Many algae produce antibiotics such as acrylic acid found in Phaeocystis poucht. This anti­biotic inhibits the growth of gram-positive organisms. The phenols found in macro — and microalgae have antimicrobial activity.

The microalga Scenedesmus obliquus has been used in postoperative recovery, assisting in coagulation of the skin surface. The extracts of the diatom Asterionella notata have an antifun­gal and antiviral activity. Toxic algae have been used as a depressant vessel, similar to tetro — dotoxin found in fish (Richmond, 1990).

Another drug obtained from microalgae is phycocyanin, a natural antioxidant that, when combined with caloric restriction, can contribute to mitigating the aging process. Free radicals are partly responsible for the human aging process (Finkel, 2003). The oxidative damage caused by free radicals has been linked to several diseases such as heart disease, atheroscle­rosis, lung problems, Alzheimer’s, and Parkinson’s. The DNA damage caused by free radicals plays an important role in the processes of mutagenesis and carcinogenesis.

Since outdoor sunlight cannot be controlled, carbon fixation by microalgae is usually studied indoors under artificial illumination. A good deal of scientific effort is being made to evaluate microalgae CO2 fixation potential. Most of these efforts focus the fixation into biomass (Chae et al., 2006; Jacob-Lopes et al., 2008; Kajiwara et al., 1997). However, these studies did not quantify the total carbon dioxide fixed effectively by microalgae (Jacob-Lopes et al., 2008; Fan et al., 2007), since there are other routes for carbon besides biomass generation, such as mineralization (formation of soluble bicarbonate and carbonate) and production of extracellular products such as polysaccharides, volatile organic compounds (Shaw et al., 2003), organohalogens (Scarratt and Moore, 1996), hormones, and others.

The determination of global rates of carbon dioxide sequestration through mass balances of CO2 in the liquid or gas phase of the systems (Eriksen et al., 2007) gives more complete data. One approximation for the rates may be obtained by evaluating dissolved inorganic carbon concentration in the culture media while monitoring the pH variation (see methodology at Valdes et al., 2012). This shows that carbon fixation by microalgae is a complex process whereby biomass production might be a part of the total carbon destination. In addition, little information is available with respect to the simultaneous research of both the global rates of

02 consumed (g/h) —— 02 Base Line (g/h)

FIGURE 4.3 Gas phase analysis carried by Sydney et al. (2011) showing the carbon consumption and oxygen production profiles.

carbon dioxide sequestration and the rates of incorporation of carbon into the microalgae biomass (Chiu et al., 2008).

Sydney et al. (2011) studied the global CO2 fixation rate of four microalgae through a mass balance of the gas phase. The experiments were carried out in a photobioreactor coupled with sensors to measure CO2 in the inlet and outlet gases. The net carbon dioxide mitigation during each microalgal cultivation was evaluated. Nutrient consumption, biomass production (and composition), and possible extracellular products were analyzed throughout the process. It was found that between 70% and 88% of the carbon dioxide consumed was used in biomass production. This finding indicates that, to explore the whole potential of microalgal mitiga­tion capacity (considering negotiations in the carbon market), carbon balance might be carried through (complex) carbon balance in the gas phase. The problem is that it is difficult to carry out this kind of analysis in open photobioreactors and to standardize this methodology. Figure 4.3 presents the profile of carbon dioxide consumption obtained during gas phase analysis during cultivation. It is interesting to note that CO2 consumption (in blue) has a com­plementary behavior with O2 production due to photosynthesis and respiration processes during light and dark cycles.

A solid-ejecting disc centrifuge provides intermittent solids ejection by regulating its valve — controlled peripheral ports using a timer or an automatic triggering device. The advantage of this centrifuge for algae harvesting is its ability to produce algal cake in a single step without chemical dosing (Mohn and Soeder, 1978; Mohn, 1980; Shelef et al., 1984). This centrifuge con­centrated various types of microalgae effectively, achieving algal cake of 12-25% solids (Mohn, 1980; Moraine et al., 1980). The extent of the algae suspension separation increases with increas­ing residence time (decreasing feed rate), and the ejected cake concentration is affected by the intervals between successive desludging (Shelef et al., 1984). A solid-ejecting disc centrifuge was found very reliable. The only reported setback was that solids finer than algae may be retained in the overflow, which reduces the separation efficiency (Moraine et al., 1980). High capital and energy costs render this separation method unappealing.

