2.2.1 Carbon Sources
Carbon sources are usually the most critical factors for the growth of microalgae. In gen- eral, microalgae can be grown under photoautotrophic, heterotrophic, and mixotrophic con- ditions using diversified carbon sources, such as carbon dioxide, methanol, acetate, glucose, or other organic compounds (Xu et al., 2006). Photoautotrophic cultivation means that microalgae use inorganic carbon (e.g., carbon dioxide or bicarbonates) as the carbon source to form chemical energy through photosynthesis (Ren et al., 2010). Some microalgae species can directly use organic carbon as the carbon source in the presence or absence of a light sup- ply. This is calledheterotrophic cultivation (Chojnacka and Noworyta, 2004). However, the most commonly used carbon source for microalgae growth and biofuels production is still carbon dioxide or bicarbonates, since using organic carbon sources would be too expensive for producing low-price products such as biofuels.
In addition, from the aspect of CO2emissions reduction, a net-zero CO2emission could be achieved when the biofuels are directly converted from using CO2as the substrate. In partic- ular, photoautotrophic growth of microalgae represents an ideal model of reutilization of CO2 coming from flue gas of power plants and industrial activities (Packer, 2009), as microalgae biomass can be further utilized to produce biofuels or other value-added products (Hsueh et al., 2007; Raoof et al., 2006). In addition, most microalgae have much higher cell growth and CO2fixation rates than terrestrial plants (around 10–50 times higher), which demon- strates another advantage of direct conversion of photoautotrophic growth of microalgae.
Therefore, it seems more reasonable from the perspectives of economic feasibility and environmental protection that microalgae-based biofuels should be produced via photoautotro- phic growth of microalgae. However, another thought is to produce biofuels from microalgae grown under heterotrophic conditions using organic carbon sources (e.g., sugars) derived from biomass. In this way, biofuel productivity could be markedly enhanced, since heterotrophic growth of microalgae is usually faster than autotrophic growth (Chen, 1996). Nevertheless, again, the high cost of obtaining the organic carbon sources from raw biomass is still a great concern.
2.2.2 Nitrogen Source
Lipid accumulation in microalgae usually occurred when microalgae are cultivated under stress conditions (e.g., nitrogen starvation, nutrient deficiency, pH variations, etc.). Among those stress conditions, nitrogen limitation is the most effective and commonly used strategy for stimulating lipid accumulation in microalgae. Recent reports demonstrated that cultivation under nitrogen starvation conditions leads to a marked increase in the oil/lipid content (Mandal and Mallick, 2009).Hu et al. (2008)collected the data of lipid contents of various microalgae and cyanobacteria species under normal growth and stress conditions in a literature 24 2. DESIGN OF PHOTOBIOREACTORS FOR ALGAL CULTIVATION
survey, indicating that under stress conditions, the lipid contents of green microalgae, diatoms, and some other microalgae species are 10–20% higher than under normal conditions. However, the lipid contents of cyanobacteria were usually very low (10%) (Hu et al., 2008).
It is thought that when microalgae are cultivated under nitrogen-starvation conditions, the proteins in microalgae will be decomposed and converted to energy-rich products, such as lipids.Siaut et al. (2011)also concluded that during microalgae growth, starch would first be synthesized to reserve energy, then lipid would be produced as a long-term storage mech- anism in case of prolonged environmental stress (such as nitrogen deficiency). Although a nitrogen-starvation strategy is very effective in increasing lipid content of microalgae, the ni- trogen deficiency conditions often lead to a significant decrease in the microalgae growth rate, thereby causing negative effects on lipid productivity. Therefore, engineering approaches should be conducted to optimize the cultivation time for the microalgae growth period (nitrogen-sufficient condition) and lipid accumulation period (nitrogen-deficient condition) to ensure high overall oil/lipid productivity.
2.2.3 Light Supply
The type of light source is known to be a critical factor affecting the growth of microalgae due mainly to the difference in the coverage of wavelength range (Terry, 1986). In addition to the type of light source, the light intensity is also very important for microalgae growth (Grobbelaar et al., 1996; Sa´nchez et al., 2008; Ugwu et al., 2008; Yoon et al., 2008). In general, the effect of light intensity on the photoautotrophic growth of microalgae could be classified into several phases, such as the light-limitation phase, the light-saturation phase, and the light- inhibition phase (Ogbonna and Tanaka, 2000). To maximize biomass productivity, the satura- tion light intensity needs to be distributed throughout the entire microalgae cultivation system.
