4. Main problems in closed photobioreactors: light supply, temperature, and oxygen accumulation

Microalgae need enough quality and quantity of light supply, and it should be taken into account as a primary critic factor to design proper PBR. Cell density can increase from 10<sup>3</sup> cells ml<sup>1</sup> to densities above 10<sup>8</sup> cells ml<sup>1</sup> ; it produces a reduction of the distance among cells over 250 times, and the cell size can reduce its size 10 times as well. By improving mix capabilities of the PBR, hydrodynamic shearing stress over the cells can be increased; also, it can reduce growth or even cell death at high stress conditions [14]. The temperature has a greater influence on respiration and photorespiration than photosynthesis; when CO2 or light is limiting for photosynthesis, the influence of temperature is negligible. In contrast, an increase in the temperature will increase significantly the respiration, but flow of carbon through the Calvin cycle increases marginally. In other words, the net efficiency of photosynthesis declines at high temperatures. This effect can worsen in culture suspension by the difference in the solubility of CO2 and O2 at high temperatures. Normal temperatures for the growth of microalgae ranged between 25 and 30°C; an increment in the temperature affects the lipid production; at higher temperatures saturated free fatty acids are produced, while low temperatures favor unsaturated free fatty acid formation [15]. High concentration of O2 can build up in closed PBR; if this happens photosynthesis can be damaged by decreasing microalgae growth, and an improvement in the PBR should be implemented as an effective gas exchange [16].

#### 5. Photobioreactor design and scale-up

The first generation of closed PBR finds limitations over 50–100 L of culture volume; this was not effective for light supply to produce higher biomass density. Several designs of light distribution over the PBR, mainly underwater lamps, optical fiber, and column-shaped photobioreactors, have been used to provide an efficient production system; however, not much success has been obtained [12]. This is the main challenge in the future to find the appropriate scaling criteria for a larger irradiate surface, mass transfer, and coupled steps upstream and downstream processes [17]. The difficulty to scale up PBRs is to establish the inherent relationship among physical parameters involved in the design and the physiology of the microalgae to be cultured. An important design rule is to define quantitatively parameters to describe the interactions between incident light, the light distribution in the PBR, cell growth, and secondary metabolite production.

To encourage the use of microalgae, it is necessary to implement a step-by-step system at different levels. The first step is the bioprospecting for selecting the most promising strain to produce a specific secondary metabolite and is the interaction of various disciplines, such as the analytical chemistry, biochemistry, molecular biology, and microbiology. The second step is the development of the culture medium, applicable to the largest volume. The third step is the strategy to analyze the scalingup; biochemical or bioprocess engineers play an important role at this point. Strain and medium selection is carried out at flask level; the type of metabolism for the desired metabolite production, namely, mixotrophic, heterotrophic, or autotrophic growth, is also defined in this step. Operation parameters are fixed at small PBR scale; once the critic factors are overcome, PBR is ready to apply a scale-up procedure, from pilot to industrial production [18]. At the same time, recovery and purification steps should be performed. The last step of scale-up process should be a feasibility economic and technological analysis, in which production costs are obtained [19]. Quinn et al. constructed and validated a scalable growth model with species-specific variables, such as light and temperature; it can be used with PBR dimensions to accurate growth modeling for life cycle analysis.

## 6. Energy efficiency received by microalgae in photobioreactors

Many aspects should be considered to obtain high concentration of biomass and secondary metabolites. Microalgae need energy from light to drive photosynthesis and growth. However, many of these organisms are able to use organic compounds as a source of chemical energy from respiratory mechanism. Although the terms of mixotrophy, autotrophy, photoheterotrophy, and heterotrophy are not welldefined, the influence of organic carbon energy and incident light energy can be quantitatively described in terms of biomass and secondary metabolite production.