Microalgae can also function in mixotrophic nutrition mode by combining both the auto­trophic and the heterotrophic mechanisms. It facilitates fixing atmospheric CO2 as well as con­suming the organic molecules and micronutrients from the growing environment (Figure 8.8). Microalgae can assimilate available organic compounds as well as atmospheric CO2 as a car­bon source in mixotrophic mode. The CO2 released by microalgae via respiration will again be trapped and reused in mixotrophic nutritional mode. It differs from photoheterotrophic nutrition mode in terms of CO2 utilization. The mixotrophs have the ability to utilize organic carbon; therefore, light energy is not a limiting factor for biomass growth (Chang et al., 2011). The acetyl-CoA pool will be maintained from both carbon sources—that is, by the CO2 fixation (Calvin cycle) and intake from outside the cell, which can further make malonyl-CoA. The photosynthetic metabolism utilizes light and CO2 for growth and organic photosynthate pro­duction, whereas respiration uses the organic photosynthates produced during photosynthe­sis. If an external carbon source is available in the system, there is a less loss of photosynthate during respiration, and the algae utilize the available excess photosynthates for biomass development. Mixotrophic cultures show reduced photoinhibition and improved growth rates over autotrophic and heterotrophic cultures (Chojnacka and Noworyta, 2004).

8.3 NUTRITIONAL MODE OF MICROALGAE

Mixotrophic Nutrition

Lipid

Glucose

FIGURE 8.8 Mixotrophic mode of nutrition in algal cells towards CO2 fixation and glucose assimilation for lipid biosynthesis

Algae have the flexibility to switch their nutritional mode based on substrate availability and light condition. If simpler carbohydrates are present in the system, algae shift towards heterotrophic nutrition from autotrophic mode to save energy. Scenedesmus obliquus readily adapted to heterotrophic growth in dark conditions utilizing glucose (Abeliovich and Weisman, 1978). Heterotrophic cells differed significantly from photoautotrophic cells with respect to several physiological properties such as the rate of photoassimilation of CO2 and the rate of incorporation of carbon and chlorophyll a concentration. Algal cells in an oxidation pond shared features common to both photoautotrophic and heterotrophic cells (Abeliovich and Weisman, 1978), associating with the mixotrophic mode of operation. Bacteria seem to play a minor role in biological oxygen demand reduction in high-rate oxidation ponds, and their role is probably confined to degradation of biopolymers, thus producing substrates for algal consumption.

The advantages of mixotrophic nutrition are its independence in terms of both photosyn­thesis and growth substrates (Kong et al., 2012). The mixotrophic growth regime is a variant of the heterotrophic growth regime, where CO2 and organic carbon are simultaneously assim­ilated and both respiratory and photosynthetic metabolism operates concurrently (Kaplan et al., 1986; Lee, 2004; Perez-Garcia et al., 2011). Mixotrophism is often observed in ecological water bodies, where the homeostatic structure and function of living systems are supported by chemical, physical, and organic activity in biota that balance the ecological status. Water ecosystems generally consist of nutrients and organic carbon as integral parts (Venkata Mohan et al., 2009), where microalgae, along with other living components, function together symbiotically. Some microalgal species are not truly mixotrophs but have the ability to switch between phototrophic and heterotrophic metabolisms, depending on environmental condi­tions (Kaplan et al., 1986). Microalgae-accumulating lipids are generally grown in natural

