The Intrinsic Growth Kinetics And Energy Utilization Of Synechococcus Sp. PCC7942 In Autophototrophic Culture

Microalgal cells and their metabolites were widely applied in food and pharmaceutical industry as well as energy supply and environmental protection. However, so far the large-scale cultivation of microalgae is still difficult and it is crucial to increase the cell density and improve the light energy efficiency for its practical industrial application. Synechococcus sp. PCC7942 is a superior typical cyanobacterium for autophototrophic cultivation and it holds a special place in the development of the photosynthesis mechanisms and light energy utilization of microalgae. In the present work, we investigated the growth characteristics and energy utilization of Synechococcus sp. PCC7942 under different culture conditions by the application of growth kinetics, chlorophyll fluorescence kinetics and metabolic flux analysis. This study will lay a foundation for revealing the intrinsic growth kinetics and improving the efficiency of light energy conversion of microalgae in autophototrophic culture.Synechococcus sp. PCC7942 was cultivated in a 1.5 L airlift photobioreactor and the characteristics of cell growth were investigated. The results showed that the growth curve of Synechococcus sp. PCC7942 cell was linear in the autotrophic culture. The linear growth rate and the maximum specific growth rate both increased while the content of chlorophyll a decreased with the elevation of incident light intensity. Furthermore, the effect of light transmission on cell growth was also studied and it was found that the average light intensity was more appropriate to serve as a key factor for cell growth. Therefore, a growth kinetic model was setup based on the conception of average light intensity and the maximum specific growth rate of Synechococcus sp. PCC7942 under autophototrophic condition was estimated to be 1.53 d-1.Continuous culture was conducted in a 2.5 L flat photobioreactor and the light absorption and utilization of Synechococcus sp. PCC7942 cell were discussed. Results demonstrated that the cell jgrowth yield and the maximum-bioenergy efficiency based on the absorbed light energy were evaluated to be 1.15×10-2 g/kJ and 24.23%, respectively.The effect of concentration of nitrogen and phosphorus on cell growth and photosynthesis were also investigated with application of the chlorophyll fluorescence analysis in the present studies.It was suggested that the appropriate increase of the concentration of NaNO3 and K2HPO4 in BG-11 medium can promote the cell growth and enhance the content of protein and chlorophyll a of Synechococcus sp. PCC7942 and high photosynthesis efficiency was achieved at the concentration of 1.5 g/L NaN03 and 60 mg/L K2HPO4.'Moreover, the photochemical reaction center of photosynthetic system (PSII) would inactivate reversibility under condition of nitrogen or phosphorus limitation.On the basis of the mechanism of photosynthesis and chlorophyll fluorescence analysis, a new light utilization model which express as Eabθ=μ/Ymaxθ+mθ(here Ymaxθand mθwere the actual maximum cell growth yield and the actual maintenance coefficient respectively) was established. According to the model, the intrinsic parameters of cell growth in autotrophic culture were obtained. The actual maximum cell growth yield (Ymaxθ) and the actual maintenance coefficient (mθ) of Synechococcus sp. PCC7942 was 0.0119 g/kJ and 0.705 kJ/(g-h). The new model can well resolve the conflict between the autophototrophic growth kinetics and the traditional growth kinetics.A comprehensive network structure for the autotrophic growth of Synechococcus sp. PCC7942 was proposed, and the carbon and energetic metabolism under different incident light intensity was investigated based on metabolic flux analysis in this paper. Results showed that the photophosphorylation and Calvin cycle were the main producer and trapper of ATP and NADPH.The values of cell growth yield and the maximum cell growth yield based on ATP generation were more stable which were 2.80 g/mol-ATP and 2.95 g/mol-ATP respectively under given conditions. The corresponding actual light conversion efficiencies were 13.65%,10.02% and 7.86%, respectively.Synechococcus sp. PCC7942 can grow in the mixotrophic culture in the presence of glucose and acetate.4.0 g/L glucose or 2.0 g/L acetate can improve the cell growth and alter the contents of carbohydrates, lipids and chlorophyll a of cell. Furthermore, the glucose acetate can also enhance the metabolic flux through glycolysis pathway and TCA cycle. On the basis of metabolic flux analysis, the cell growth yield and energy conversion efficiency under the mixotrophic conditions was higher than that in the photoautotrophic condition.In the present work, the intrinsic growth kinetics and light utilization were discussed based on the cell growth characteristics, photosynthetic mechanism and metabolic flux analysis. The results will establish a solid foundation for microalgae mass cultivation.