Assuming that autotrophy growth occurred in cells absorbing incident light on the irradiate surface of the reactor, growth depends on the specific energy yield (YkJA), in other words, the amount of energy required to produce an amount of biomass; it can be defined by the following equation for continuous cultures:

$$Y\_{k\!\!\!A} = \frac{DX\_A V}{I\_O A} \tag{13}$$

Mixotrophic growth can be described according to Figure 1. On the left-hand

The balance for energetic yield (YX/kJ) can be described as follows: (i) autotrophic growth; total energy comes from incident light, and the biomass is formed

> ΔXA kJhv

Heterotrophic growth, energy, and biomass are provided by an organic carbon

ΔXH kJGlu

Then, in mixotrophic growth, energy is supplied by both incident light and chemical energy from the organic carbon source and biomass from both inorganic

YMkJ <sup>¼</sup> <sup>Δ</sup>XM

Drawing describing the interaction of heterotrophy and autotrophy during mixotrophic growth. A, biomass

from autotrophy; H, biomass from heterotrophy, modified from [5].

kJhv þ kJGlu

ΔATPT ¼ ΔATPhv (15)

ΔATPT ¼ ΔATPGlu (17)

ΔATPT ¼ ΔATPhv þ ΔATPGlu (19)

(16)

(18)

(20)

side, in the photosynthetic growth by the consumption of CO2 from culture medium, in the presence of light, biomass is produced, and oxygen is produced as well. On the other side, biomass is also produced but from organic carbon source consumption. The photosynthesis and respiration rates depend on several factors, such as microalgae species, O2 and CO2 availability, light supply, organic carbon source availability, temperature, pH, etc., but the main factor is the ability of

microalgae to use O2 and CO2 at the same time [5].

Microalgae Cultivation for Secondary Metabolite Production

DOI: http://dx.doi.org/10.5772/intechopen.88531

from inorganic carbon:

and organic carbon sources.

source:

Figure 1.

199

Moreover, microalgae growing in mixotrophic mode and energy from carbon source can be included in Eq. (13), as follows:

$$Y\_{k|M} = \frac{DX\_M V}{I\_o A + \Delta H\_S D V (S\_o - S)}\tag{14}$$

Yield equations can be achieved in continuous cultivation, where D is the dilution rate (h�<sup>1</sup> ). Energy efficiency in batch and continuous cultures for Spirulina was calculated in values of 5.0 � <sup>10</sup>�<sup>3</sup> g biomass kJ�<sup>1</sup> [11] and 2.4–4.8 � <sup>10</sup>�<sup>3</sup> g biomass kJ�<sup>1</sup> [20]. There are two different points of view concerning energy efficiency which should be mentioned: one recently, a photocolor spirometer has been used for direct measurements of photosynthesis (calorimetry) and oxygen evolution at different light intensities [21], and the other one uses a photobioreactor to measure the overall light and carbon energy necessary to produce biomass and secondary metabolites [5]. The first can be useful to provide a potential energetic yield measurement because it considers the metabolic energy flows in the cells; the second provides data necessary for bioengineering purposes, specifically for the photobioreactor design and scaling-up procedures to follow.

Microalgae Cultivation for Secondary Metabolite Production DOI: http://dx.doi.org/10.5772/intechopen.88531

promising strain to produce a specific secondary metabolite and is the interaction of various disciplines, such as the analytical chemistry, biochemistry, molecular biology, and microbiology. The second step is the development of the culture medium, applicable to the largest volume. The third step is the strategy to analyze the scalingup; biochemical or bioprocess engineers play an important role at this point. Strain and medium selection is carried out at flask level; the type of metabolism for the desired metabolite production, namely, mixotrophic, heterotrophic, or autotrophic growth, is also defined in this step. Operation parameters are fixed at small PBR scale; once the critic factors are overcome, PBR is ready to apply a scale-up procedure, from pilot to industrial production [18]. At the same time, recovery and purification steps should be performed. The last step of scale-up process should be a feasibility economic and technological analysis, in which production costs are obtained [19]. Quinn et al. constructed and validated a scalable growth model with species-specific variables, such as light and temperature; it can be used with PBR

dimensions to accurate growth modeling for life cycle analysis.