water bodies; therefore, ecological water bodies embedded with diverse microalgae species can be considered as potential reservoirs for harnessing biodiesel. In this regard, an attempt was made to explore the ability and potential of mixed microalgae cultures derived from dif­ferent water bodies in extracting lipids, which can be further transesterified to biodiesel. The study also focused on the economic mode of lipid production from the treatment of domestic sewage. The growth of algae was shown to be highest under mixotrophic conditions, with higher biomass productivity under photoautotrophic conditions (Bhatnagar et al., 2010; 2011). Mixotrophic cultivation was shown to be a good strategy to obtain a large biomass and high growth rates (Ogawa and Aiba, 1981; Lee and Lee, 2002), with the additional benefit of producing photosynthetic metabolites (Chen, 1996; Perez-Garcia et al., 2011). Solazyme, a renewable oil company in the United States, has developed an integrated algal cultivation process by dark heterotrophic mechanisms, giving carbon sources externally. The company is using various forms of waste material as feedstock for the cultivation of algae in fermenters and harnessing as much as 75% of oil on the basis of dry cell weight. The company is antic­ipating in selling algal oil to commercial refineries by the end of 2013.

Open-pond culture systems and enclosed bioreactor facilities have been used commer­cially in the recently evolved alga biomass biotechnology, but bioreactor design in algal hy­drogen production is still in the research and development stage. Because biohydrogen metabolism is primarily the domain of microalgae, diatoms, or cyanobacteria, the design of a photobioreactor depends on microbiological processes associated with bacteria and microalgae (Show et al., 2008; 2011; 2012). Although these photoheterotrophic bacteria differ in photochemical efficiency, absorption coefficient, and size, the light regime, including light and dark cycles, is assumed to be much more determining than biological factors (Akkerman et al., 2003). Hence the productivity of a photobioreactor is light-dependent, and a large surface-to-volume ratio is a prerequisite for a productive photobioreactor for optimal light exposure of the algae. Provisions for thermal control and monitoring of factors, including flow rates, pH, and dissolved oxygen, sulfur, and hydrogen are essential. Technical develop­ment is now moving toward devising gas-tight systems, engineered microalgae culturing, and computer-controlled systems for monitoring and automatic nutrient delivery and culture dilution.

Photobioreactors have been designed to achieve an economical, rapid multiplication and high algal biomass density (>1012 cells per cubic meter of culture) (Evens et al., 2000). Various photobioreactor designs, including flat plate, tubular, pond, and pool type, have been inves­tigated (Akkerman et al., 2003). The photobioreactor process, whether of batch or continuous flow, should be designed for optimal light exposure to the algae. Sufficient light supply is vital for adequate biomass growth in achieving high-density culture and for photosynthetic generation of hydrogen. Light conversion efficiencies are low (limited to 10% theoretically) and tend to decrease at higher light intensities because of the light saturation effect (Akkerman et al., 2003). The reason for this inefficiency is that, at high solar intensities, the rate of photon absorption by the chlorophyll antenna of the upper layers of algal biomass far exceeds the rate at which photosynthesis can consume. This phenomenon is attributable to the fact that algae have an intrinsic tendency to accumulate a large assembly of photon­absorbing chlorophyll antenna molecules as a survival strategy. The overabsorption of light by the chlorophyll antenna results in loss of excess photon as heat or other rays. Moreover, cells at the upper layers of the algal mass are subject to severe photoinhibition of photosyn­thesis due to the high rate of photon absorption (Baroli and Melis, 1996; Melis, 1999).

Research is underway in improving further algal photosynthetic capacity using an molec­ular engineering approach, whereas algal strains have been manipulated to increase hydro­gen production (Hankamer et al., 2007; Beer et al., 2009). It has been reported that a truncated chlorophyll antenna size of the photosystems in the chloroplast of the microalgae could alle­viate the optical shortcomings associated with a fully pigmented chlorophyll antenna (Melis et al., 1999; Neidhardt et al., 1998). The work on the truncated chlorophyll antenna size in maximizing solar conversion efficiencies is delineated in Section 9.5. Mutant algae with less chlorophyll were cultured and are able to distribute more sunlight to deeper layers in the algal biomass for large-scale applications (Hankamer et al., 2007). In this manner, sunlight is made available for more algal cells to generate hydrogen. Hence, for efficient photoproduction of hydrogen, it is critical to dilute the light and distribute it over the entire reactor volume and to mix the culture at high rates so that cells are exposed to the light for only a short time.

