Effects Of Aqueous CO₂ Concentration On Growth And Carbon Sequestration Of Dominant Algal Blooms

At present,due to the extensive use of fossil fuels,the concentration of CO₂ in the atmosphere has exceeded 400ppm.It is estimated that the concentration of CO₂ in the atmosphere will exceed 700ppm by the end of the 21st century.The increase of CO₂concentration in the atmosphere will lead to a series of changes in the water carbonic acid system.The direct impact is the increase of CO₂ concentration in the water phase,which has the most significant impact on the growth of algae.Compared with the marine water system,fresh water,such as lakes and rivers,has less alkalinity and buffering capacity.Therefore,the increase of CO₂ concentration in the water phase will have a greater impact on the carbonation system of fresh water.CO₂is one of the limiting factors of algae growth,so the change of CO₂ concentration in aqueous phase will have a direct and crucial impact on algae.There are few in-situ studies on the effects of different aqueous CO₂ concentrations on the Growth Physiological Characteristics and carbon sequestration capacity of mixed algae.Therefore,the experimental area of this study is located in Pinghu Lake of Gaoyang in Pengxi River Basin of the Three Gorges Reservoir area.Four water phase CO₂concentration gradients were designed to control algae in situ.A well-designed continuous flow device was used to conduct indoor culture and controlled experiments on water samples and algae species in the same area,and several representative fluorescence parameters were measured.The physiological indexes,community succession and carbon sequestration capacity of Dominant Algae in water bloom under different water phase CO₂ concentrations were comprehensively analyzed.The main conclusions are as follows:（1）Both in situ and indoor experiments,the photosynthesis of mixed algae can effectively buffer the p H change of water environment caused by the increase of aqueous CO₂concentration,and maintain the p H change in the weak alkaline range suitable for its own growth and reproduction.（2）In the indoor experiment,with the increase of CO₂ concentration in water phase,the biomass and chlorophyll-a concentration of mixed algae increased first and then decreased.In situ experiment,the biomass and chlorophyll-a concentration had no obvious decreasing trend.In the two experiments,the concentration of CO₂ in aqueous phase was 20.41μmol/L When the concentration of CO₂ in water phase was27.21μmol/L,the growth of algal cells was promoted.There are two main reasons:one is the inhibition of algae cell growth by high concentration of aqueous CO₂;the other is the inhibition of algae cell growth by high concentration of aqueous CO₂;Secondly,low concentration of aqueous CO₂ stimulated the CCM mechanism of algal cells,which made algal cells grow under carbon stress.（3）In the indoor experiment,the increase of CO₂ concentration in water phase changed the community structure of algae cells,showing that the proportion of mat algae decreased,while the proportion of Microcystis,Anabaena and Chlorella increased,in which Cyanophyta was the dominant algae.In the two experiments,the concentration of CO₂ in aqueous phase was 6.80μmol/L.The results showed that the CCM mechanism of cyanobacteria was more efficient than that of green algae,so cyanobacteria could use bicarbonate directly,and the growth of cyanobacteria in low aqueous CO₂ concentration water environment increased significantly,which led to the rapid succession of algae community in the reactor and became the dominant species.In situ experiments,the proportion of Chlorophyta cells increased significantly under the condition of high aqueous CO₂ concentration,but this phenomenon did not occur in indoor experiments,which may be because the increase of carbonate concentration expanded the competitive advantage of Chlorophyta in water.（4）In the two experiments,the concentration of CO₂ in aqueous phase was20.41μmol/L.The biomass and nitrogen use efficiency of algae were the highest when the concentration of CO₂ in aqueous phase was 27.21μmol/L.The maximum fluctuation of TP（cv=0.24）and the highest utilization rate of TP were observed when the concentration of CO₂ in the water phase was higher than that in the water phase,which may be because the algae consumed more phosphorus when they grew.In the laboratory experiment,the molar ratios of C:N,C:P and N:P increased on the 7th day when the concentration of CO₂ in aqueous phase was 20.41μmol/L.When the concentration of CO₂ in aqueous phase was 27.21μmol/L,the growth of the molar ratio of C:P and N:P in algal cells was the smallest.（5）It was found that the increase of CO₂ concentration in water phase had a significant effect on the photosynthetic activity parameters of microalgae,but there were some differences among different photosynthetic activity parameters.Fv/Fm increases with the increase of CO₂ concentration in aqueous phase;With the increase of CO₂ concentration in aqueous phase,α,r ETRmax and I_k first increased and then decreased,both at the concentration of 20.41μmol/L.（6）The carbon sequestration and CO₂ fixation rate of algae cells per unit water body increased with the increase of CO₂ concentration in water phase,and the CO₂concentration in water phase was 20.41μmol/L.The carbon sequestration and CO₂fixation rate of algal cells per unit water body decreased significantly when the concentration of carbon dioxide in algal cells was higher than that in algal cells.The concentration of CO₂ in aqueous phase is 6.80μmol/L.However,the CO₂ fixation rate of algae cells did not change significantly.