A photosynthetic species, isolated from an alkaline hot spring in eastern Taiwan, was applied to enhance the dissolved inorganic carbon (DIC) uptake capability under high alkaline and temperature conditions and was identified. The strain was found to be close to Thermosynechococcus elongatus BP-1 and Synechococcus elongatus based on phylogenetic analysis of 16S rRNA genes. The result was verified with morphological observations. This strain was named as Thermosynechococcus sp. CL-1 (TCL-1). A study of the effects of pH and DIC on the content variations of four elements (C, N, H, and O), lipids (LI), proteins (PR), carbohydrates (CA), and the bioenergy potential 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 then shifted to inorganic compounds 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 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 Fourier transform infrared spectroscopy (TG-IR), the transformation of xylan, cellulose, and lignin contents was observed under various pHs and DIC concentrations.