Algal photobioreactors can be designed to regulate light inputs to the algal culture to im­prove its photon conversion efficiency. A substantial increase in light utilization efficiency of up to 15% has been reported (Tetali et al., 2007; Laurinavichene et al., 2008). Conversion ef­ficiency between 10% and 13% is feasible using engineered microbial culture to better utilize the solar energy (Turner et al., 2008). However, improvements must be made to the solar con­version efficiency of the algae for commercial purposes. Critical issues such as the optical shortcomings associated with the chlorophyll antenna size and the light saturation of photo­synthesis must be addressed under mass culture conditions (Melis et al., 1999). Technological advancement addressing these issues is discussed in Section 9.5.

Appropriate configuration of the bioreactor needs to be established for the most effective use of light and surface area. Biomass mixing is hence significant to ensure uniform disper­sion of nutrients and light illumination in the culture as well as to prevent agglomeration and sedimentation of algal biomass (Melis, 2002). Modular design of experimental systems should be allowed for possible scale-up. Such commercial scale should achieve sustainable gas out­put and high hydrogen yields with compact configuration. Trapping and withdrawal of hy­drogen gas in the system are also important design considerations for photobioreactors. Given the current advancement in photobiohydrogen production, technical and economic strategies for cycling the microalgae between sulfur deprivation and supply must be devel­oped (Laurinavichene et al., 2008).

Various types of photobioreactors had been investigated in a study by Janssen (2002). Small-scale flat panel reactors consisting of a rectangular transparent box were mixed with gas introduced via a perforated tube at the bottom of the reactor. To create a high degree of turbulence, 3 to 4 liters of air per liter of reactor volume per minute must be provided. The panels were illuminated from one side by direct sunlight, and the panels are placed vertically or inclined toward the sun. Light/dark cycles were short in flat panel reactors, and this is probably the key factor leading to high photochemical efficiency. A disadvantage of flat panel reactors systems is that the power consumption of aeration (or mixing with another gas) is high, although mixing is always necessary in any reactor. The large-scale flat plate reactor consists of a rectangular airlift photobioreactor with a large number of light — redistributing plates fixed a few centimeters from each other. Mixing was provided by air injected between adjacent plates, and the culture liquid rises in between.

Tubular photobioreactors consist of long transparent tubes with diameters ranging from 3 to 6 cm and lengths ranging from 10 to 100 m (Janssen, 2002). The culture liquid is pumped through these tubes by means of mechanical or airlift pumps. The tubes can be positioned in many different ways: in a horizontal plane as straight tubes with a small or large number of U-bends; vertical, coiled as a cylinder or a cone; in a vertical plane, positioned in a fence-like structure using U-bends or connected by manifolds; or horizontal or inclined, parallel tubes connected by manifolds. In addition, horizontal tubes can be placed on different reflective surfaces with a certain distance between the tubes. Although tubular reactor design is rather diverse, the predominant effect of the specific designs on the light regime is a difference in the photon flux density incident on the reactor surface. The shape of the light gradient in the tubes is similar in most designs. Also with respect to liquid mixing, the circumstances in most de­signs are similar. The length of the tubes is limited because of accumulation of gas, though this might not be so important for nitrogenase-based processes, since they may be less inhibited by hydrogen. The way to scale up is to connect a number of tubes via manifolds. Flat panel reactors normally show a high photochemical efficiency or biomass yield on light energy, while biomass density is also high. Tubular bioreactors in theory should show better efficiencies because of the shorter average light/dark cycles.