However, this is impossible in practical cultivation systems, since the light distribution inside the photobioreactor normally decreases significantly along with the distance due to the light shading effects (seeFigure 2.1), especially when the cell concentration gets very high or when
Light intensity
Specific growth rate No cell growth Light limitation Light saturation Light inhibition
FIGURE 2.1 Effect of light intensity on specific growth rate of microalgae under phototrophic cultivation (Ogbonna and Tanaka, 2000).
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2.2 FACTORS AFFECTING MICROALGAE GROWTH AND BIOFUELS PRODUCTION
significant biofilm formation on the surface of the reactor vessel occurs (Chen et al., 2008). Im- proving the mixing of the cells can reduce the effects of light shading or photoinhibition at dif- ferent zones of the photobioreactor. Some literature describes the effect of light intensity on the lipid content of microalgae.Lv et al. (2010)demonstrated that in comparing low and high light intensity (i.e., light-limitation and light-saturation conditions), using a light intensity of 60mmol/m2/s led to an increase in biomass concentration and lipid content of Chlorella vulgaris, along with changes in pH, NADPH, and Mg2þconcentration (Lv et al., 2010).
2.2.4 Temperature
In commercial microalgae cultivation systems, temperature is also an important environ- mental factor for microalgae growth and target-product production. For outdoor microalgae cultivation, variations in temperature greatly depend on the light exposure (i.e., day/night cycle) and seasonal changes. In Taiwan, for example, the temperature variation range is be- tween 25C and 45C. Appropriate cultivation temperature could promote microalgae growth, whereas at a high temperature, microalgae biomass production would decrease, primarily due to denature of essential proteins/enzymes as well as inhibitory effects on cellular physiology.
The effect of temperature on the growth rate of microalgae has been reported for a variety of microalgae species. For instance, the growth rate ofChaetoceros pseudocurvisetusreached a max- imum level when it was grown at 25C (Yoshihiro and Takahashi, 1995).Renaud et al. (2002) also reported that when the operation temperature was controlled at 25–30C, theChaetoceros sp. had a higher growth rate. Thus, the operation temperature has a significant effect on biomass production. In addition,Hu et al. (2008) also indicated that the environmental temperature can affect the degree of saturation of the microalgae lipid, since an increase in saturated fatty acids has been observed when the culture temperature was increased. In addition, for some microalgae (e.g.,Nannochloropsis SalinaandOchromonas danica), increasing the cultivation temperature may also lead to an increase in the lipid content (Aaronson, 1973; Boussiba et al., 1987).
2.2.5 pH
The pH is also an important environmental parameter for microalgae growth and target- product formation. The optimal pH for most cultured microalgae species is between 7 and 9 (Ho et al., 2011). The pH of the culture medium normally affects the biochemical reaction characteristics of microalgae. It is crucial to maintain culture pH in the optimal range because complete culture collapse may occur due to the disruption of cellular processes by extreme pH. Meanwhile, the feeding of CO2obviously affects the culture pH as well as microalgae growth. When the CO2from the gas phase (molecular CO2) is transferred into- the culture medium, some of the CO2gas will dissolve and become soluble phase (HCO3), and the conversion of CO2to HCO3 is greatly dependent on the pH value in the culture.
The HCO3 is then utilized by microalgae via Ci-concentrating mechanisms (CCMs) (Miller et al., 1990).
Liu et al. (2007)reported that growth ofC. marinaremained unchanged in the normal range of pH (pH 7.5 to 8.5), whereas a significant reduction in microalgae growth was observed when pH was increased beyond 9.0 (Liu et al., 2007). Belkin and Boussiba found that a 26 2. DESIGN OF PHOTOBIOREACTORS FOR ALGAL CULTIVATION
cyanobacterium Spirulina platensis exhibited optimal growth at pH 9.0 to 10.0 (Belkin and Boussiba, 1991). Apparently the suitable pH range for the growth of microalgae and cyanobacteria is greatly species-dependent.
2.2.6 Salinity
The ability of microalgae to survive in marine environments has received considerable attention. It was found that microalgae can produce some metabolites to protect salt injury and to balance the influence of osmotic stresses of the surroundings. The microalgae, bacteria, and cyanobacteria can tolerate up to 1.7 M of salt concentration in marine medium.
The salinity condition may stimulate the production of specific components in microalgae.
For instance, Fazeli and his colleagues reported that the highest carotenoid contents (11.72 mg/L) ofDunaliella tertiolectaDCCBC26 occurred when the culture medium contained 0.5 M NaCl (Fazeli et al., 2006). However, salinity conditions may cause negative effects on the microalgal growth. It was reported that a salinity of 35% (standard seawater) or higher led to a reduction in the growth rate and the efficiency of photosynthesis and dark respiration (Jacob et al., 1991).