(micronutrients) are important, such as boron (B), copper (Cu), manganese (Mn),
zinc (Zn), molybdenum (Mo), cobalt (Co), vanadium (V) and selenium (Se).
2.2.2 PH
During the photosynthetic CO 2 fixation, the hydroxide ion (OH
− ) accumulates in
the growing medium, leading to a gradual increase of pH (Richmond 2004). This
shifts the chemical equilibrium of the inorganic carbon present in the medium
towards the formation of carbonates (CO 3
2− ). However, they are not the preferred
carbon source for microalgae (Lower 1999). On the other hand, a decrease on the
solution pH shifts the chemical equilibrium towards the formation of CO 2 , which is
one of the preferred carbon sources for microalgae. Nevertheless, this process can
lead to the release of CO 2 into the atmosphere, decreasing the concentration of this
nutrient extremely important for the cultivation of microalgae.
With regard to nitrogen, when it is provided in the form of ammonium, an
increase on the solution pH can result in a decrease on the concentration of nitrogen
available for microalgae (Guštin and Marinšek-Logar 2011; Cai et al. 2013). High
pH values move the chemical equilibrium of ammonium for the production of
ammonia that can be released into the atmosphere due to the aeration of the culture,
reducing the availability of nitrogen for microalgae.
The concentration of phosphorus in culture medium can also be influenced by
elevated pH, as it can lead to precipitation of phosphate (in the forms of calcium
phosphate, iron phosphate and aluminium phosphate) and therefore limit the
amount of phosphorus available for microalgae (Wang and Nancollas 2008; Cai
et al. 2013).
The pH can directly affect the microalgae, as the pH of microalgal cytoplasm is
neutral or slightly alkaline, and enzymes are pH-sensitive and may be inactive in
acidic conditions (Chiranjeevi and Mohan 2016). Therefore, extreme pH conditions
can cause the disruption of many cellular processes, which may lead to the collapse
of culture (Jia et al. 2016).
Tripathi et al. (2015) studied the effect of pH on the growth of Scenedesmus
sp. microalgae in a range of 7–10 and concluded that the optimal pH for this species
was 8. Munir et al. (2015) evaluated the pH effect on the growth of two species of
microalgae (Spirogyra sp. and Oedogonium sp.) in a range of 6.5–9.0, achieving the
highest growth at pH 7.5 for both species.
2.2.3 Light Intensity and Temperature
The light energy received by microalgae is a function of the photon flux density that
reaches the surface of the culture (Richmond 2004). The cells absorb only a fraction
of the photon flux, which is influenced by several factors, such as (i) cell density;
3 Process Integration Applied to Microalgal Biofuels Production
39
zinc (Zn), molybdenum (Mo), cobalt (Co), vanadium (V) and selenium (Se).
2.2.2 PH
During the photosynthetic CO 2 fixation, the hydroxide ion (OH
− ) accumulates in
the growing medium, leading to a gradual increase of pH (Richmond 2004). This
shifts the chemical equilibrium of the inorganic carbon present in the medium
towards the formation of carbonates (CO 3
2− ). However, they are not the preferred
carbon source for microalgae (Lower 1999). On the other hand, a decrease on the
solution pH shifts the chemical equilibrium towards the formation of CO 2 , which is
one of the preferred carbon sources for microalgae. Nevertheless, this process can
lead to the release of CO 2 into the atmosphere, decreasing the concentration of this
nutrient extremely important for the cultivation of microalgae.
With regard to nitrogen, when it is provided in the form of ammonium, an
increase on the solution pH can result in a decrease on the concentration of nitrogen
available for microalgae (Guštin and Marinšek-Logar 2011; Cai et al. 2013). High
pH values move the chemical equilibrium of ammonium for the production of
ammonia that can be released into the atmosphere due to the aeration of the culture,
reducing the availability of nitrogen for microalgae.
The concentration of phosphorus in culture medium can also be influenced by
elevated pH, as it can lead to precipitation of phosphate (in the forms of calcium
phosphate, iron phosphate and aluminium phosphate) and therefore limit the
amount of phosphorus available for microalgae (Wang and Nancollas 2008; Cai
et al. 2013).
The pH can directly affect the microalgae, as the pH of microalgal cytoplasm is
neutral or slightly alkaline, and enzymes are pH-sensitive and may be inactive in
acidic conditions (Chiranjeevi and Mohan 2016). Therefore, extreme pH conditions
can cause the disruption of many cellular processes, which may lead to the collapse
of culture (Jia et al. 2016).
Tripathi et al. (2015) studied the effect of pH on the growth of Scenedesmus
sp. microalgae in a range of 7–10 and concluded that the optimal pH for this species
was 8. Munir et al. (2015) evaluated the pH effect on the growth of two species of
microalgae (Spirogyra sp. and Oedogonium sp.) in a range of 6.5–9.0, achieving the
highest growth at pH 7.5 for both species.
2.2.3 Light Intensity and Temperature
The light energy received by microalgae is a function of the photon flux density that
reaches the surface of the culture (Richmond 2004). The cells absorb only a fraction
of the photon flux, which is influenced by several factors, such as (i) cell density;
3 Process Integration Applied to Microalgal Biofuels Production
39