Although much of the research has been focused on single-stage photobioreactor systems, multistage bioreactors entailing three or even four bioreactors in biohydrogen production have also been examined (see Figure 9.2) (U. S. DOE, 2007; Wang et al., 2011; Show et al., 2011). Sunlight is first filtered through first-stage direct photolysis, in which visible light is utilized by blue-green algae, and the unfiltered infrared ray is used by photosynthetic mi­crobes in the second-stage photofermentative reactor. The effluent from the second-stage photofermentation, together with the biomass feedstock, is fed into a third-stage dark fermen­tation reactor, where the microorganisms convert the substrate into hydrogen and organic acids. As the effluent is enriched with organic acids, a supply of external organic acids for the photofermentative process can be eliminated. The fourth stage involves the use of a microbial electrolysis cell to convert the organic acids generated from the dark fermentation into hydrogen in a light-independent process. This stage thus can be operated during the night or in low-light conditions.

The increasing attention on hythane has led to research interest in hydrogen production by dark fermentation of biomass in hybrid or multistage bioreactors. Hythane, a mixture of hydrogen and methane, is a highly efficient and ultraclean-burning alternative fuel that is probably the most promising biogas for industrial applications (Cavinato et al., 2009). How­ever, there are issues to be addressed before multistage bioreactors can be put to practical applications. Integration of multiple biochemical conversion processes poses significant chal­lenges for multistage reactor engineering, system design, process control, operation, and maintenance. Major challenges with the simultaneous production of hydrogen and oxygen from photolytic hydrogen production include respiration-to-photosynthetic-capacity ratio, co-culture balance, and concentration and processing of cell biomass (Holladay et al., 2009).

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

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

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

Apart from the perspective of energy balance, economic feasibility, coupled with techno­logical innovations, plays a critical role in ensuring the successful production of algal biofuels on a commercial scale. Based on an ideal assumption that algae could grow at a very rapid rate (typically requiring less than 10 days to reach the stationary growth phase) and could accumulate a high lipid content (30-70%) inside the cells, the resulting algal biodiesel should be able to compete or at least be on par with the current petrol-diesel price.

A preliminary cost analysis conducted by Chisti (2007) revealed that oil recovered from algal biomass produced in closed photobioreactors cost approximately $2.80/L, assuming that the algal biomass contained 30% oil and the oil-recovery process contributed 50% to the total production cost. However, in comparison to the average petrol-diesel price of $1.20/L gross delivered in the year 2010 (McHenry, 2012), the estimated algal biodiesel price is still much too high for commercial use, unless the algal biomass contains 70% oil, which could further reduce the price to $0.72/L (Chisti, 2007).

In a recent algal technological road map reported by the U. S. Department of Energy, a more robust system of modeling and comprehensive techno-economic analyses for algal biofuels should be developed to reach the goal of commercialization in the next 5 to 15 years (Fishman et al., 2010). An economic breakdown of multiple algal processing units with dif­ferent integration systems could help address the techno-economic feasibility of algal biofuels before reaching the commercial scale (Amer et al., 2011). An in-depth understanding of the techno-economic feasibility of algal biofuels is required to not only maximize profits and minimize investment risk but also to stimulate the consideration of the "bigger picture" in identifying the critical problems for scaling up this process and recommending specific corrective measures (Davis et al., 2011; Delrue et al., 2012; Harun et al., 2011; Sun et al., 2011). The following sections describe some significant results from recent techno-economic studies of algal biofuels.

13.2.1 Production of High-Value Carotenoids

Carotenoids such as astaxanthin and beta-carotene are examples of high-value products obtained from microalgae. Astaxanthin is a carotenoid that is naturally synthesized in some plants and bacteria but especially in the microalga Haematococcus pluvialis. It is widely used in aquaculture for salmon and trout farming as well as in dietary supplements (Guerin et al., 2003; Higuera-Ciapara et al., 2006).

Astaxanthin can be produced synthetically at a cost of $1,000/kg, its market size being more than $200 million per year, with the market price above $2,000/kg (Olaizola, 2003). However, because synthetic astaxanthin is derived from petrochemicals, its use is only permitted in aquaculture. It is not allowed for human consumption nor in animal feed other than in aquaculture applications, so for these other uses natural astaxanthin production is required (Li et al., 2011).

