| Summary: | 博士 === 國立成功大學 === 環境工程學系碩博士班 === 95 === Carbon dioxide mass transfer is a key factor in cultivating photosynthetic microorganisms besides the light limitation of photosynthesis. Firstly, this study offers a comparison of CO2 assimilation with photosynthesis via combining with and without alkaline absorption. The two photosynthetic microorganisms adopted as the test samples were Nannochloropsis oculta (NAO) and hot spring cyanobacterium (HSC). HSC was isolated from an alkaline hot spring (pH 9.3, 62°C), Chin-Lun hot spring, in eastern Taiwan and grows well over pH 11.5 and 50°C. The growth of NAO and HSC was better when combined with alkaline absorption than without it. The integration of alkaline absorption and photosynthetic bio-fixation provides a higher performance for CO2 assimilation than photosynthetic bio-fixation alone. In addition, HSC on the growth and alkaline adaptation were better than NAO under suitable temperature and pH. After analysis of HSC phylogeny with 16S rDNA and its morphological characteristics, the species is found to be close to Thermosynechococcus elongates BP-1 in the bootstrap tree. This strain is named as Thermosynechococcus sp. CL-1 (TCL-1).
In order to assess the available of TCL-1 on the caboxylation, a Monod equation of TCL-1 with varied DIC (dissolved inorganic carbon) concentrations was proposed. The maximum growth rate (μmax) was 3.85 ± 0.07 d-1; affinity constant (KS) was 1.95 ± 0.28 mM. We also proposed an equation of CO2 assimilation rate that ranged in temperature from 40 to 55�薡 of temperatures. The assimilation constant (θ) was 1.02.
We also performed the alkaline absorption of CO2 with a packed tower and then carboxylation with TCL-1 to do the preliminary test of integrated system of these two mechanisms, TCL-1. CO2 removal efficiencies in a packed tower increase about 5-fold in a suitable growth condition compared to that of without adding any sodium hydroxide. In addition, TCL-1 also exhibits high growth rates under the controlled pHs from 7 to 11. The integrated system is, therefore, more feasible to treat CO2 in the flue gases using the species with higher alkaline affinity such as TCL-1 in small volume bioreactors.
Alkaline conditions enhance the water’s absorption capacity of CO2, but the DIC carboxylation mechanisms under different pHs and DIC concentrations may change the composition of the biomass. Hence a study on the effects of pH and DIC regarding the content variations of four elements (C, N, H, O), lipids (LI), proteins (PR), and carbohydrates (CA) was carried out. The concentrations of PR and LI were the highest under the cultivation of pH 7 and CA was at 10.5. According to the analysis of three compositions, the production pathway of LI might be shifted to CA from pH 7 to 10.5, and shifted to inorganic compound from pH 10.5 to 11. Regarding the effect of DIC at pH 9, the results revealed that the uptake pathway shift (such as metals uptake) might happen while DIC is less than 18.9 mM. From 18.9 to 47.2 mM of DIC, the production pathway of LI shifted to CA and the contents of CA increased quickly from 47.2 to 94.3 mM without a further decrease of LI. Regarding the pyrolysis experiments with a thermogravimetric analyzer coupled with FT-IR (TG-IR), the transformation of xylan, cellulose, and lignin contents was observed under various pHs and DIC concentrations.
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