Carbon bio-fixation by photosynthesis of Thermosynechococcus sp. CL-1 and Nannochloropsis oculta

There is a great potential to assimilate CO₂ and produce bio-energy from cellular component by utilizing carbon fixation of photosynthetic microorganisms. Two different types of photosynthetic microorganisms were used in the present study. The strain Thermosynechococcus sp. CL-1 (TCL-1) was previously isolated from a hot spring while Nannochloropsis sp. Oculta (NAO) from sea water. Two types of inorganic carbon were used (gaseous CO₂ and dissolved inorganic carbon, DIC) with nitrate as N source under different temperature conditions. The Monod model was used to relate its growth rate and DIC concentration. Additionally, lipid and carbohydrate of cell component, which can be used as bio-energy precursors, as function of CO₂ and DIC concentrations is quantified. The growth rate of TCL-1 decreased as CO₂ concentrations increased from 10% to 40% due to low pH inhibition with the maximum value 2.7 d^-1 at 10% CO₂. As for NAO, the maximum growth rate of about 1.6 d^-1 was obtained at 5% and 8% CO₂ (pH between 5.5 and 7 at 30 °C). Regarding the cultivation of TCL-1 under various DIC concentrations, the maximum growth rate of TCL-1 was 3.5 d^-1 at the initial DIC 94.3 mM, pH 9.5 and 50 °C. The carbohydrate content of TCL-1 increased from 2.1% to 33% as DIC concentration increased from 4.7 to 94.3 mM. However, the 33% carbohydrate content at 94.3 mM DIC was much less than 61% at 10% CO₂. That may be due to the fact that the cultivation at 94.3 mM DIC can not supply adequate amounts of DIC to produce carbohydrate under N-limiting conditions. Conversely, enough amounts of DIC supplied from washing flue gas for cultivating TCL-1 would provide a higher performance of carbon bio-fixation and carbohydrate production.