Astaxanthin is produced from Haematococcus pluvialis using a two-step strategy. First, green vegetative cells are produced under optimal growth conditions; they are then put un­der stress to trigger the accumulation of astaxanthin (Guerin et al., 2003; Olaizola, 2003). Because Haematococcus pluvialis is easily contaminated with other fast-growing strains such as Scenedesmus or Chlorella, the production is ideally performed in discontinuous mode. To enhance the process yield, repeated batches or semicontinuous cultures can be used to produce green vegetative cells, but this has always to be carried out using closed photobioreactors to avoid contamination problems. The second step is performed for a short time, 5-10 days, under nutrient deprivation conditions and high irradiance, so this step is usually carried out in cheaper open photobioreactors. Although a one-step production tech­nique has been reported at the pilot scale, no commercial production using this methodology exists (Del Riio et al., 2008; Garcia-Malea et al., 2009).

One of most extensive cost analyses carried out on astaxanthin production from Haematococcus pluvialis has recently been published (Li et al., 2011). Cost analysis data were obtained from the operation of a pilot-scale facility consisting of an 8,000 L airlift tubular photobioreactor and a 100 m2 raceway photobioreactor located in Shenzhen, China. Biomass is harvested by sedimentation and centrifugation, and then it is stabilized in a dryer, then additionally disrupted by pulverization (see Figure 14.3). The production capacity of the pilot

FIGURE 14.3 Block diagram of the process for the production of astaxanthin from Haematococcus. (Adapted from Li et al., 2011.)

plant is estimated at 140kg/year of dry Haematococcus pluvialis biomass with 2.5% astaxanthin content: meaning an astaxanthin production capacity of 3.5 kg/year. From these data, the authors extrapolate the production cost of a projected facility, located at a different location under better environmental conditions, producing 900 kg/year of astaxanthin (36 t/year of biomass); that is, a 260 times greater production capacity than demonstrated. Scale-up is performed by multiplying the number of units equal to that used on the pilot scale; thus a total of 30 airlift tubular photobioreactors and 200 raceway photobioreactors were con­sidered. From this analysis the total fixed capital required to build up the facility is close to $1.5 million, the direct production cost including manpower being close to $0.5 million/year. Thus, the expected biomass production cost is $14/kg and $718/kg for biomass and astaxanthin, respectively. These costs are much lower than usually reported for this process (which range from $2,000-3,000/kg); the authors attributing this fact to the low cost of the photobioreactors used and of manpower in China. Thus, if the same production facility were located in the United States, labor would cost approximately $600/kg of astaxanthin, com­pared to only $120/kg of astaxanthin in China.

Data from Li et al. (Li et al., 2011) demonstrated that to produce high-value biomass, the complexity of the process is greater and the use of reactors with adequate control systems is mandatory. In this case, depreciation represents more than 23% of the total production cost, although major costs relate to utilities (33%) and labor (30%) (see Figure 14.4). The utility cost is mainly a result of the facility’s high power consumption, whereby temperature is controlled by cooling the culture volume in tubular photobioreactors; the cost of water is not relevant. Raw material cost is principally due to fertilizer use, representing up to 54% of raw material cost, in addition to pure CO2, which represents up to 39%. The depreciation cost is mainly a function of tubular photobioreactor and raceway pond costs, representing 25% and 16% of total fixed capital, respectively. Machinery cost related to harvesting is low because Haematococcus pluvialis is easily separated from the supernatant by sedimentation.

From these data, it is clearly shown that a reduction in power consumption is a major factor in reducing the production cost of this facility. Consequently, one third of power consump­tion comes from the cooling of the tubular photobioreactors, one third is related to raceway power consumption, and the rest is consumed in gas supply and harvesting (including drying). Any reduction in cooling requirements or improvements in raceway reactor fluid dynamics can help significantly improve the economic viability of the system.

raw materials, the fertilizer cost cannot be reduced, so the only possibility is to reduce their consumption. Finally, with regard to the depreciation cost, it is possible to reduce the cost of tubular photobioreactors by increasing their size instead of installing multiply units. The pre­cise total production cost obtained using these improvements can be evaluated only if their viability is previously demonstrated not to influence the overall process yield, but it can reach up to 30% of total production.

