1. Introduction

Industrial reactors for microalgae cultivation have been generally constructed using channels with movement and adapted for a better gas exchange. One of the biggest problems in this culture system is the low density of microalgae cells; they are constructed between 15 and 30 cm deep along the canal, limiting therefore the available light in addition to increasing the potential for contamination. A system proposed to solve the problem of low density and pollution has been found in closed polyethylene pipe systems, having the geometric design of the reactor as its main objective. Some strategies addressing three aspects have been developed to improve cultivation of microalgae in photobioreactors and produce fine chemicals:

Microalgae - From Physiology to Application

(1) the culture medium design—it is necessary to fix the nutrient composition to provide the right source of carbon and energy depending on the microalgae strain and secondary metabolite to be produced; (2) reducing adverse conditions for culture, such as oxygen accumulation, CO2 efficient supply, and sufficient light distribution. For this purpose, studies on the photobioreactor prototype should be performed; (3) once that photobioreactor prototype works well, critical factor criteria for scale-up bioengineering process should be fixed [1–3].

respectively, under strict control of culture conditions; mixotrophic biomass concentration and growth rates resulted in the sum of the biomass or growth rates of heterotrophic and autotrophic cells in growing culture in parallel. The yields produced approximately the same amount of biomass produced in mixotrophic condi-

Assuming that mixotrophic growth is derived from cells growing in autotrophic and heterotrophic conditions, it can be described mathematically by defining the contribution of both metabolic growth modes. This can be simplified in Eq. (1):

Supposing that α is the heterotrophic fraction in mixotrophic growth, XH (bio-

Combining Eqs. (1) and (2), biomass in mixotrophic (XM) conditions can be

XM <sup>¼</sup> XA

in which XA is the biomass from autotrophy; the growth rate ratio can be

<sup>1</sup><sup>≈</sup> dXM=dt

ΔXM Δti

> ΔXM Δti

> > ΔXM<sup>¼</sup>

<sup>α</sup><sup>i</sup> <sup>¼</sup> <sup>1</sup> � Klo

ðXMiþ<sup>1</sup> XMi

And then, α<sup>i</sup> at any time can be calculated:

<sup>¼</sup> <sup>1</sup> 1 � α<sup>i</sup>

By replacing in Eq. (5) of autotrophic growth, the equation included incident

<sup>¼</sup> <sup>1</sup> 1 � α<sup>i</sup>

> Klo 1 � α<sup>i</sup>

XMiþ<sup>1</sup> � XMi

ðtiþ<sup>1</sup> ti

ΔXA Δti

The α coefficient is low when the incident light reaches all cells in the photobioreactor, where cell density should be low enough to avoid hiding among cells. Once that cell density increases, heterotrophic fraction increases as well, in the presence of an organic substrate as carbon source. To estimate α at any time, the value of α can be represented by an α<sup>i</sup> which can be constant for a period

XM ¼ XA þ XH (1)

<sup>1</sup> � <sup>α</sup> (3)

Kl<sup>0</sup> (6)

Δti (7)

ð Þ tiþ<sup>1</sup> � ti (8)

(5)

dXA=dt <sup>þ</sup> dXH=dt � � g d�<sup>1</sup> � � (4)

XH ¼ αXM with the condition 0<α <1 (2)

tions; a mathematical approach can be summarized in Eqs. (1)–(8) [8, 10].

mass from heterotrophy) may be expressed as follows:

Microalgae Cultivation for Secondary Metabolite Production

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

Then, Eq. (4) can be expressed as follows:

expressed as follows:

expressed as follows:

of time, Δti.

light:

195

where <sup>K</sup> <sup>¼</sup> YkJA

V . Integrating Eq. (6) results in
