Ocean Acidification Alleviates Cadmium Stress In Phaeodactylum Tricornutum

Ocean acidification changes the carbonate system of seawater and affects the growth and photosynthesis of marine organisms.Diatoms take up inorganic carbon and fix it into organic compounds,this process consumes a lot of energy.Ocean acidification increases the inorganic carbon content in seawater,which promotes the growth of diatoms.Cadmium?Cd?pollution is a widespread threat to marine life.Cd is non-biodegradable and accumulates in the food chains,threaten the safety of seafood and human health.Due to human activities,a large number of Cd pollutants was discharged into the ocean,resulting in excessive concentrations of Cd in estuaries and coastal areas.As a major contributor to marine primary productivity,diatoms are the initial link of the food chain in marine.Cd exposure could induce ROS production that can interact with lipids,leading to lipid peroxidation and decrease growth and photosynthetic rate.The ongoing ocean acidification?OA?is predicted to impact bio-toxicity of Cd compounds.However,the cascading effects of changed Cd toxicity to marine primary producers are to date not well characterized.Here,we studied the impact of OA on Cd toxicity responses in a globally important diatom Phaeodactylum tricornutum over a long-term selection under both ambient and elevated p CO₂ conditions.In the indoor experiments,we found that elevated pCO₂increased the growth rate in P.tricornutum both in short-term and long-term trearments.After 720 days selection,we also found that algal fitness increased lineally at 400?atm.P.tricornutum showed adaptation responses to OA after long-term selection,where cells pre-selected under elevated pCO₂ had lower growth than those pre-selected under ambient pCO₂ when tested under ambient pCO₂ condition.P.tricornutum reduces the absorption of Cd and enhances the discharge of Cd under elevated pCO₂.The results showed that respiration was enhanced under elevated pCO₂ in P.tricornutum.Results of outdoor experiments are consistent with the results of indoor experiments,OA increased growth rate and decreased cadmium accumulation in P.tricornutum.Transcriptome analysis showed that long-term selected lineages under elevated pCO₂expressed a specific Cd detoxification strategy,which included down-regulation of transmembrane transporters of ZUPT and TPC1A to reduce Cd²⁺uptake,up-regulation of?-ECS,GS,and PCS to enhance Cd²⁺chelation as less toxic organic forms of Cd-PC_n,up-regulation of VIT and ABCT to transport Cd-PC_n into vacuole,up-regulation of HMA to increase Cd²⁺efflux from cell.In addition,we also tested the activity of antioxidant enzymes in P.tricornutum,and the results that the activity of antioxidant enzymes was significantly enhanced under OA when exposure to Cd.We further investigated the impact of OA conditions the transfer of Cd to the scallop Argopecten irradians,a major consumer of P.tricornutum.We found a significant reduction in the levels of Cd transferring from P.tricornutum to A.irradians which could have broader impacts on the bio-geochemical cycle of Cd in marine ecosystem.