The Cultivate And Screening Technique Of Microalgae With Fixed Co₂ In High Density

With the rapid development of global economy, the demand of fossil fuel is growing. Consequently, the concentration of CO₂ in the atmosphere is gradually increasing, which induced a series of environmental problems that seriously affected the development of human beings and the progress of society. Thus, it is very urgency to control the CO₂ concentration in the atmosphere. The technique of biological fixing CO₂ by photosynthesis of microalgae is economical and feasible, but the screening work of algal species with high efficiency fixing CO₂ is also in the initial stage. In order to solve the bottle-neck promble, we have selected two kinds of fresh microalgae which were isolated from environmental samples to fix CO₂ after domesticated and isolated in laboratory in this paper. The main research results as follows:1. Five nature water samples and one soil sample were fetched from environment (pond, river, polluted water and paddy field). After precultivated,domesticating cultivated and isolation cultivated in laboratory, 15 kinds of fresh microalgae which could grow under highly CO₂ concentration (15%, v/v) were obtained at last, and two best growth microalgae were chosen as the objects after comparing the growth status of these microalgae. According to principium analysis, one was identified as genus Chlorella sp. named KX-1 strain; the other was identified as genus Selenastrum sp. named KX-2 strain.2. Microalgae biomass was expressed by absorbency, and the standard curves of absorbency of target microalgae VS cells concentration/dry weight were built up, too. The cells carbon content was determined by elemental analyzer, and the cells average carbon content of KX-1 strain was 50.064%, while KX-2 strain was 47.316%.3. The growth condition of KX-1 strain and KX-2 strain were changed to confirm the best culture term. And, it was necessaries to minutely calculate the CO₂ fixation rate under each condition. The results as follows:(1) When the CO₂ concentration was up to 15%, the growth rate and the CO₂ fixation rate were maximal of KX-1 strain and KX-2 strain, but decreased as CO₂ concentration increased. Under the 100% CO₂ circumstance, the algae cells concentration of KX-1 strain was a fixed value within five days, but the algae cells concentration of KX-2 strain was negative at the same time.(2) The vaccination density (absorbency) was changed between 0.05 and 0.57. The maximal growth rate and CO₂ fixation rate of KX-1 strain appeared when the vaccination density was 0.115 and KX-2 strain was 0.175.(3) Culture temperature was controlled between 20℃and 40℃, increasing by 5℃grades. CO₂ fixation rate could maintain a higher state with the optimum temperatures in the range of 25℃-30℃of KX-1 strain and 20℃-25℃of KX-2 strain. The maximal CO₂ fixation rate of KX-1 strain and KX-2 strain were both observed when the temperature was 25℃.(4) The initial pH of culture medium was adjusted between 3.0 and 8.0. The studies showed that CO₂ fixation rate could maintain a higher state with pH 5.0-6.0 of KX-1 strain and 6.0-7.0 of KX-2 strain. The maximal CO₂ fixation rates of KX-1 strain and KX-2 strain were both observed when the initial pH was about 6.0.(5) Gas flow rate was controlled between 0.2 L/min and 1.5 L/min. The CO₂ fixation rate of KX-1 strain and KX-2 strain was increasing while the gas flow rate was increasing, and reached maximal value when gas flow rate was 1.0 L/min. But CO₂ fixation rate became to decrease when the gas flow rate was up to 1.5 L/min.(6) The intensity of illumination was changed between 1000Lux and 50000 Lux. The study results showed that CO₂ fixation rate could maintain a higher state with the intensity of illumination in the range of 15000 -25000 Lux of KX-1 strain and KX-2 strain. The maximal CO₂ fixation rate of KX-1 strain and KX-2 strain were observed when the intensity of illumination was 25000 Lux.(7) Two ways of supply gas were choosed in this experiment. The continuously supply gas was changed into semi-continuously supply which included 3h:1h,2h:1h and 1h:1h. The CO₂ fixation rate with continuously supply gas obviously preceded semi-continuously supply.(8) Under the optimum conditions of culture, the maximal CO₂ fixation rates of KX-1 strain was 1.75 gCO₂/(L·d), while the maximal CO₂ fixation rates of KX-2 strain was 1.40 gCO₂/(L·d). KX-1 strain and KX-2 strain were used to fix CO₂ among the simulated flue gas (proportion: CO₂: 15%; O2: 4%; N2: 81%), and the CO₂ fixation rates of them were 2.33 gCO₂/(L·d) and 1.82 gCO₂/(L·d). The study results showed that KX-1 strain and KX-2 strain which were isolated from environmental samples were fit for fixing CO₂ from industrial exhaust gas, and the CO₂ fixation effect of KX-1 strain was better than that of KX-2 strain.