrates, proteins and pigments) in microalgae requires different cultivation conditions and operation strategies.

These would rely on more understanding on the

photosynthetic metabolism and physiology of the microalgae used and more advanced engineering technology that could help design more appropriate photobioreactors and establish better environment for microalgae growth and product formation. Finally, high-efficiency and low-cost downstream processing, which accounts for the majority of operating cost, should be developed. For example, some innovative microalgae harvesting approaches have been proposed (Show et al. 2012; Lee et al. 2012; Cheng et al. 2011). A novel one-step transesterification method for producing microalgae-based biodiesel using wet oil-bearing microalgae biomass was also reported (Tran et al. 2012a,b). This method allows biodiesel synthesis from microalgae without dewatering and oil extraction steps. In addition, appropriate treatment of the wastes produced from microalgae systems as well as recycling of water used during microalgae cultivation processes are also critical issues. Finally, life cycle analysis, energy balance and cost assessment should also be performed to justify the economic feasibility and environmental impacts. In this review, we provide up-to-date information on the formation mechanisms, production and applications of those microalgae-based components (namely, carbohydrates, lipids, pigments and proteins) (Olguin, 2012). For example, microalgal carbohydrate can serve as a carbon source to replace traditional crop carbohydrates in the fermentation industry, while lipids extracted from the microalgal biomass could be used as a potential feedstock for biodiesel production. In addition, some long chain fatty acids (such as DHA and EPA) are important health food supplements, and 5

several proteins (Aikawa et al., 2012) and pigments found in microalgae have been applied in the pharmaceutical industry in the treatment of diseases. Therefore, microalgae could play important roles in producing biofuels and bio-based chemicals based on both their natural components and refined (or fermented) products. This review aims to provide a comprehensive report on the production and application of the natural or refined products of microalgae in order to aid the greater the utilization of microalgal biomass (also known as a third generation feedstock) for different purposes.

2. Carbohydrates from microalgae Carbohydrates are one of the most important sources of energy and biological nutrients. Algae have a relatively high photo conversion efficiency, and are able to accumulate a high carbohydrate content (potentially higher than 50% of its dry weight) (Ho et al., 2012). In general, the algal carbohydrates are mainly composed of starch, glucose, cellulose/hemicelluloses, and various kinds of polysaccharides. Of these, algal starch/glucose is conventionally used for biofuel production, especially for bioethanol (John et al., 2011) and hydrogen (Chochois et al., 2009), while polysaccharides have various important biological functions in algae cells, mainly as storage, protection and structural molecules (Arad & Levy-Ontman, 2010). Recently, algal polysaccharides (e.g. seaweed) have come to be regarded as new bioactive materials, due to their novel structures and distinct biological functions. The monosaccharides of fructose, galactose, glucose, mannose and xylose, consist of microalgal polysaccharides in different ratios. Currently, algal polysaccharides represent a class of high-value compounds with many downstream applications in food, cosmetics, textiles, stabilizers, emulsifiers, lubricants, thickening agents and 6

clinical drugs (Arad & Levy-Ontman, 2010). In particular, algal polysaccharides contain sulfate esters called sulfated polysaccharides (e.g., fucoidan, carrageenans and agarans), and are gaining wide attention due to their unique medical applications . Although the pharmaceutical mechanism of algal polysaccharides is still under investigation, the basic mechanism of their therapeutic effects is based on macrophage stimulation and modulation. As a general rule, the biological activity of sulfated polysaccharides changes with their sugar composition and degree of sulfation (Kim et al., 2012). Algal sulfated polysaccharides have been shown to exhibit a wide range of pharmacological activity, including acting as antioxidant, antitumor, anticoagulant, anti-inflammatory, antiviral and immunomodulating agents (Table 1) (Chen et al., 2010; Guzman et al., 2003; Kim et al., 2012; Matsui et al., 2003; Mohamed, 2008; Park et al., 2011; Tannin-Spitz et al., 2005). Many microalgal polysaccharides can modulate the immune system via activating the functions of macrophages and inducing the production of reactive oxygen species (ROS), nitric oxide (NO) and various kinds of cytokines/chemokines (Schepetkin & Quinn, 2006). Macrophages are able to regulate various innate responses, as well as secrete connecting cytokines and chemokines, such as interleukin (IL)-6, IL-8, IL-β and the tumor necrosis factor (TNF-α), which are the signaling molecules for the immune system and inflammatory reactions (Park et al., 2011). For example, Tannin-Spitz and his colleagues mentioned that the main function of the cell-wall sulfated polysaccharide from the red microalga Porphyridium sp. is to overcome extreme environmental factors, and thus it has a protective response against the oxidative stress imposed by ROS (representing antioxidant activity) (Tannin-Spitz et al., 2005). The sulfated polysaccharides derived from Porphyridium sp. have significant potential for use in anti-inflammatory skin treatments because of their 7

