Microalgal Metabolism and their Utilisation 47
concentrations of over 150% of air saturation are usual during the day and pH can rise to above pH 9 and
this can inhibit photosynthesis by 40% or more.
Most algae have evolved a diverse range of mechanisms collectively called carbon concentrating
mechanisms (CCMs) that overcome the deficiencies in Rubisco (Beardall and Raven 2016). These
mechanisms may involve the primary use of HCO 3
–
or active CO 2 uptake. Cyanobacteria have a CCM
based on CO 2 or HCO 3
–
transport either at the plasmalemma or the thylakoid membrane delivering HCO 3
–
to the cytosol (Ogawa and Kaplan 2003). The HCO 3
–
then diffuses into the carboxysomes where carbonic
anhydrase generates the CO 2 for Rubisco (Price et al. 2002). In eukaryotic algae with CCMs, the active
transport mechanisms of dissolved inorganic carbon (CO 2 , HCO 3
–
) are located on the plasmalemma or
the inner plastid envelope or both (Giordano et al. 2005). The equilibration of the CO 2 and HCO 3
–
in
the various cell compartments (periplasmic space, thylakoid lumen, chloroplast stroma) also involves
a number of carbonic anhydrases. Diatoms such as Thalassiosira weissflogii appear to show C 4 -type of
photosynthesis where the inorganic carbon is added to a C 3 carrier to form a C 4 intermediate that is then
decarboxylated at the site of Rubisco generating CO 2 at the active site; however there is still some conflict
in the experimental data available (Raven 2010; Valenzuela et al. 2012).
Irrespective of the CCM mechanisms, almost all intensive algae cultures are carbon-limited and the
addition of CO 2 and/or bicarbonate will stimulate growth (Olaizola et al. 1991; White et al. 2013).
Organic carbon utilisation
The ability to utilize organic carbon sources mixotrophically and/or heterotrophically is widespread
amongst the algae, especially the green algae (Neilson and Lewin 1974; Perez-Garcia et al. 2011). Some
algae, such as the thraustochytrids used commercially for the production of long chain polyunsaturated
fatty acids, are obligate heterotrophs (Barclay et al. 1994, 2010). Mixotrophic growth overcomes some of
the C-limitation usually found in algae grown photoautotrophically.
The uptake and metabolism of organic C sources such as glucose or acetate by several microalgae
(e.g., Chlorella, Chlamydomonas, Haematococcus, Phaeodactylum, Nannochloropsis, etc.), has been
extensively studied with different algae utilising different organic C sources (Chu et al. 1995; Tanner
2000; Perez-Garcia et al. 2011).
In heterotrophic culture, cell densities of more than 100 g.L
−1
cell dry weight, were achieved with
Chlorella, Crypthecodinium, and Galdieria species (de Swaaf et al. 2003; Graverholt and Eriksen 2007;
Doucha and Lívanský 2012). Not surprisingly, heterotrophic growth has an effect on the composition
of the algal cells. For example, heterotrophically grown Chlorella saccharophila (=Chloroidium
saccharophilum) had about 3x the lipid content of autotrophically grown cells (Isleten-Hosoglu et al.
2012). The lipid composition can also be manipulated further by changing the C/N ratio. For example,
in C. sorokiniana, low C/N ratios favoured a high proportion of trienoic fatty acids at the expense of
monoenoic acids (Chen and Johns 1991) and in the diatom, Nitzschia laevis heterotrophically grown cells
had a higher eicosapentaenoic acid content (Wen and Chen 2000).
Several studies have shown that the optimum concentration of organic C for mixotrophic growth
varies between strains and that too high concentrations of the organic substrate can be inhibitory (Chu
et al. 1995; Liang et al. 2009), probably because of substrate inhibition and possibly also because of
osmotic effects. In many, but not all algae, mixotrophic growth is the sum of photoautotrophic growth and
heterotrophic growth (Martínez and Orús 1991; Cheirsilp and Torpee 2012). Species and strain variation
in the types of sugars which can be used and their effects on mixotrophic and heterotrophic growth has
been reported in the genera Chlorella and Auxenochlorella (Shihira and Krauss 1965; Kessler 1976).
Glucose has been found to inhibit photosynthetic CO 2 fixation in Chlorella in experiments where pH
was not controlled (Lalucat et al. 1984), and this is probably because glucose inhibits carbonic anhydrase
induction (Shiraiwa and Umino 1991; Villarejo et al. 1997). However, specific growth rates are still
doubled by the addition of glucose, and further enhanced by the addition of 2% CO 2 (Martínez and Orús
1991). Excess CO 2 may however inhibit mixotrophic growth (Sforza et al. 2012). In the light glucose
also affects N uptake, stimulating nitrate uptake in Ankistrodesmus, especially at low CO 2 concentrations
(Eisele and Ullrich 1977), and also in Chlorella (Schlee et al. 1985).
concentrations of over 150% of air saturation are usual during the day and pH can rise to above pH 9 and
this can inhibit photosynthesis by 40% or more.
