Characterization of a heat-tolerant Chlorella sp. GD mutant with enhanced photosynthetic CO2 fixation efficiency and its implication as lactic acid fermentation feedstock

Tse-Min Lee; Yu-Fei Tseng; Chieh-Lun Cheng; Yi-Chuan Chen; Chih-Sheng Lin; Hsiang-Yen Su; Te-Jin Chow; Chun-Yen Chen; Jo-Shu Chang

doi:10.1186/s13068-017-0905-y

Characterization of a heat-tolerant Chlorella sp. GD mutant with enhanced photosynthetic CO₂ fixation efficiency and its implication as lactic acid fermentation feedstock

Biotechnol Biofuels. 2017 Sep 12:10:214. doi: 10.1186/s13068-017-0905-y. eCollection 2017.

Authors

Tse-Min Lee^{1

2}, Yu-Fei Tseng¹, Chieh-Lun Cheng³, Yi-Chuan Chen¹, Chih-Sheng Lin⁴, Hsiang-Yen Su^{2

5

6}, Te-Jin Chow⁶, Chun-Yen Chen⁷, Jo-Shu Chang³

Affiliations

¹ Department of Marine Biotechnology and Resources, National Sun Yat-sen University, Kaohsiung, 80424 Taiwan.
² Doctoral Degree Program in Marine Biotechnology, National Sun Yat-sen University, Kaohsiung, 80424 Taiwan.
³ Department of Chemical Engineering, National Cheng-Kung University, Tainan, 70146 Taiwan.
⁴ Department of Biological Science and Technology, National Chiao Tung University, Hsinchu, 30068 Taiwan.
⁵ Doctoral Degree Program in Marine Biotechnology, Academia Sinica, Taipei, 11529 Taiwan.
⁶ Department of Biotechnology, Fooyin University, Kaohsiung, 83102 Taiwan.
⁷ University Center for Bioscience and Biotechnology, National Cheng-Kung University, Tainan, 70146 Taiwan.

Abstract

Background: Fermentative production of lactic acid from algae-based carbohydrates devoid of lignin has attracted great attention for its potential as a suitable alternative substrate compared to lignocellulosic biomass.

Results: A Chlorella sp. GD mutant with enhanced thermo-tolerance was obtained by mutagenesis using N-methyl-N'-nitro-N-nitrosoguanidine to overcome outdoor high-temperature inhibition and it was used as a feedstock for fermentative lactic acid production. The indoor experiments showed that biomass, reducing sugar content, photosynthetic O₂ evolution rate, photosystem II activity (F_v/F_m and F_v'/F_m'), and chlorophyll content increased as temperature, light intensity, and CO₂ concentration increased. The mutant showed similar DIC affinity and initial slope of photosynthetic light response curve (α) as that of the wild type but had higher dissolved inorganic carbon (DIC) utilization capacity and maximum photosynthesis rate (P_max). Moreover, the PSII activity (F_v'/F_m') in the mutant remained normal without acclimation process after being transferred to photobioreactor. This suggests that efficient utilization of incident high light and enhanced carbon fixation with its subsequent flux to carbohydrates accumulation in the mutant contributes to higher sugar and biomass productivity under enriched CO₂ condition. The mutant was cultured outdoors in a photobioreactor with 6% CO₂ aeration in hot summer season in southern Taiwan. The harvested biomass was subjected to separate hydrolysis and fermentation (SHF) for lactic acid production with carbohydrate concentration equivalent to 20 g/L glucose using the lactic acid-producing bacterium Lactobacillus plantarum 23. The conversion rate and yield of lactic acid were 80% and 0.43 g/g Chlorella biomass, respectively.

Conclusions: These results demonstrated that the thermo-tolerant Chlorella mutant with high photosynthetic efficiency and biomass productivity under hot outdoor condition is an efficient fermentative feedstock for large-scale lactic acid production.

Keywords: CO2 utilization efficiency; Chlorella sp.; Lactic acid; Light conversion efficiency; Mutagenesis; N-methyl-N′-nitro-N-nitrosoguanidine (MNNG); Photosynthesis.