304
P. Prabakaran et al.
As a result, the microalgae cells can maintain high respiration activity that exceeds
the rate of photosynthesis, rendering an anaerobic condition in the cultivation system
that allows biohydrogen production for 4 to 5 days (Melis 2007).
During biohydrogen production, it was found that the protein and starch content in
Chlamydomonas reinhardtii were increased 40% and 60%, respectively, from their
initial value for the first 24 h and 30 h of sulfur deprivation, but gradually declined
thereafter (Zhang et al. 2002). This observation clearly shows that substantial protein
and starch catabolism occurs under sulfur limitation condition, and subsequently
utilization of these endogenous substrates by microalgae to sustain the production of
biohydrogen. For the interest of bioethanol production, once the microalgae cells have
accumulated optimum starch concentration under sulfur deprivation, the microalgae
need to be immediately harvested instead of extending the cultivation for biohydrogen
production.
Apart from that, sulfur deprivation method was found to give more promising
result than deprivation of other macronutrients, such as nitrogen and phosphorus
(Brányiková et al. 2011). In a study done by Brányiková et al. (2011), when Chlorella
vulgaris was cultivated with limited nitrogen or phosphorus nutrients, the starch
accumulation in the microalgae cells could reached to 55% and 38%, respectively;
however, after 20 h of cultivation, the microalgae stopped to synthesize starch and its
content began to decrease (Brányiková et al. 2011). On the other hand, under sulfur
limited condition, the Chlorella vulgaris was able to maintain 60% of starch content
even after 32 h that allowed it to attain optimum cell density (stationary phase). In
addition, the study also revealed plausible result with sulfur limitation method in
a scaled-up open door thin-layer solar photobioreactor (working volume was 250
L). Under outdoor condition, Chlorella vulgaris successfully accumulated 50% of
starch content, although the cultivation was only exposed to sunlight for 11 h with
inconsistent of surrounding temperature.
2.3 Effect of Nitrogen
Nitrogen is an essential element of photosynthesis components and usually participating in DNA, proteins, amino acids, enzymes and pigments (Markou et al. 2012).
Hence, sufficient nitrogen concentration is always necessary to accelerate the growth
of microalgae and to enhance biomass productivity. However, recent studies have
shown that when microalgae are cultivated under nitrogen deficient condition, higher
accumulation of lipid or carbohydrate could be attained. This is because protein
synthesis pathway is diverted to form lipid or carbohydrate as the main energy
reserve components in microalgae cells. However, the main drawback of this cultivation method is low biomass yield, predominantly due to the decrease in the synthesis
of photosynthesis pigments (chlorophyll) that subsequently inhibit the growth of
microalgae (Berges et al. 1996). Thus, two-stage cultivation method has been recommended to enhance microalgae biomass productivity with high lipid or carbohydrate content (Widjaja et al. 2009). In the first stage, microalgae are cultivated in
P. Prabakaran et al.
As a result, the microalgae cells can maintain high respiration activity that exceeds
the rate of photosynthesis, rendering an anaerobic condition in the cultivation system
that allows biohydrogen production for 4 to 5 days (Melis 2007).
During biohydrogen production, it was found that the protein and starch content in
Chlamydomonas reinhardtii were increased 40% and 60%, respectively, from their
initial value for the first 24 h and 30 h of sulfur deprivation, but gradually declined
thereafter (Zhang et al. 2002). This observation clearly shows that substantial protein
and starch catabolism occurs under sulfur limitation condition, and subsequently
utilization of these endogenous substrates by microalgae to sustain the production of
biohydrogen. For the interest of bioethanol production, once the microalgae cells have
accumulated optimum starch concentration under sulfur deprivation, the microalgae
need to be immediately harvested instead of extending the cultivation for biohydrogen
production.
Apart from that, sulfur deprivation method was found to give more promising
result than deprivation of other macronutrients, such as nitrogen and phosphorus
(Brányiková et al. 2011). In a study done by Brányiková et al. (2011), when Chlorella
vulgaris was cultivated with limited nitrogen or phosphorus nutrients, the starch
accumulation in the microalgae cells could reached to 55% and 38%, respectively;
however, after 20 h of cultivation, the microalgae stopped to synthesize starch and its
content began to decrease (Brányiková et al. 2011). On the other hand, under sulfur
limited condition, the Chlorella vulgaris was able to maintain 60% of starch content
even after 32 h that allowed it to attain optimum cell density (stationary phase). In
addition, the study also revealed plausible result with sulfur limitation method in
a scaled-up open door thin-layer solar photobioreactor (working volume was 250
L). Under outdoor condition, Chlorella vulgaris successfully accumulated 50% of
starch content, although the cultivation was only exposed to sunlight for 11 h with
inconsistent of surrounding temperature.
2.3 Effect of Nitrogen
Nitrogen is an essential element of photosynthesis components and usually participating in DNA, proteins, amino acids, enzymes and pigments (Markou et al. 2012).
Hence, sufficient nitrogen concentration is always necessary to accelerate the growth
of microalgae and to enhance biomass productivity. However, recent studies have
shown that when microalgae are cultivated under nitrogen deficient condition, higher
accumulation of lipid or carbohydrate could be attained. This is because protein
synthesis pathway is diverted to form lipid or carbohydrate as the main energy
reserve components in microalgae cells. However, the main drawback of this cultivation method is low biomass yield, predominantly due to the decrease in the synthesis
of photosynthesis pigments (chlorophyll) that subsequently inhibit the growth of
microalgae (Berges et al. 1996). Thus, two-stage cultivation method has been recommended to enhance microalgae biomass productivity with high lipid or carbohydrate content (Widjaja et al. 2009). In the first stage, microalgae are cultivated in