Ecofriendly Approach for Bioethanol Production …
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nutrients-rich medium until sufficient biomass is produced, and then transferred to
nitrogen-deficient medium to trigger the lipid or carbohydrate accumulation.
Apparently, the competition between lipid and carbohydrate accumulation in
microalgae cells under nitrogen deficient condition is still unclear as these two
compounds shared similar synthesis pathway (Li et al. 2008). However, some recent
studies have reported that cultivation time played a significant role in manipulating
the accumulation of lipid or carbohydrate in microalgae cells. For example, the carbohydrate content in Scenedesmus obliquus was increased from 20.90% to 49.36% after
two days of nitrogen starvation, while the protein content decreased from 50.43%
to 27.38% (Ho et al. 2013). Nevertheless, for the subsequent days of cultivation, the
carbohydrate content was decreased to 46.98% whereas the lipid content was started
to increase from 11.72% to 13.66%. Similar results are also observed in Chlorella
vulgaris and Chlorella zofingiensis.
2.4 Effect of Phosphorus
Phosphorus is essential macronutrient that regulates microalgae growth and
metabolism. It is mainly incorporated into nucleic acid and phospholipids, an important modifier of protein function, and involved in generating metabolic energy (Yao
et al. 2013). Up to now, the effect of phosphorus on carbohydrate accumulation in
microalgae cells is not well understood and it is strongly believed to be speciesspecific (González-Fernández and Ballesteros 2012). Nevertheless, it is well established that carbohydrate accumulation started to increase when intracellular phosphorus content decreased to a critical level (Cade-Menun and Paytan 2010). In a
recent study by Brányiková et al. (2011), Chlorella vulgaris tended to accumulate higher starch content (55%) under phosphorus starvation condition instead of
nitrogen starvation condition (38%) (Rodolfi et al. 2009). In addition, an added advantage of inducing phosphorus starvation instead of nitrogen starvation to microalgae
cultivation is that higher biomass productivity can be attained, that subsequently
resulted to higher carbohydrate yield. However, in some studies, phosphorus starvation only caused lipid accumulation with no significant effect on carbohydrate
accumulation. For example, cultivation of Tetraselmis subcordiformis under phosphorus starvation did not increase the carbohydrate content in the microalgae but the
concentration remained constant throughout 8 days of cultivation (Ji et al. 2011).
2.5 Effect of Carbon Source
Inorganic carbon
It is a common practice to cultivate phototrophic microalgae by supplying CO 2 as the
main carbon source. Through photosynthesis process, CO 2 is absorbed by microalgae
cells to support their growth by converting the carbon to build proteins, nucleic acids,
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nutrients-rich medium until sufficient biomass is produced, and then transferred to
nitrogen-deficient medium to trigger the lipid or carbohydrate accumulation.
Apparently, the competition between lipid and carbohydrate accumulation in
microalgae cells under nitrogen deficient condition is still unclear as these two
compounds shared similar synthesis pathway (Li et al. 2008). However, some recent
studies have reported that cultivation time played a significant role in manipulating
the accumulation of lipid or carbohydrate in microalgae cells. For example, the carbohydrate content in Scenedesmus obliquus was increased from 20.90% to 49.36% after
two days of nitrogen starvation, while the protein content decreased from 50.43%
to 27.38% (Ho et al. 2013). Nevertheless, for the subsequent days of cultivation, the
carbohydrate content was decreased to 46.98% whereas the lipid content was started
to increase from 11.72% to 13.66%. Similar results are also observed in Chlorella
vulgaris and Chlorella zofingiensis.
2.4 Effect of Phosphorus
Phosphorus is essential macronutrient that regulates microalgae growth and
metabolism. It is mainly incorporated into nucleic acid and phospholipids, an important modifier of protein function, and involved in generating metabolic energy (Yao
et al. 2013). Up to now, the effect of phosphorus on carbohydrate accumulation in
microalgae cells is not well understood and it is strongly believed to be speciesspecific (González-Fernández and Ballesteros 2012). Nevertheless, it is well established that carbohydrate accumulation started to increase when intracellular phosphorus content decreased to a critical level (Cade-Menun and Paytan 2010). In a
recent study by Brányiková et al. (2011), Chlorella vulgaris tended to accumulate higher starch content (55%) under phosphorus starvation condition instead of
nitrogen starvation condition (38%) (Rodolfi et al. 2009). In addition, an added advantage of inducing phosphorus starvation instead of nitrogen starvation to microalgae
cultivation is that higher biomass productivity can be attained, that subsequently
resulted to higher carbohydrate yield. However, in some studies, phosphorus starvation only caused lipid accumulation with no significant effect on carbohydrate
accumulation. For example, cultivation of Tetraselmis subcordiformis under phosphorus starvation did not increase the carbohydrate content in the microalgae but the
concentration remained constant throughout 8 days of cultivation (Ji et al. 2011).
2.5 Effect of Carbon Source
Inorganic carbon
It is a common practice to cultivate phototrophic microalgae by supplying CO 2 as the
main carbon source. Through photosynthesis process, CO 2 is absorbed by microalgae
cells to support their growth by converting the carbon to build proteins, nucleic acids,