Influence Of High CO₂ Concentration On Growth And Physiological And Photosynthetic Response Of Chlamydomonas Reinhardtii Cultivated In Primary Piggery Wastewater

Microalgae can use CO₂ as carbon source for photosynthesis and transform the organic compounds in wastewater at the same time.This proceeding of wastewater reuse coupled with CO₂ fixation for biomass production of microalgae is expected to be a sustainable economic production mode.In this study,Chlamydomonas reinhardtii CC-125was selected to establish a culture system for treatment of primary piggery wastewater coupled with carbon capture.The photosynthetic response of C.reinhardtii cultured in the primary piggery wastewater for high concentrations of CO₂ was investigated.The research content is as follows:?1?Under autotrophic condition,C.reinhardtii was cultured in TAP medium with aeration of different CO₂ concentration(5%,10%,15%and 20%of CO₂ in air)to investigate the CO₂ tolerance of the microalgae.The ambient air was used as control.It was found that C.reinhardtii can resist 15%of CO₂ and grow rapidly with 5%-15%of CO₂concentration.The aeration of elevated CO₂ content gas is an effective way to adjust medium pH at the same time.?2?C.reinhardtii was cultured with aeration of 5%,10%and 15%of CO₂ mixed with sterile air coupled with the treatment of primary piggery wastewater with an initial COD of1000mg/L.The results showed C.reinhardtii grow well in the primary piggery wastewater with 5%-15%of CO₂ mixed aeration.With elevated CO₂ aeration in the primary piggery wastewater,the growth of C.reinhardtii increased significantly compared with the culture in TAP medium.The biomass with 5%,10%,and 15%of CO₂ mixed aeration was 1.76,1.66 and 1.53 times of control,respectively.The nutrient removal of the primary piggery wastewater was enhanced remarkably with elevated CO₂ aeration,especially of ammonium and total phosphorus,which respectively achieved 96.3%and 94.8%of removal rate.Meanwhile,the utilization of total nitrogen was improved effectively.?3?The photosynthetic response of C.reinhardtii cultivated in primary piggery wastewater compared with TAP medium was investigated by measuring the rate of photosynthesis and respiration with a Clark-type oxygen electrode in log phase of the microalgae.And the content of chlorophyll was determined simultaneously.In group of primary piggery wastewater with elevated CO₂ aretation,the rate of photosynthesis and respiration was significantly higher than the control of ambient air.However,the increase of the rate of hotosynthesis and respiration in TAP medium was not obvious.The chlorophyll fluorescence?Fv/Fm?of C.reinhardtii was measured with FMS2 pulse amplitude modulation fluorometer.The value of Fv/Fm of C.reinhardtii cultured in the primary piggery wastewater stabilized at the range of 0.67 to 0.8 with elevated CO₂aeration.Under this condition,the normal growth of C.reinhardtii was achieved by maintaining the photosynthesis efficiency.?4?In order to research the response of C.reinhardtii to elevated CO₂ in environment,the expression of CCM-related genes?CAH1?HLA3?LCIA?RBCL?in C.reinhardtii at elevated CO₂ concentrations was detected with the realtime fluores-cence quantitative PCR.In primary piggery wastewater,the expression level of CAH1,HLA3,and LCIA at elevated CO₂ aeration was down regulated remarkably compared with ambient air.But the expression level of RBCL,which coding the diphosphate ribulose carboxylase/oxygenase?RubisCO?,remained stable.This illustrated that C.reinhardtii could adapt to elevated CO₂concentrations by changing the CCM,and CO₂ might be directly fixed with the action of RubisCO.And yet the down-regulation of CAH1,HLA3,and LCIA at elevated CO₂concentrations in TAP medium was not as big as that in the primary piggery wastewater.It indicated that composition of culture medium also affects the CCM response of C.reinhardtii to elevated CO₂ concentrations.