Effect of light intensity and nitrogen starvation on CO2 fixation and lipid/carbohydrate production of an indigenous microalga Scenedesmus obliquus CNW-N
Highlights
► Scenedesmus obliquus CNW-N is a good candidate for CO2 fixation and biofuels production. ► A light intensity of 420 μmol m−2 s−1 favors CO2 fixation and lipid/sugar production. ► Carbohydrate/lipid production was enhanced by nitrogen starvation strategies. ► Accumulation of lipid/carbohydrate can be adjusted by the nitrogen starvation time.
Introduction
The global environment is currently threatened by a sharp rise in CO2 emissions, which have a significant impact on climate change (Lopez et al., 2009). In light of this, more attention is being paid to efforts to reduce such emissions, and various physical, chemical, and biological methods have been applied to capture CO2 (Chen et al., 2011). Among these, the biological method using microalgae is considered one of the most effective and environmental-friendly approaches to fixing CO2 (de Morais and Costa, 2007a). Microalgae are the fastest growing plants on earth, can grow about 10–50 times faster than terrestrial plants, with a very high CO2 fixation rate (Chen et al., 2011). Moreover, the organic biomass produced by photosynthetic microalgae and cyanobacteria can be transformed into a wide range valuable products, such as biofuels, food additives, health-care products, and so on, giving additional benefits from the microalgal CO2 reduction process (Ho et al., 2011).
Recently, liquid biofuels have received wide attention since they are made from non-toxic, biodegradable, and renewable resources, and their use can lead to a decrease in the emission of harmful air pollutants. The most common liquid biofuels to date are biodiesel and bioethanol, which have been mostly produced from food crops (e.g., rapeseed, soybean, and sugarcane). However, producing enough liquid biofuels from these sources to satisfy the existing demand would have a serious impact on food supplies, rainforests, or arable land, and thus there is an urgent need to find new feedstocks (Siaut et al., 2011). Today, many researches are looking to microalgae, which are considered as “third generation biofuels” with several favorable characteristics, such as high biomass and biofuel productivity. In addition to their extremely high cell growth rate and seasonal tolerance, microalgae have relatively higher photon conversion efficiency (photosynthesis efficiency) to fix CO2, and can effectively accumulate large quantities of lipids (for biodiesel) and carbohydrates (for bioethanol), along with other valuable end-products (Subhadra and Edwards, 2010). Therefore, microalgae could be a promising alternative feedstock for the next generation of biofuels, because they have a relatively high content of energy-rich compounds (carbohydrate/lipids), as well as a high growth rate via cultivation on non-arable land and with non-potable water. In addition, there are no seasonal culture limitations, as they can be harvested daily (Siaut et al., 2011).
To enhance the economic feasibility of microalgal-based biofuel production, it is necessary to improve the microalgal biomass productivity, lipid/carbohydrate content, and overall lipid/carbohydrate productivity. The ideal process is that the microalgae are able to produce biofuels (biodiesel/bioethanol) at the highest productivity with the highest lipid/carbohydrate content. Unfortunately, this is not always achievable, because high lipid/carbohydrate contents usually occur under environmental stress (typically nutrient deficiency), which is often associated with relatively low biomass productivity, and thus low overall lipid/carbohydrate productivity (Dragone et al., 2011). As a result, strategies that could achieve the best combination of lipid/carbohydrate content and biomass production rate should be applied, leading to optimization of overall lipid/carbohydrate productivity.
In this study, a microalgal isolate (i.e., Scenedesmus obliquus CNW-N) was used to fix CO2 and also to produce lipid/carbohydrate suitable for making liquid biofuels (Ho et al., 2010). Irradiance intensity and nitrogen starvation were applied to optimize CO2 fixation and lipid/carbohydrate productivity. Combining CO2 mitigation and biofuel production systems by using microalgae may provide an innovative alternative to current carbon-reduction and biofuel-production strategies (Subhadra and Edwards, 2010).
Section snippets
Microalga strain and growth medium
The microalgal species used in this work were obtained from freshwater located in southern Taiwan. The microalgae were identified as S. obliquus by their morphology as well as by 23S rDNA sequence matching (details described in previous study). A modified version of Detmer’s Medium (DM) was used to grow the pure culture of S. obliquus. The medium consisted of (g L−1): Ca(NO3)2·4H2O, 1.00; KH2PO4, 0.26; MgSO4·7H2O, 0.55; KCl, 0.25; FeSO4·7H2O, 0.02; EDTA·2Na, 0.2; H3BO3, 0.0029; ZnCl2 1.1 × 10−4;
Identification of biomass production, CO2 fixation, and biochemical composition of S. obliquus isolates
Microalgae, as third generation biofuels, have received considerable attention due to their high CO2 fixation efficiency and high energy yields (Mata et al., 2010). Selection of fast-growing and high-energy-content microalgal strains is of fundamental importance to the success of commercial applications of microalgae, in particular for low-value products like biofuels (Griffiths and Harrison, 2009). The screening of microalgal species with high biomass productivity and lipid/carbohydrate
Conclusions
This work demonstrates that lipids/carbohydrate production of S. obliquus CNW-N was significantly enhanced by using appropriate light intensity (i.e., 420 μmol m−2 s−1) and nitrogen starvation strategies. The length of nitrogen starvation is an important factor influencing the accumulation of lipids/carbohydrates. The highest CO2 consumption rate and productivity of biomass, lipid, and carbohydrate obtained were 1420.6, 840.6, 140.4, and 383.4 mg L−1 d−1, respectively, which are better than most of
Acknowledgements
The authors gratefully acknowledge financial support from Taiwan’s National Science Council under grant numbers NSC 100-3113-E-006-016, NSC 100-3113-E-006-017, 101-3113-E-006-015, and NSC 99-3113-P-110-001. The support from top university grant (or known as “5-year-50-billion” grant) of National Cheng Kung University is also appreciated.
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