ability to inhibit the migration and adhesion of polymorphonuclear leukocytes (PMNs) (Matsui et al., 2003). The sulfated polysaccharides of Haematococcus lacustris significantly stimulate murine macrophages to secrete the pro-inflammatory cytokine, indicating its potent immune stimulating activities (Park et al., 2011), although the detailed molecular mechanisms of macrophage activation are not currently known . Several types of microalgal sulfated polysaccharides show wide-spectrum antiviral activity because they are capable of specific interactions with viral particles or cellular surface molecules, resulting in the unique inhibition of virus-type or host cell-type independent activity (Kim et al., 2012). The sulfated polysaccharide p-KG03 of G. impudium has specific antiviral activities, because it not only inhibits the binding site of the influenza A virus to host cells (virus-cell interaction), but also prevents cellular internalization of the virus (virus-cell fusion) (Kim et al., 2012). Therefore, microalgal polysaccharides have attracted considerable attention as sources of biologically active molecules, in particular as natural therapeutic agents, cosmetic additives and functional food ingredients.

3. Lipids from microalgae Many lipids are accumulated by algae (normally accounting for 30-50% of their content by weight) under several specific cultural conditions, such as in a high C/N medium or under stress conditions. Microalgal lipids are classified into two types according to their carbon numbers, with fatty acids containing 14−20 carbons used for biodiesel production, and polyunsaturated fatty acids (with over 20 carbons) used as health food supplements. Two parameters are generally considered for the evaluation of lipid accumulation for biofuel production: one is the lipid content (% lipid per dry weight of biomass), and the other is the lipid productivity (amount of lipid produced 8

per liter of working volume per day). Both the lipid content and biomass production rate should be considered simultaneously to ensure efficient microalgal lipid production, with a carbon number in the range of 14−20 being suitable for biodiesel production. In contrast, polyunsaturated fatty acids (PUFAs) with more than 20 carbons are high value compounds for use in the health food market, as they are essential nutrients that cannot be synthesized by higher eukaryotes. Among all of the commercially produced microalgal PUFAs, eicosapentaenoic acid (EPA, 20:5, ω-3) and docosahexaenoic acid (DHA, 22:6, ω-3) are reported to have levels of bioactivity, and thus are of particular interest. Studies have shown that microalgae may contain large quantities of high-quality EPA and DHA, and thus they are considered a good potential source of these valuable fatty acids (Spolaore et al., 2006; Vazhappilly & Chen, 1998).

3.1. Lipid biosynthesis metabolism of microalgae Microalgae are able to produce a wide variety of biofuels. The main energy-rich compounds stored in microalgae are triacylglycerol (TAG) and starch, which can be used to produce biodiesel and bioethanol, respectively. Recently, more attention has been paid to producing lipids from microalgae for biodiesel synthesis. There are three major steps in the synthesis pathway of TAG (Fig. 1): (1) the conversion of acetyl-CoA to malonyl-CoA, catalyzed by acetyl-CoA carboxylase (ACCase); (2) the elongation of the carbon chain of fatty acids; and (3) TAG formation (Huang et al., 2010). The conversion of acetyl-CoA and CO2 into malonyl-CoA, considered the first phase in fatty acid synthesis, occurs in chloroplasts (Hu et al., 2008). This reaction has two steps, and is catalyzed by a single enzyme complex. In the first step, CO2 (from 9

