Dissecting The Effect Of PCO₂ And PH On Trichodesmium IMS 101 And Its Mechanisms

The dissolution of rising atmospheric carbon dioxide(CO2)in the ocean leads to a shift in seawater carbonate chemistry,including an increase in partial pressure of carbon dioxide(pCO2)and a decrease in pH,changes collectively referred to as ocean acidification.Many researches have been conducted to investigate the impact of ocean acidification on Trichodesmium,a colonial nitrogen(N2)-fixing marine cynobacterium that plays an important role in the structure and function of the ecosystem in oligotrophic oceans.To the best of our knowledge,however,how pCO2 and pH individually may affect N2 fixation and growth of Trichodesmium remains essentially unknown.Here,we manipulated the carbonate chemistry of an artificial seawater medium by varying the concentration of dissolved inorganic carbon(DIC)and meanwhile forcing the pH with organic buffers.We then assessed the growth and N2 fixation of Trichodesmium IMS 101 in response to different pH/pCO2 matrix[i.e.,constant pH(8.1 and 7.8)orpCO2(400,atm and 800 μatm)].The effects of the increasing pCO2 and decreasing pH on Trichodesmium were evaluated by observing the growth rate,nitrogenase activity,carbon fixation rate,cytosolic pH and ribulose bisphosphate carboxylase oxygenase(RubisCO)’s half-saturation constant(Km).The main results obtained are as follows:1.Method of manipulate the pCO2 and pH independently.In this paper,the method of adjusting the concentration of DIC and total alkalinity(TA)in the artificial seawater was applied by using NaOH/HCl while adjusting the amount of sodium bicarbonate(NaHCO3)in the artificial seawater.2.Trichodesmium’s growth rate and nitrogenase activity under different/pCO2/pH matrix.On the one hand,increasing pCO2 promoted the growth rate and nitrogenase activity of Trichodesmium.On the other hand,the decrease of pH decreased the growth rate and nitrogenase activity.In addition,the change of the quantities of organic carbon/nitrogen(POC/PON)in the cells was the same as that of the growth rate and nitrogenase activity.In general,the negative effect of pH overwhelmed the positive effect of pCO2,resulting in inhibitory effect of ocean acidification on the growth and nitrogen fixation of Trichodesmium.3.The reason for the change in the growth rate and nitrogenase activity.(1)With the increasing concentration of CO2 in seawater,RubisCO’s Km of Trichodesmium was significantly increased,which indicated that the carbon concentrating mechanisms(CCM)was significantly down-regulated under high pCO2 condition,thus the energy could be saved for other important processes.(2)The decrease in seawater pH decrease decreased the cytosolic pH of Trichodesmium,which may affect cellular pH homeostasis and the growth and N2 fixation of Trichodesmium.In short,the negative effect of seawater pH will overwhelm the positive effect of the pCO2 in the future marine environment.The significance of this work is that(1)a multifactorial approach is adopted at constant pCO2 or pH whilst varying DIC concentration to determine the reponses of Trichodesmium to individual carbonate system parameters;(2)it is showed that Trichodesmium is more sensitive to changes in pH than in CO2.So this work emphasis the reason for different phenomenon found in Trichodesium’s feedback on ocean acidification.In the future,the method to adjust the artificial seawater carbonate system can also be applied to other phytoplankton and even marine organisms to distinguish the physiological effects of sea water pCO2 and pH changes in order to gain a deeper understanding of the effect of ocean acidification on marine biota.