Size-dependent Response Of Phytoplankton To Increase Of Seawater CO₂ Concentration

Phytoplankton are the basis of marine food chains and ecosystems and are the main source of organic matter in marine ecosystem and therefore the key to the regulation of marine biogeochemical process.Inorganic carbon is synthesized into organic carbon by photosynthesis.The ways of phytoplankton absorbing inorganic carbon mainly include passive diffusion of carbon dioxide(CO2),which basically consumes no cell energy,and active transport of bicarbonate(HCO3-),which significantly consumes cell energy.With the continuous CO2 emission from human activities since the industrial revolution,the CO2 concentration of seawater increases,which causes the rate of CO2 passive diffusion in phytoplankton cell increasing.Theoretically,phytoplankton can reduce the active transport of HCO3-,which can save cell energy and promote phytoplankton growth.Previous studies have suggested that phytoplankton with large particle size have a stronger response to the rise of CO2 concentration in seawater and a larger growth rate due to their small specific surface area and greater limitation of CO2 diffusion in photosynthesis.However,by collecting and analyzing the historical experimental data of diatoms' response to CO2 rise,it was found that the growth rate of diatoms with medium particle size(diameter 10-20 ?m)increased the most.In this paper,a theoretical model was further established to study the change of the relative Growth Rate Response(GRR)of phytoplankton with different cell size under the condition of CO2 increase in seawater.The model simulated the equilibrium between the physical diffusion of CO2 and the carbonate system in seawater.What's more,we also simulated the supply,use and leakage rate of inorganic carbon(CO2 and HCO3-)of cells with different sizes,the optimal distribution of cell energy under the active transport of HCO3-and the cell growth.When CO2 concentration increases from the current typical concentration of the ocean surface(10?M,LC)to 20 ?M(HC),the simulation results confirm the conclusion of the historical data analysis:(1)For diatoms with small particle size(<12 ?m),consistent with the previous conjecture,as the particle size of the cell increases,the specific surface area decreases,the CO2 under LC gradually fails to meet the needs of the cell inorganic carbon,the energy consumption of HCO3-active transport increases,and the cell growth rate decreases.However,the CO2 supply under HC can still fully meet the demand of inorganic carbon,so the GRR response of cell growth rate increases with the particle size.(2)However,as the particle size of the cells futher increased,GRR continued to weaken.This is because the proportion of CO2 in the total inorganic carbon supply of the cells continued to decrease and the proportion of HCO3-continued to increase,and the cells were gradually insensitive to the increase of CO2 concentration in seawater.The study in this paper shows that there is an optimal cell particle size(or range),and the GRR of phytoplankton on seawater CO2 concentration rise is the highest.Parameter sensitivity experiments showed that the peak GRR was mainly affected by the CO2 leakage rate of cells.The higher the peak GRR,the higher the leakage rate.The optimal cell particle size corresponding to the peak GRR is mainly affected by the degree of increase of CO2 concentration.The more the CO2 concentration increases,the larger the optimal cell particle size is.Parameters such as maximum cell growth rate,temperature and co-action of extracellular carbonic anhydrase(eCA)also had great influence on the model results.The global prediction results based on the global change scenario of RCP 8.5 in this century show that under the influence of CO2 concentration,rising degree of CO2 and growth rate of diatoms in different sea areas,the GRR of diatoms with medium particle size is the highest in tropical regions,while the response of diatoms with large particle size is the highest in temperate and cold zones.Although the response of phytoplankton to rising CO2 concentration in seawater is also influenced by other factors,the model of this paper examines the most basic and direct effect of rising CO2 concentration on phytoplankton,and establishes a hypothesis on the relationship between phytoplankton particle size and CO2 concentration that can be further tested by laboratory studies.If the conclusion of this study is correct,in the tropical ocean,the increase of CO2 concentration may not cause a significant shift of phytoplankton community structure to large particle size,and the size and sedimentation rate of organic carbon particles may not increase.This presents a further challenge for studying and simulating the effects of rising CO2 concentration on the rate of carbon deposition in the ocean,known as the biological pump,and even the feedback mechanisms associated with a warming climate.