HCO3-) is transferred to nitrogen by the biotin carboxylase prosthetic group of ACCase in a biotin prosthetic group attached to the ε-amino group of lysine residue, which is ATP-dependent. In the second step, the activated CO2 is transferred from biotin to acetyl-CoA to form malonyl-CoA, which is catalyzed by carboxyltransferase . The fatty acid elongation in the fatty acid synthesis cycle condenses malonyl-CoA molecules and acetyl-CoA; after several repeated reaction steps, saturated C16 and C18-ACP are formed, and then the ACP-thioesterase cleaves the acyl chain and liberates the fatty acid (Courchesne et al., 2009). The first step of TAG formation is the condensation of glycerol-3-phosphate with acyl-CoA and the formation of lysophosphatidic acid. This reaction is catalyzed by GPAT (glycerol-3-phosphate acyltransferase), which exhibits the lowest specific activity of the TAG synthesis pathway, and is suggested to be the rate-limiting step (Courchesne et al., 2009). After this, phosphatidic acid, diacylglycerol and TAGs are synthesized by a series of catalytic reactions.

3.2. Strategies to enhance microalgal lipid production In both lab-scale and pilot-scale microalgae cultivation systems, growth characteristics and microalgae composition are known to significantly depend on the environmental factors and medium composition. The performance of lipid production should be evaluated in two ways: one is the lipid contents (% lipid per dry weight of biomass) and the other is the lipid productivity (amount of lipid produced per liter of working volume per day). Both lipid content and biomass production rates should be considered, simultaneously, to reach efficient microalgal lipid production. Lipid accumulation in microalgae usually occurs at the expense of slower cell growth, due to the stress conditions. Therefore, it is more reasonable to evaluate lipid production 10

performance using the concept of lipid productivity, which is more important from an engineering aspect (Chen et al., 2011). The most well known factors influencing lipid production efficiency include light illumination strategy, temperature and nitrogen source availability (e.g., nitrogen starvation). These factors can affect either the lipid accumulation or cell growth rate, or both. First, the type of light source is known to affect the growth of microalgae, due mainly to the difference in the coverage of the wavelength range. Light intensity is also known to be a critical factor in microalgae growth. In order to obtain maximum productivity, the saturation light intensity needs to be distributed throughout the entire photobioreactor; however, this is impractical in practice, since the light distribution inside the photobioreactor will decrease rapidly due to the light shading effects arising from the increased concentration of cells and products, or from the formation of biofilm on the surface of the reactor vessels (Chen et al., 2008), although mixing the cells well can reduce the impacts of these. In addition, some reports investigated the effect of light intensity on the lipid contents of microalgae. Under a high light intensity the total polar lipid content decreases, while the contents of neutral storage lipids (TAGs) increases (Hu et al., 2008). The culture temperature can also affect the degree of saturation, and an increase in the content of saturated fatty acids was observed when the temperature was increased (Hu et al., 2008). The lipid content is also sometimes influenced by temperature. For instance, the lipid content of Nannochloropsis salina and Ochromonas danica increased with an increase in the culture temperature (Boussiba et al., 1987). Nevertheless, other studies showed that there was almost no difference in the lipid contents of Chlorella Sorokiniana when it was grown at various temperatures (Patterso, 1970). Lipid accumulation in microalgae usually occurs when they are cultivated under stress conditions. The major physical stimuli are temperature and light intensity, while 11

the major chemical stimuli are pH, salinity, mineral salts and nitrogen source (Hu et al., 2008). Among these stress conditions, nitrogen limitation is the most widely used and reliable strategy for increasing the lipid content of microalgae. Hu and his colleagues collected the data for the lipid content of various classes of microalgae and cyanobacteria under normal growth and stress conditions (Hu et al., 2008). Their results clearly showed that the lipid content of green microalgae, diatoms and some other species under stress conditions was 10−20% higher than under nitrogen sufficient conditions. However, the lipid content of cyanobacteria produced under stress conditions was usually less than 10%, making it unsuitable for lipid production . Various nitrogen limitation strategies have been applied to foster lipid accumulation, and these can mainly be divided into two types. The first is a two-stage nitrogen limitation method in which the microalgae cells are cultivated under nitrogen-sufficient conditions to stimulate cell growth for a set amount of time. They are then collected and switched to nitrogen-starving conditions for lipid accumulation. The second method is a one-stage approach in which the initial nitrogen concentration is adjusted to a desirable level to control the time required to reach nitrogen starvation. The microalgae culture will then automatically advance to the nitrogen starvation stage when the nitrogen content in the medium is depleted. The effects of the two types of nitrogen starvation on lipid production efficiency seem to be microalgal species-dependent.

