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P. Prabakaran et al.
carbohydrate and lipids (Beer et al. 2009). However, it is interesting to note that the
carbohydrate content in microalgae cells usually does not increase linearly with the
CO 2 concentration. Based on several recent studies, the increment of carbohydrate
content in Chlorella, Scenesdusmus obliquus and Chlamydomonas reinhardtii was
not significantly increased although high CO 2 concentration was supplied (Thyssen
et al. 2001). In a study carried out by Izumo et al. (2007), the carbohydrate content in
Chlorella was reduced when switching the CO 2 supply from 0.04% (atmospheric air)
to 3%. It was observed that when supplying low concentration of CO 2 , the location
of starch was shifted from stroma to pyrenoid. As a result, the expression of granulebound starch synthase (GBSS) was increased, leading to higher accumulation of
starch content under low concentration of CO 2 . However, it is well established that
supplying high concentration of CO 2 to microalgae cultivation could accelerate the
biomass productivity and thus, shorten the overall cultivation time (Kumar et al.
2010). Subsequently, by inducing appropriate stress conditions to the microalgae
cultivation, such as nitrogen depletion, the overall carbohydrate yield could be further
increased as high biomass productivity were already attained previously (Ho et al.
2013).
Organic carbon
Besides CO 2 , specific microalgae species could utilize organic substrate (e.g. glucose,
acetate, glycerol and etc.) as carbon and energy source to grow (Mata et al. 2010).
Usually, this is referred as heterotrophic cultivation (independent of light). The
consumption of organic substrates by microalgae is achieved through several mechanisms, such as phosphorylation (glucose), simple diffusion (glycerol) or using
membrane transporter proteins (organic acids) (Markou et al. 2012). Up to now,
several microalgae species such as Chlorella protothecoide (Cheng et al. 2009),
Chlorella vulgaris (Liang et al. 2009), Crypthecodinium cohnii (Couto et al. 2010),
Neochloris oleoabundans (Morales-Sánchez et al. 2013) and Schizochytrium limacinum (Johnson and Wen 2009) have been studied under heterotrophic growth
conditions to achieve high biomass productivity.
However, most heterotrophic cultivation studies are focused on lipid productivity
and studies related to carbohydrate production is rarely reported in the literature.
This is because the commonly used organic carbon source in growing heterotrophic
microalgae is glucose, which is a derivative of carbohydrate after subjected to hydrolysis process. In other words, the net production of carbohydrate in heterotrophic cultivation could be significantly lower than phototrophic cultivation. In a recent study
by Morales-Sánchez et al. (2013), 54 wt % of carbohydrate content (comprised of
90% glucose) was observed when Neochloris oleoabundans was cultivated under
fed-batch heterotrophic condition using glucose as carbon source. Nevertheless, in
term of glucose balance, the overall glucose produced by Neochloris oleoabundans
was estimated to be 7.67 g/L (based on biomass yield of 14.2 g/L), while the total
glucose consumed by the microalgae to grow was 50 g/L. Thus, it is clear that high
carbohydrate content attained in microalgae under heterotrophic cultivation does
not directly indicates the potential of this approach, but the overall mass balance of
carbohydrate should be taken into consideration.
P. Prabakaran et al.
carbohydrate and lipids (Beer et al. 2009). However, it is interesting to note that the
carbohydrate content in microalgae cells usually does not increase linearly with the
CO 2 concentration. Based on several recent studies, the increment of carbohydrate
content in Chlorella, Scenesdusmus obliquus and Chlamydomonas reinhardtii was
not significantly increased although high CO 2 concentration was supplied (Thyssen
et al. 2001). In a study carried out by Izumo et al. (2007), the carbohydrate content in
Chlorella was reduced when switching the CO 2 supply from 0.04% (atmospheric air)
to 3%. It was observed that when supplying low concentration of CO 2 , the location
of starch was shifted from stroma to pyrenoid. As a result, the expression of granulebound starch synthase (GBSS) was increased, leading to higher accumulation of
starch content under low concentration of CO 2 . However, it is well established that
supplying high concentration of CO 2 to microalgae cultivation could accelerate the
biomass productivity and thus, shorten the overall cultivation time (Kumar et al.
2010). Subsequently, by inducing appropriate stress conditions to the microalgae
cultivation, such as nitrogen depletion, the overall carbohydrate yield could be further
increased as high biomass productivity were already attained previously (Ho et al.
2013).
Organic carbon
Besides CO 2 , specific microalgae species could utilize organic substrate (e.g. glucose,
acetate, glycerol and etc.) as carbon and energy source to grow (Mata et al. 2010).
Usually, this is referred as heterotrophic cultivation (independent of light). The
consumption of organic substrates by microalgae is achieved through several mechanisms, such as phosphorylation (glucose), simple diffusion (glycerol) or using
membrane transporter proteins (organic acids) (Markou et al. 2012). Up to now,
several microalgae species such as Chlorella protothecoide (Cheng et al. 2009),
Chlorella vulgaris (Liang et al. 2009), Crypthecodinium cohnii (Couto et al. 2010),
Neochloris oleoabundans (Morales-Sánchez et al. 2013) and Schizochytrium limacinum (Johnson and Wen 2009) have been studied under heterotrophic growth
conditions to achieve high biomass productivity.
However, most heterotrophic cultivation studies are focused on lipid productivity
and studies related to carbohydrate production is rarely reported in the literature.
This is because the commonly used organic carbon source in growing heterotrophic
microalgae is glucose, which is a derivative of carbohydrate after subjected to hydrolysis process. In other words, the net production of carbohydrate in heterotrophic cultivation could be significantly lower than phototrophic cultivation. In a recent study
by Morales-Sánchez et al. (2013), 54 wt % of carbohydrate content (comprised of
90% glucose) was observed when Neochloris oleoabundans was cultivated under
fed-batch heterotrophic condition using glucose as carbon source. Nevertheless, in
term of glucose balance, the overall glucose produced by Neochloris oleoabundans
was estimated to be 7.67 g/L (based on biomass yield of 14.2 g/L), while the total
glucose consumed by the microalgae to grow was 50 g/L. Thus, it is clear that high
carbohydrate content attained in microalgae under heterotrophic cultivation does
not directly indicates the potential of this approach, but the overall mass balance of
carbohydrate should be taken into consideration.