Enhanced biofuel production potential with nutritional stress amelioration through optimization of carbon source and light intensity in Scenedesmus sp. CCNM 1077
Graphical abstract
Introduction
In recent years, with the increase in human population, our planet is facing many environmental problems like depletion in fossil fuel reserve, high GHG emission, water shortage, etc. To overcome increasing energy demand as well as reduce various environmental problems, researchers are currently focusing on development of renewable and sustainable biofuel alternatives. Microalgae are one of the most promising options due to their simple growth requirement, higher growth rate and photosynthetic ability compared to other terrestrial energy crop plants. Microalgae are known to accumulate high amount of lipid and carbohydrate under various nutritional starvation conditions (Chisti and Yan, 2011, Maurya et al., 2014).
Generally, microalgae are grown photoautotrophically which yield comparatively lower biomass, lipid and carbohydrate content. To make microalgae based biofuel economically viable, its biomass, lipid and carbohydrate productivity as well as downstream processing should be optimized. Mixotrophic or heterotrophic growth of microalgae utilizing various types of organic substrates is a viable option to increase their biomass and biofuel production capabilities (Yang et al., 2014). Many microalgae like Chlorella, Scenedesmus and Nannochloropsis have the ability to grow mixotrophically and heterotrophically utilizing various organic carbon sources.
Under mixotrophic conditions, microalgae get advantage of photosynthesis as well as organic carbon supplementation to produce higher biomass, lipid and carbohydrates, which makes them better resource for biofuel production compared to heterotrophically and photoautotrophically grown microalgal cultures. However, mixotrophic cultivation is comparatively costly due to high cost of organic carbon sources than photoautotrophic cultivation. Therefore, utilization of cheap carbon source can make overall process economically viable (Moon et al., 2013).
Different industrial processes generate large amount of byproducts, which can be used as an organic carbon source for cultivation of microalgae. For example, large amount of crude glycerol is easily available as a byproduct of biodiesel; glucose and fructose can be obtained through chemo-enzymatic hydrolysis of various agriculture and lignocellulosic wastes; lactose and galactose can be found in dairy wastewater (Morales-Sánchez et al., 2013). Finding out a microalgae capable of utilizing various types of organic substrates will make a viable alternative to remediate wastes generated in various industrial processes along with a potential source to generate bioenergy from the microalgal biomass (Wu et al., 2014). Different nutritional starvation conditions in microalgae yield higher lipid and carbohydrate contents but very limited reports are available on effects of organic carbon supplementation under various stress conditions. To fill this gap, in the present study, capability of the microalgae Scenedesmus sp. CCNM 1077 to utilize various types of organic sugars was evaluated followed by optimizing different concentration of best carbon source for higher biomass production. Further, effects of light intensity were also evaluated under optimized glucose concentration. At last, the effects of supplementation of organic carbon source under various stress conditions viz. nitrate, phosphate and salinity stress were evaluated to determine the possible role of addition of organic carbon supplementation on stress amelioration, which helps in enhancement of biofuel production capability of microalgae Scenedesmus sp. CCNM 1077.
Section snippets
Microalgae and experimental conditions
The microalgae used in this study, Scenedesmus sp. CCNM 1077, was cultured in BG-11 medium as described previously (Pancha et al., 2014). The entire study was carried out in four sections which are described below.
In the first experiment, various organic carbon sources viz. glucose, fructose, galactose, maltose, sucrose, lactose and glycerol were supplemented in BG-11 medium at concentration of 10 gm/L to determine their effects on biomass and biochemical composition of microalgae Scenedesmus
Effect of various organic carbon source on biomass and biochemical composition of microalgae Scenedesmus sp. CCNM 1077
To identify ideal carbon source for higher biomass, lipid and carbohydrate production, Scenedesmus sp. CCNM 1077 was grown in BG-11 media with seven different carbon sources viz., three monosaccharides (glucose, fructose, galactose), three disaccharides (maltose, lactose and sucrose) and one complex sugar glycerol at initial concentration of 10 g/L under mixotrophic growth condition. Results showed that microalgae Scenedesmus sp. CCNM 1077 have capability to grow mixotrophically and ability to
Conclusion
In the present study, addition of different sugars significantly enhanced the biomass and biofuel production capability of Scenedesmus sp. CCNM 1077. Addition of 4 g/L glucose in BG-11 medium and 150 μmol m−2 s−1 light intensity is optimized condition for higher biomass, lipid and carbohydrates accumulation under mixotrophic cultivation. Addition of glucose under various nutritional stress conditions also helps in improving biomass, lipid and carbohydrate content of Scenedesmus sp. CCNM 1077.
Acknowledgements
CSIR-CSMCRI Registration Number: 194/2014. IP and KC would like to acknowledge CSIR for awarding Senior Research Fellowship as well as financial support from CSC 0203 and AcSIR for their Ph.D. enrolment. The continuous support from Dr. Arvind Kumar, DC, SMC and the entire staff of the division is gratefully acknowledged. The authors would like to thank Dr. Parimal Paul, DC, ADCIF, CSIR-CSMCRI, Bhavnagar for their help during the analysis. Authors would also like to acknowledge Mr. Rahul Kumar
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