3.3. Producing biodiesel from microalgal lipids Plant or microalgal oil, which contains triglycerides, is used to make biodiesel via the transesterification process. The triglycerides react with methanol in a reaction called transesterification or alcoholysis and then methyl esters of fatty acids 12

(biodiesel) and glycerol are produced. Each mole of triglyceride requires 3 moles of alcohol to produce 1 mole of glycerol and 3 moles of methyl esters. In industrial processes, 6 moles of methanol are used for each mole of triglyceride to ensure that the reaction is driven in the direction of methyl esters . Because the alkali-catalyzed transesterification is about 4,000 times faster than the acid-catalyzed reaction, alkalis such as NaOH and KOH are commonly used as the catalyst. However, there is a major problem when using an alkali-catalyzed process. A saponification reaction occurs when free fatty acids are present in the triglyceride feed. Since high-quality feedstock needs to be produced to prevent this problem, the cost of raw materials usually accounts for 60−75% of the total cost of the biodiesel fuel (Huang et al., 2010). Acid catalysts, which can be used for feedstock with high free fatty acid content, could overcome the saponification problems that arise in an alkali-catalyzed process. Nevertheless, reactions catalyzed by acid catalysts have lower reaction rates and yields than those catalyzed by an alkali process.

Therefore,

a two-stage biodiesel production, which converts free fatty acids to methyl esters under an acid-catalyzed process in the primary stage followed by an alkali-catalyzed process, has been examined in a number of studies (Behzadi & Farid, 2007).

Biodiesel is now produced mainly from vegetable oils; however, these are mostly derived from food crops, raising the issue of food-fuel competition. In addition, because of the increasing demand for these oil crops, their market price has increased, thus raising the cost of producing biodiesel. Alternative feedstocks thus need to be developed to solve this problem. Among the various candidates for this, microalgae appear to be one of the most promising ones (Trzcinski et al., 2012), due to their high lipid content, fast growth rate and large lipid production capacity. The lipid content in 13

microalgal biomass can range from 16% to 77%, with levels of over 30% commonly seen in a variety of microalgal species. The advantages of using microalgae as a source of biodiesel feedstock are as follows (Um & Kim, 2009): 1. The growth of microalgae is extremely fast compared with terrestrial plants, and the biomass can be doubled within 24 hrs. 2. The oil content of the microalgal biomass can reach over 50% of the dry cell weight. 3. The oil yield by cultivated area is larger than that of the oilseed crops. 4. Microalgae are aquatic microorganisms, and thus do not compete for the land needed for agriculture crops. 5. The production of microalgae does not compete with human food production. 6. Microalgae are able to grow under conditions that are not suitable for conventional crops. 7. Microalgae can convert CO2 into biomass, and may reduce the CO2 concentration in the atmosphere. 8. The biofuels produced from microalgae do not contain sulfur, and are non-toxic and highly biodegradable. However, there are several critical issues regarding the production of biodiesel from microalgae. Compared to terrestrial oil crops, the harvest of, and oil extraction from, microalgae is much more difficult, leading to a higher production cost for microalgae-based biodiesel. Normally, the dry microalgal biomass only accounts for 0.1−1% by weight of the microalgae culture. As a result, concentration of the microalgal biomass and the dewatering processing are both costly and energy intensive. In addition, since microalgae are small in size, the microalgal lipid cannot be easily extracted by conventional mechanical methods used for obtaining oil from 14

oil crops, such as palm trees or soybeans, raising another technical hurdle for converting microalgal lipid into biodiesel (Chen et al., 2011). Recently, wet extraction approaches have been used to enhance the efficiency and economic feasibility of lipid extraction from microalgae. In this way, FAME yields of 84% from microalgae can be achieved under a water content of 50% (w/w) using methanol as the co-solvent (Wahlen et al., 2011). Another approach is using dry microalgae powder as the starting material to make biodiesel with a direct or in-situ transesterification process . In these approaches, the lipid extraction and transesterification can be done simultaneously in one step, leading to a simplified process and reduced production costs. However, intensive dewatering of the microalgal biomass is still required to avoid the problems associated with transesterification reactions. More recently, (Tran et al., 2012a,b) demonstrated an innovative approach using wet microalgae (up to 90 wt% water content) as the feedstock to produce biodiesel in the presence of a solvent (i.e., hexane), an excessive amount of methanol, and immobilized lipase via a one-step lipid extraction and transesterification process. They achieved a high biodiesel conversion of 97.3% when using a wet biomass of Chlorella vulgaris microalgal biomass (lipid content = 63% per dry weight) in the presence of 71% water content (Tran et al., 2012a,b).

