Preliminary Study On The Physiological Effects Of Marine Heatwaves On Pinctada Maxima

With climate change,marine heatwaves(MHWs)will increase in frequency,duration and intensity,posing a serious threat to marine ecosystems and fisheries.However,the impact of MHWs on marine bivalves remains largely unknown.Here,we take Pinctada maxima-one of the most ecologically and economically significant bivalves in coastal ecosystems of Beibu Gulf-as research object,to investigate the response of bivalves to short-lasting and repeatedly-occurring MHWs scenarios.The results as follows:(1)Rapidly increased the water temperature from 24℃ to 28℃,32℃ and 36℃,and the mortality of P.maxima increased after 3d short-term MHWs exposure,and especially they suffered 100% mortality at 36℃.Activities of enzymes including acid phosphatase(ACP),alkaline phosphatase(AKP),glutathione(GSH)and level of malondialdehyde(MDA)increased significantly with increasing intensity and duration of MHWs,indicating thermal stress responses.When exposed to repeatedly-occurring MHWs scenarios,thermal stress responses were alleviated,as exemplified by mortality rates of pearl oysters increased slightly and significant decreases in ACP,AKP,GSH and MDA activities compared with those during short-lasting MHWs scenarios,demonstrating the potential of P.maxima to acclimate rapidly to MHWs.(2)To explore whether P.maxima has acclimation potential after exposure to MHWs of different durations and frequencies,RNA-seq studies were performed using gill tissues.GO enrichment analysis of DEGs found that both short-and long-term MHWs exposure,DEGs between the experimental groups(28℃ and 32℃)and control group(24℃)were mainly enriched in the cellular component.KEGG enrichment found that after acute MHWs exposure,DEGs were mainly enriched in “Protein processing in endoplasmic reticulum”,and after long-term MHWs exposure,DEGs were mainly enriched in metabolism and environmental information processing.related signaling pathways.Eight genes,HSP70,HSP90,CRT,ATPase,ND5,Cat L,GST and UBC significantly related to marine heat waves were screened for expression verification.The analysis confirmed that the expression levels of CRT,HSP70 and HSP90 were significantly up-regulated after acute MHWs exposure,and were positively correlated with the temperature.After long-term and repeatedly-occurring MHWs exposure,P.maxima evident obvious acclimation responses.The results showed that except for ATPase,which had no significant difference throughout the experimental period,the expression levels of other 7 genes were significantly down-regulated during the recovery and the second MHWs period,and the expression level between the experimental groups and the control group has no significant difference.Therefore,long-term and recurrent MHWs are believed to alleviate heat stress in P.maxima.(3)Biomarkers of P.maxima in response to were developed,the full-length c DNAs of HSP70 and HSP90 in P.maxima using RACE technology,and the expression patterns of Pm HSP70 and Pm HSP90 under different duration and intensity of MHWs were analyzed using real-time fluorescence quantitative technology.Tissue expression results showed that Pm HSP70 and Pm HSP90 were expressed in all tissues,with the highest expression in gill tissue and hepatopancreas,respectively.The time-series expression results showed that under the scenario of acute MHWs,the expression of Pm HSP70 and Pm HSP90 were significantly highly expressed at 32 and 36℃.The expression levels of Pm HSP70 and Pm HSP90 were significantly reduced in recurrent MHWs.Sensitivity and indicative roles of Pm HSP70 and Pm HSP90 to MHWs were confirmed.In conclusion,P.maxima can respond to MHWs by increasing the activities of antioxidant-related enzymes and up-regulating the expression of HSPs and other related genes in the face of acute MHWs.Long-lasting and repeatedly-occurring MHWs can alleviate these thermal stresses.These findings deepen our understanding of how marine bivalves respond to scenarios of MHWs of varying duration,frequency and intensity.