Chlorella spp. are simple, nonmotile, unicellular, aquatic green microalgae. They were one of the first algae to be isolated as a pure culture. The Chlorella microalga measures between 5 and 10 micrometers and, under an optical microscope one, can observe its green color and spherical shape.

Compared to higher plants, Chlorella has a high concentration of chlorophyll and photo­synthetic capacity. The microalga Chlorella is classified as a species according to the shape of the cells, characteristics of chlorophyll, and other variables. There are 20-30 species, some of which are Chlorella vulgaris, Chlorella pyrenoidosa, and Chlorella ellipsoidea. The species are differentiated within the group, known as strains (Illman et al., 2000).

The first pure culture of microalga to be scientifically proven was Chlorella vulgaris in 1890 by the microbiologist M. W. Beijerinck. In 1919, Otto Warburg published articles on the use of this microalga in culture to study its physiology. After years of research with Chlorella and other microalgae, he found that these microorganisms grow under specific conditions and can be used to produce compounds with nutritional benefits to human health.

One of the most important characteristics of Chlorella is its protein content. Depending on the culture conditions, this microalga can provide 60% of protein with essential amino acids for human consumption. Chlorella has approximately three times more protein than the same amount of red meat, which is one of the most concentrated sources of protein. Due to its high protein concentration, Chlorella is used as a food supplement. This microalga has 23% carbo­hydrates, 9% fat, and 5% minerals (Henrikson, 1994).

Chlorella is also rich in B vitamins, especially B12, which is vital in the formation and re­generation of blood cells. Because it also has a high iron content, this microalga is a product indicated for the treatment and prevention of anemia. In order for its nutrients to be fully uti­lized by the body, cells of Chlorella, which are protected by a cell wall, must be disintegrated during the drying process to enable its nutrients to be fully absorbed by the metabolism (Henrikson, 1994).

Production of sustainable biofuels from microalgae is a high-potential option for develop­ing renewable energy. Unfortunately, the production cost of microalgae-based biofuels is still too high, which prevents them from becoming commercially feasible. One of the major obsta­cles that impedes the commercialization of microalgal biofuels is the high cost of photobioreactors and the high demand of auxiliary systems or intensive energy input re­quired during the cultivation of microalgae. Basic conceptual designs for a photobioreactor for the autotrophic cultivation of microalgae are to provide efficient mixing, appropriate light intensity, and rapid gas transport (Singh and Sharma, 2012).

In light of these demands, photobioreactor designs can be generally classified as open sys­tems and closed systems (Table 2.1). Open systems can be divided into natural waters (lakes, lagoons, ponds) and artificial ponds or containers, which are presented in very different ways. Apparently, open systems are potentially subject to contamination resulting from the free gas exchange from the environment to the cultivation system. The cultivation

conditions of open systems are usually poorly controlled, and the estimated growth rate of microalgae will be mostly lower than that in closed systems.

In terms of technical complexity, open systems are more widespread than closed systems. From the aspect of operation, closed systems are more suitable for the cultivation of algae for the production of high-value products. In closed systems, the productivity of desired prod­ucts can be enhanced by controlling the microalgae cultivation under optimal operating con­ditions. The design of closed photobioreactors must be carefully optimized for each individual algal species according to its unique physiological and growth characteristics. Providing appropriate light intensity and efficient hydrodynamic mixing are key issues in the success of a productive autotrophic cultivation system (Kumar et al., 2011).

Given the advantages of closed systems over open systems, several different photo­bioreactor designs with closed systems have also been proposed, ranging from lab scale to industry scale. More detailed descriptions of microalgae cultivation in open and closed systems are presented in the following sections.