3.4. Polyunsaturated fatty acids Besides using the fatty acids of 14−20 carbons for biodiesel production, the polyunsaturated fatty acids (PUFAs) with more than 20 carbons are high value compounds in the health food market. Higher forms of plants and most animals lack the required enzymes to synthesize PUFAs from more than 18 carbons, although they are essential for good functioning, conferring flexibility, fluidity and selective 15

permeability properties to membranes. PUFAs consist of three or more double bonds on a fatty acid skeletal chain containing 18 or more carbons, and they are further classified into ω-6 and ω-3 forms, depending on the position of the last double bond proximal to the methyl end of the fatty acid. Fish and fish oils are common sources of long-chain PUFAs, and since the PUFAs found in fish originate from digested microalgae in oceanic environments, it is reasonable to consider microalgae as a direct potential source of these substances. Among all the commercially produced microalgal PUFAs, eicosapentaenoic acid (EPA, 20:5, ω-3) and docosahexaenoic acid (DHA, 22:6, ω-3) are reported to have bioactivities of particular interest. Moreover, microalgae contain large quantities of high-quality EPA and DHA, and are thus considered a potential source of this important fatty acid. Numerous strategies have been investigated for commercial production of EPA and DHA using microalgae, including screening of high EPA and DHA-yielding microalgal strains, improvement of strains by genetic manipulation, optimization of culture conditions (such as pH, temperature and the ratio of C/N), and the development of efficient cultivation systems. The productivity and yield figures shown in Tables 2 and 3 reveal that the production of EPA and DHA by algae is a potential replacement for the conventional extraction of EPA and DHA from fish meats. The effect of aeration on the performance of docosahexaenoic acid (DHA) production by Schizochytrium sp. was investigated in a 1,500-L bioreactor using fed-batch fermentation. The aeration rate was controlled at a 0.4 volume of air per volume of liquid per min (vvm) for the first 24 h, then shifted to 0.6 vvm for 96 h, and finally switched back to 0.4 vvm until the end of the fermentation. High cell density (71 g/L), high lipid content (35.75 g/L) and a high DHA percentage (48.95%) were achieved by using this strategy, and DHA productivity reached 119 mg/L h, which 16

was 11.21% over the best results obtained with a constant aeration rate (Ren et al., 2010). In order to achieve lipid accumulation in a microorganism, it is necessary to grow it in a medium with excess carbon substrates and limited amounts of other nutrients, usually nitrogen. However, different nutritional conditions are likely to be required for the stages of growth and DHA accumulation. Therefore, a two-stage cultivation process (growth and production) was performed for DHA production by Aurantiochytrium limacinum SR21, which produced 154 mg DHA/L/h (Rosa et al., 2010).

4. Pigments and Proteins from microalgae The colorful appearance of algae is derived from their pigments, which absorb visible light and initiate photosynthesis reactions. The three major classes of photosynthetic pigments that appear in algae are chlorophylls, carotenoids and phycobilins (Table 4) (Hall & Rao, 1999). Chlorophyll a is the primary pigment in all photosynthetic organisms, and it absorbs the most energy from the wavelengths of violet-blue and orange-red light, and then serves as a primary electron donor in the electron transport chain . The other accessory pigments include chlorophyll b (also c and d in different algae), carotenoids (such as beta-carotene in most algae) and phycobilins (in cyanobacteria and red algae) (Hall & Rao, 1999).

4.1. Chlorophylls Chlorophylls are greenish pigments which contain a porphyrin (tetrapyrrole ring), a central magnesium atom, side chains and a phytol tail . Chlorophyll is situated in the chloroplast lamellae in a form where the porphyrin head binds to the protein layers, and a phytol tail extends into the lipid layers. 17