Most algae have evolved a diverse range of mechanisms collectively called carbon concentrating
mechanisms (CCMs) that overcome the deficiencies in Rubisco (Beardall and Raven 2016). These
mechanisms may involve the primary use of HCO 3
–
or active CO 2 uptake. Cyanobacteria have a CCM
based on CO 2 or HCO 3
–
transport either at the plasmalemma or the thylakoid membrane delivering HCO 3
–
to the cytosol (Ogawa and Kaplan 2003). The HCO 3
–
then diffuses into the carboxysomes where carbonic
anhydrase generates the CO 2 for Rubisco (Price et al. 2002). In eukaryotic algae with CCMs, the active
transport mechanisms of dissolved inorganic carbon (CO 2 , HCO 3
–
) are located on the plasmalemma or
the inner plastid envelope or both (Giordano et al. 2005). The equilibration of the CO 2 and HCO 3
–
in
the various cell compartments (periplasmic space, thylakoid lumen, chloroplast stroma) also involves
a number of carbonic anhydrases. Diatoms such as Thalassiosira weissflogii appear to show C 4 -type of
photosynthesis where the inorganic carbon is added to a C 3 carrier to form a C 4 intermediate that is then
decarboxylated at the site of Rubisco generating CO 2 at the active site; however there is still some conflict
in the experimental data available (Raven 2010; Valenzuela et al. 2012).
Irrespective of the CCM mechanisms, almost all intensive algae cultures are carbon-limited and the
addition of CO 2 and/or bicarbonate will stimulate growth (Olaizola et al. 1991; White et al. 2013).
Organic carbon utilisation
The ability to utilize organic carbon sources mixotrophically and/or heterotrophically is widespread
amongst the algae, especially the green algae (Neilson and Lewin 1974; Perez-Garcia et al. 2011). Some
algae, such as the thraustochytrids used commercially for the production of long chain polyunsaturated
fatty acids, are obligate heterotrophs (Barclay et al. 1994, 2010). Mixotrophic growth overcomes some of
the C-limitation usually found in algae grown photoautotrophically.
The uptake and metabolism of organic C sources such as glucose or acetate by several microalgae
(e.g., Chlorella, Chlamydomonas, Haematococcus, Phaeodactylum, Nannochloropsis, etc.), has been
extensively studied with different algae utilising different organic C sources (Chu et al. 1995; Tanner
2000; Perez-Garcia et al. 2011).
In heterotrophic culture, cell densities of more than 100 g.L
−1
cell dry weight, were achieved with
Chlorella, Crypthecodinium, and Galdieria species (de Swaaf et al. 2003; Graverholt and Eriksen 2007;
Doucha and Lívanský 2012). Not surprisingly, heterotrophic growth has an effect on the composition
of the algal cells. For example, heterotrophically grown Chlorella saccharophila (=Chloroidium
saccharophilum) had about 3x the lipid content of autotrophically grown cells (Isleten-Hosoglu et al.
2012). The lipid composition can also be manipulated further by changing the C/N ratio. For example,
in C. sorokiniana, low C/N ratios favoured a high proportion of trienoic fatty acids at the expense of
monoenoic acids (Chen and Johns 1991) and in the diatom, Nitzschia laevis heterotrophically grown cells
had a higher eicosapentaenoic acid content (Wen and Chen 2000).
Several studies have shown that the optimum concentration of organic C for mixotrophic growth
varies between strains and that too high concentrations of the organic substrate can be inhibitory (Chu
et al. 1995; Liang et al. 2009), probably because of substrate inhibition and possibly also because of
osmotic effects. In many, but not all algae, mixotrophic growth is the sum of photoautotrophic growth and
heterotrophic growth (Martínez and Orús 1991; Cheirsilp and Torpee 2012). Species and strain variation
in the types of sugars which can be used and their effects on mixotrophic and heterotrophic growth has
been reported in the genera Chlorella and Auxenochlorella (Shihira and Krauss 1965; Kessler 1976).
Glucose has been found to inhibit photosynthetic CO 2 fixation in Chlorella in experiments where pH
was not controlled (Lalucat et al. 1984), and this is probably because glucose inhibits carbonic anhydrase
induction (Shiraiwa and Umino 1991; Villarejo et al. 1997). However, specific growth rates are still
doubled by the addition of glucose, and further enhanced by the addition of 2% CO 2 (Martínez and Orús
1991). Excess CO 2 may however inhibit mixotrophic growth (Sforza et al. 2012). In the light glucose
also affects N uptake, stimulating nitrate uptake in Ankistrodesmus, especially at low CO 2 concentrations
(Eisele and Ullrich 1977), and also in Chlorella (Schlee et al. 1985).