Short CommunicationCharacterization of the thermal-tolerant mutants of Chlorella sp. with high growth rate and application in outdoor photobioreactor cultivation
Introduction
Global warming, which has been a concern in world-wide, is due to the increasing carbon dioxide (CO2) level in atmosphere. The global atmospheric concentration of CO2 has increased from a pre-industrial value of about 280–379 ppm in 2005. Microalgae have very efficient photosynthesis, grow faster than other plants and are able to convert CO2 to biomass efficiently. In recent years, biological CO2 fixation using microalgal photosynthesis has emerged as a potential option because of its effectiveness and economical in CO2 reduction. Among the microalgae which were studied in CO2 fixation, Chlorella sp. has been used in many studies and shows high CO2 fixation rate (Cheng et al., 2006, Chiu et al., 2009b, de Morais and Costa, 2007).
During outdoor cultivation with solar as the light source, biomass productivity is strongly affected by environmental factors such as irradiation and temperature (Ugwu et al., 2007). The temperature of microalgal culture broth in photobioreactors can increase to about 40 °C by irradiation of sunlight in subtropical zones. The microalgal growth would be highly inhibited at such high temperature if the cultivation is not provided with cooling system. Thermal-tolerant species could grow well under high temperature and would significantly reduce the cooling costs (Ono and Cuello, 2007).
Many thermal-tolerant microalgal strains have been isolated from hot springs (Hsueh et al., 2007, de Bashan et al., 2008). However, it is time consuming to purify the cultures from other microorganisms. In the present study, two thermal-tolerant mutant strains of Chlorella sp. were isolated by mutagenic chemical treatment. The growth pattern, CO2 fixation rate and lipid content of the microalgae were determined for characterizations of the isolated Chlorella sp. mutant strains.
Section snippets
Methods
The microalga (wild type) Chlorella sp. was obtained from Taiwan Fisheries Research Institute, Tung-Kang, Taiwan. The microalga was cultured in artificial sea water in each batch culture with the medium which has the composition (per liter) of 750 mg NaNO3, 44.11 mg NaH2PO4·H2O, 43.6 mg Na2·EDTA, 31.6 mg FeCl3·6H2O and micronutrients (trace elemental solution) including 1.8 mg MnCl2·4H2O, 0.1 mg CoCl2·6H2O, 0.1 mg CuSO4·5H2O, 0.23 mg ZnSO4·7H2O, 0.06 mg Na2MoO4, 1 mg vitamin B1, 5 μg vitamin B12 and 5 μg
Results and discussion
Fig. 1 demonstrates that the μ of Chlorella sp. mutant MT-7 and MT-15 in indoor cultivation were 1.4- and 1.8-fold at 25 °C and 3.3- and 6.7-fold at 40 °C cultivation compared with those of the wild type during an 8-day cultivation, respectively. The wild type cultures did grow poor at 35 and 40 °C. The mutant strains showed the maximum μ at 30 °C cultivation and moderate growth at 35 and 40 °C. The optimal temperature for most microalgal species was in a range of 22–28 °C. This was confirmed that 25
Conclusion
The mutant strains Chlorella sp. MT-7 and MT-15 isolated in this study are thermal-tolerant, grow fast with a high density (i.e., high biomass concentration) at temperature of 40 °C, and could capture CO2 with a significantly high efficiency compared to their wild type. In addition, MT-7 and especially MT-15 have the potential to be applied at outdoor cultivation in subtropical region without cooling system, thereby reducing the cost of outdoor cultivation.
Acknowledgements
This work was supported by the grants from the National Science Council (NSC) and “Aim for the Top University Plan” of the National Chiao Tung University and Ministry of Education, Taiwan.
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