Green algae have the highest chlorophyll content among all algae, and this well-commercialized green alga belongs to the Chlorella species. Chlorella is produced worldwide, with an annual production of 2,000 metric tons of dry powder, over 50% of which is manufactured in Taiwan (Milledge, 2011). Pheophorbide a is a chlorophyll derivative obtained from removing the phytol and the central magnesium atom. A high dose of pheophorbide a will cause an allergic reaction in humans, including inflammation and a red rash on the skin. As a result, the pheophorbide content in food containing algae (such as Chlorella, Spirulina and Nori (a purple laver product)) is limited by food safety and hygiene regulations. These levels have been set by the Department of Health in Taiwan (<0.8 mg/g for Chlorella and <1 mg/g for Spirulina) and Japan (0.8 mg/g) for more than 20 years (Lembi & Waaland, 1988), with both species being widely consumed in these countries for health reasons. The photosensitizer characteristics of pheophorbide mean that it has been applied in photodynamic therapy (PDT) since the 1990s (Busch et al., 2009). Three key components of modern PDT applications are photosensitizers, light sources and tissue oxygen. The process occurs as follows: a light source with the appropriate wavelength excites the photosensitizer to produce a reactive oxygen species, and then the reactive singlet oxygen rapidly reacts with any nearby molecules. These destructive reactions will kill the target, such as malignant cancers, through apoptosis or necrosis (Chen et al., 2002). Pheophorbide a-based PDT has been utilized in treating human uterine sarcoma line MES-SA, human colon adenocarcinoma, human hepatoma (Hep3B cells) and rat pancreatic cancer (Busch et al., 2009). There are currently two FDA-approved photosensitizers, Photofrin® and aminolevulinic acid (ALA), which induce protoporphyrin IX (PpIX). Commercial pheophorbide a is now available from Frontier Scientific, Inc. (Logan, UT, USA) (Logan, UT, USA). 18

4.2. Carotenoids Carotenoids are yellow, orange or red pigments found in most photosynthetic organisms. Carotenoids belong to the group of tetraterpenoids with a 40-carbon skeletal formation built up from isoprene subunits. Two small six-carbon rings are connected to the carbon skeleton. The known structures of carotenoids mentioned in this review are shown in (Takaichi, 2011). Carotenoids are insoluble in water and are usually attached to membranes within cells, being situated in the chloroplast in most algae or photosynthetic lamellae of cyanobacteria . Carotenoids serve as photo-protectors against the photo-oxidative damage resulting from excess energy captured by light-harvesting antenna (Xiao et al., 2011). Astaxanthin is an oxygenated carotenoid, also a kind of ketocarotenoid, known for its powerful antioxidant activity. The reported antioxidant activity of astaxanthin is 10 times higher than that of β-carotene and more than 500 times that of α-tocopherol (Dufossé, 2007). It is also a potent quencher of reactive oxygen and nitrogen species, including singlet oxygen and single- and two-electron oxidants, as shown in several in vitro studies (Fassett & Coombes, 2011) . Astaxanthin has many benefits in the prevention and treatment of various conditions, such as chronic inflammatory diseases, eye diseases, skin diseases, cardiovascular diseases, cancers, neurodegenerative diseases, liver diseases, metabolic syndrome, diabetes, diabetic nephropathy and gastrointestinal diseases . This powerful antioxidant activity means that there have been a wide range of applications of astaxanthin in the food, dietary supplements (or nutraceuticals), cosmetics and food industries. The United States Food and Drug Administration (US FDA) approved astaxanthin as a food additive for use in the aquaculture industry in 1987, and in 1999 astaxanthin was further approved 19

for use as a dietary supplement (Guerin et al., 2003). Natural sources of astaxanthin are microalgae, yeast, shrimp, krill and plankton. Astaxanthin also provides the red color of salmon meat and the feathers of some birds after these animals have ingested it. Haematococcus pluvialis is a freshwater green alga that can synthesize and accumulate astaxanthin under oxidative stress.