Microalgae - From Physiology to Application

6. Energy efficiency received by microalgae in photobioreactors

Many aspects should be considered to obtain high concentration of biomass and secondary metabolites. Microalgae need energy from light to drive photosynthesis and growth. However, many of these organisms are able to use organic compounds as a source of chemical energy from respiratory mechanism. Although the terms of mixotrophy, autotrophy, photoheterotrophy, and heterotrophy are not welldefined, the influence of organic carbon energy and incident light energy can be quantitatively described in terms of biomass and secondary metabolite production. Assuming that autotrophy growth occurred in cells absorbing incident light on the irradiate surface of the reactor, growth depends on the specific energy yield (YkJA), in other words, the amount of energy required to produce an amount of biomass; it can be defined by the following equation for continuous cultures:

YkJA <sup>¼</sup> DXAV

Moreover, microalgae growing in mixotrophic mode and energy from carbon

Yield equations can be achieved in continuous cultivation, where D is the dilu-

calculated in values of 5.0 � <sup>10</sup>�<sup>3</sup> g biomass kJ�<sup>1</sup> [11] and 2.4–4.8 � <sup>10</sup>�<sup>3</sup> g biomass kJ�<sup>1</sup> [20]. There are two different points of view concerning energy efficiency which should be mentioned: one recently, a photocolor spirometer has been used for direct measurements of photosynthesis (calorimetry) and oxygen evolution at different light intensities [21], and the other one uses a photobioreactor to measure the overall light and carbon energy necessary to produce biomass and secondary metabolites [5]. The first can be useful to provide a potential energetic yield measurement because it considers the metabolic energy flows in the cells; the second

provides data necessary for bioengineering purposes, specifically for the

photobioreactor design and scaling-up procedures to follow.

). Energy efficiency in batch and continuous cultures for Spirulina was

YkJM <sup>¼</sup> DXMV

source can be included in Eq. (13), as follows:

tion rate (h�<sup>1</sup>

198

IOA (13)

IoA <sup>þ</sup> <sup>Δ</sup>HSDV Sð Þ <sup>o</sup> � <sup>S</sup> (14)

Mixotrophic growth can be described according to Figure 1. On the left-hand side, in the photosynthetic growth by the consumption of CO2 from culture medium, in the presence of light, biomass is produced, and oxygen is produced as well. On the other side, biomass is also produced but from organic carbon source consumption. The photosynthesis and respiration rates depend on several factors, such as microalgae species, O2 and CO2 availability, light supply, organic carbon source availability, temperature, pH, etc., but the main factor is the ability of microalgae to use O2 and CO2 at the same time [5].

The balance for energetic yield (YX/kJ) can be described as follows: (i) autotrophic growth; total energy comes from incident light, and the biomass is formed from inorganic carbon:

$$
\Delta \text{ATP}\_T = \Delta \text{ATP}\_{hv} \tag{15}
$$

$$\frac{\Delta X\_A}{kf\_{hv}}\tag{16}$$

Heterotrophic growth, energy, and biomass are provided by an organic carbon source:

$$
\Delta \text{ATP}\_T = \Delta \text{ATP}\_{\text{Glu}} \tag{17}
$$

$$\frac{\Delta \mathbf{X}\_H}{k \mathbf{J}\_{\rm Glu}} \tag{18}$$

Then, in mixotrophic growth, energy is supplied by both incident light and chemical energy from the organic carbon source and biomass from both inorganic and organic carbon sources.

$$
\Delta \text{ATP}\_T = \Delta \text{ATP}\_{hv} + \Delta \text{ATP}\_{Glu} \tag{19}
$$

$$Y\_{M\_{k\parallel}} = \frac{\Delta X\_M}{kf\_{hv} + kf\_{Glu}} \tag{20}$$

#### Figure 1.

Drawing describing the interaction of heterotrophy and autotrophy during mixotrophic growth. A, biomass from autotrophy; H, biomass from heterotrophy, modified from [5].