Its life

cycle consists of four cell stages: vegetative cell growth, encystment, maturation and germination (Collins et al., 2011; Kobayashi et al., 1997). The vegetative cells are motile flagellated cells with an optimum growth temperature of 22–25°C. Stress conditions, such as nutrient deprivation, high salinity, strong irradiance, high temperature or combinations of these stresses, trigger carotenogenesis and cell differentiation from the vegetative cells stage to cyst cells. The cyst cells are nonmotile aplanospores with thick walls. Astaxanthin is contained in aplanospore (Lorenz & G.R., 2000), and its content can reach up to 4% of the total cellular dry weight (Collins et al., 2011; Kobayashi et al., 1997). Other natural sources of astaxanthin are crustacean exoskeletons and yeast Xanthophyllomyces dendrorhous (Phaffia rhodozyma); however, the former is in limited supply and has a low astaxanthin content, while the latter has a astaxanthin content (0.4%-2.5%) much lower than that seen in microalgae (Muntendam et al., 2009). The annual worldwide aquaculture market for astaxanthin was estimated at US$200 million in 2004, with estimations of the global astaxanthin market rising to US$257 million in 2009 (Del Campo JA, 2007). Synthetic astaxanthin is valued at US$2500/kg, while the natural product is sold for over US$7000/kg. Although 95% of market is met by synthetic astaxanthin, consumer demand for natural products provides an opportunity for the sale of Haematococcus astaxanthin. There are currently five major producers of Haematococcus astaxanthin: Algatechnologies, Ltd. 20

(Kibbutz Ketura, Israel), BioReal Inc. (Kihei, Hawaii, U.S.A., a subsidiary of Fuji Chemical Industry, Toyama, Japan), Cyanotech Corporation (Kailua-Kona, Hawaii, U.S.A.), Mera Pharmaceuticals (Kailua-Kona, Hawaii, U.S.A.) and Parry’s Pharmaceuticals (Chennai, India) (Dufossé, 2007).

4.3. Phycobilins/phycobiliproteins from algae The chemical structures of phycobilins are related to those of chlorophylls. Each phycobilin is a linear tetrapyrrole with four pyrrole rings that are linked together by single-atom bridges. Phycobilins are covalently attached to polypeptides to form water-soluble phycobiliproteins. The phycobilins further give phycobiliproteins a distinct absorption spectra, ranging from 460 to 670 nm, in which chlorophyll a has a low absorption. Phycobiliproteins are classified into three types by their phycobilin energy (absorption spectra): high-energy ones are phycoerythrins (PEs) or phycoerythrocyanins (PECs), with their main absorption at 480−580 nm; intermediate-energy ones are phycocyanins (PCs), with their main absorption at 600−640 nm; and low-energy ones are allophycocyanins (APCs), with their main absorption at 620−660 nm. Purified phycobiliproteins are highly fluorescent because there are no nearby acceptors to receive the harvested energy. The phycobiliproteins have several unique features, such as a broad absorption in a visible light spectrum, a high extinction coefficient, high fluorescence quantum efficiency, a large Stokes shift and very little fluorescence quenching. These features make phycobiliproteins promising fluorescent labeling reagents that can be employed in flow cytometry, fluorescence immunoassay, fluorescence microscopy, immuno-histochemistry and other biomedical research purposes ((Matamala et al., 2007; Waggoner, 2006). Blue phycobiliproteins, APC, 21

can be produced by Spirulina sp., and red ones, B-PE and R-PE, are produced by red microalgae, such as Porphyridium sp., Rhodella sp. and Bangia sp., on a kilogram scale per year. The global market was estimated to be approximately US$50 million in 1997, with prices varying from US $3/mg to US $25/mg (Milledge, 2011). The leading manufacturers and suppliers of phycobiliproteins as a fluorescent labeling reagent are Far East Bio-Tec Co., Ltd. (Taiwan, also known as FEBICO with the product brand name of Flogen®) and ProZyme, Inc. (Canada).

5. Conclusions The components of microalgae are valuable, with a wide range of applications. The carbohydrates present in microalgae are considered an appropriate feedstock for microbial growth and for the production of various fermentation products. The high lipid content in algal biomass makes it promising for biodiesel production, while the related long-chain fatty acids, pigments and proteins have their own nutraceutical and pharmaceutical applications. Therefore, microalgal biorefinery processes deserve further investigations. In particular, economic feasibility and life cycle assessments of such processes should be conducted to confirm the commercial viability of converting the components of microalgae into biofuels and other valuable products.

Acknowledgments The authors gratefully acknowledge the financial support from the top university project of NCKU and by Taiwan’s National Science Council under grant numbers NSC 101-3113-P-110-002, NSC 101-3113-E-006-015, and NSC 101-3113-E006-016.

22

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27

Table 1: The pharmacological functions found in the microalgae. Microalgae

Polysaccharide extracts

Pharmacological significance

Reference

C. vulgaris

Crude polysaccharide

Antioxidant

(Mohamed, 2008)

S. quadricauda

Crude polysaccharide

Antioxidant

(Mohamed, 2008)

Porphyridium sp.

Crude polysaccharide

Antioxidant

(Tannin-Spitz et al., 2005)

Porphyridium sp.

Sulfated polysaccharide

Anti-inflammatory

(Matsui et al., 2003)

H. lacustris

Water-soluble polysaccharide

Immunostimulating

(Park et al., 2011)

G. impudium KG-03

Sulfated polysaccharide

Antiviral

(Kim et al., 2012; Lee et al., 2009)

R. reticulata

Extracellular polysaccharide

Antioxidant

(Chen et al., 2010)

C. stigmatophora

Crude polysaccharide

Anti-inflammatory/ Immunomodulating

(Guzman et al., 2003)

P. tricornutum

Crude polysaccharide

Anti-inflammatory/ Immunomodulating

(Guzman et al., 2003)

28

Table 2: DHA production by microalgae in the literature

Strain

DHA Productivity (g/L day)

DHA Content (mg DHA/g biomass)

DHA (% of total fatty acids)

Reference

Schizochytrium sp.

3.3

277

35.6

(Yaguchi et al., 1997)

1.15

174

31.1

(Swaaf et al., 2003)

Schizochytrium sp.

-

130

32.5

(Wu et al., 2005)

Schizochytrium limacinum

0.51

170

33.6

(Chi et al., 2007)

Schizochytrium sp

3

100

40

(Ganuza et al., 2008)

Schizochytrium limacinum

-

173

-

(Chi et al., 2009)

Schizochytrium sp.

2.86

246

49

(Ren et al., 2010)

Aurantiochytrium limacinum

3.7

153

23.9

(Rosa et al., 2010)

Aurantiochytrium sp.

2.90

175

40

(Hong et al., 2011)

Aurantiochytrium

-

290

39.7

(Yang et al., 2012)

Crypthecodinium cohnii

29

Table 3: EPA production by microalgae in the literature

Strain

EPA Productivity (mg/L day)

Phaeodactylum tricornutum 6

EPA Content (mg EPA/g biomass)

EPA (% of total fatty acids)

-

25.8

(Reis et al., 1996)

Reference

Navicula saprophila

0.89

13.6

20.1

(Kitano et al., 1997)

Rhodomonas salina

0.04

-

15.4

(Kitano et al., 1997)

Nitzschia sp.

0.18

-

24.7

(Kitano et al., 1997)

Monodus subterraneus

-

38

34.2

(Vazhappilly & Chen, 1998)

Chlorella minutissima

-

37

31.3

(Vazhappilly & Chen, 1998)

Phaeodactylum tricornutum -

22

21.4

(Vazhappilly & Chen, 1998)

Monodus subterraneus

44-56

35-43

31.5-31.8

(Lu et al., 2001)

Nitzschia laevis

175

26

22.4

(Wen & Chen, 2001)

Nitzschia laevis

33.5

30

11

(Wen et al., 2002)

Monodus subterraneus

9

23-32

25-30

(Lu et al., 2002)

30

Table 4. The photosynthetic pigments Pigments

Characteristic absorption maxima (nm)

Chlorophylls

(in organic solvents)

Occurrence

Chlorophyll a

420, 660

All higher plants and algae

Chlorophyll b

435, 643

All higher plants and green algae

Chlorophyll c

445, 625

Diatoms and brown algae

Chlorophyll d

450, 690

Red algae

Carotenoids

(in organic solvents)

α-carotene

420, 440, 470

Most plants and some algae

β-carotene

425, 450, 480

Higher plants and most algae

Luteol

425, 445, 475

Green algae, red algae, and higher plants

Fucoxanthol

425, 450, 475

Diatoms and brown algae

Peridinin

375, 495

Dinophytes

Phycobilins

(in water)

Phycoerythrins

490, 546, 576

Red algae and some cyanobacteria

Phycocyanins

618

Cyanobacteria and some red algae

Allophycocyanins

650

Cyanobacteria and red algae

31

Figure Captions Fig 1. Microalgae metabolic pathways for different kinds of biofuel production

32

Fig. 1

33

Highlights > Components of microalgal biomass are suitable for biofuels production and biorefineries > Characterization and application of microalgae-based lipids, carbohydrates, pigments and proteins are elucidated > Critical comments were made on the role of microalgae in fermentation, food, and pharmaceutical industries

34