| Temperature may determine spatial and temporal distribution of species,while global warming and extreme heat events are threatening the survival of species and changing biogeographic distributions.It is well known that physiological adaptation is an important mechanism for organisms to cope with heat stress,however,we lack systematic studies on the underlying mechanisms of thermal adaptation,especially for some species living in extremely high temperatures.Species in the intertidal zone often suffer from serious and rapidly changing heat stress,and therefore they are suggested to possess high thermal tolerance,which makes them appropriate study organism for investigating mechanisms of extreme thermal tolerance.In the present study,we selected several typical high-intertidal gastropods,and conducted a multi-level study,including cardiac physiology,enzyme activity,membrane stability,and targeted and untargeted metabolomic analyses,to elucidate physiological adaptation of organisms to extreme thermal stress.1.Impact of heating rate on cardiac performance of intertidal gastropodsIn this study,we measured the heart rates of two splash-zone gastropods(Echinolittorina malaccana,E.radiata),one high-intertidal gastropod(Littorina brevicula)and one mid-intertidal gastropod(Nerita yoldii)at different heating rates(0.1℃ min-1 and 0.5℃ min-1),and monitored the ambient temperature in the splash zone.The results show that the organisms in the splash zone suffered extreme heat stress in summer.Heating rate would affect physiological responses of high-intertidal gastropods.At high heating rate(0.5℃ min-1),the metabolic depression that helps organisms to resist heat stress was eliminated or attenuated.High heating rate affected FLTs and ABTs in E.malaccana and E.radiata,but not altered thermal tolerance of L.brevicula.The physiological response patterns and thermal tolerance of N.yoldii were not affected by different heating rates.Thus,the intertidal gastropods have different physiological responses to different heating rates,and physiological responses were related to tidal levels.2.Metabolomic analysis on thermal adaptations of the thermophilic snail Echinolittorina malaccanaWe conducted an innovative investigation,including enzyme activity,and targeted and untargeted metabolomic analysis,to elucidate a spectrum of adaptations to extreme heat All systems examined showed heat intensity-dependent responses.The snail E.malaccana would depress cardiac activity and enter a state of metabolic depression under moderate heat stress(from 37 to 45℃).The global metabolomic analyses and enzymatic analysis showed that the depressed metabolic state under moderate heat stress entailed production of metabolites characteristic of oxygen-independent pathways of ATP generation(lactate and succinate),which suggests that anaerobic metabolism was the main energy supply pathway under heat stress(from 37 to 52℃).The metabolomic analyses also revealed alterations in glycerophospholipid metabolism under extreme heat stress of 52℃,which likely reflected adaptive changes to maintain membrane structure.Small molecular mass organic osmolytes(glycine betaine,choline,and carnitine)showed increases in concentration that were consistent with a role of these protein-stabilizing solutes in protection of the proteome under heat stress.Thus,this thermophilic species can deploy a wide array of adaptive strategies to cope with cell stress damage,membrane damage,and protein methylation and to acclimatize to extremely high temperatures.3.Effects of thermal stress on the lysosomal membrane stability in two highintertidal snailsIn this study,lysosomal neutral red retention time(NRRT)of lysosomal membrane of hemocytes of high intertidal gastropods(E.malaccana and L.brevicula)distributed in different latitudes and tidal levels were measured.The results show that lysosomal membrane stability of hemocytes of E.malaccana under thermal stress was higher than that of L.brevicula.Under both acute heat stress(ATS)and gradual heat stress(GTS),the NRRT of the southern periwinkle(E.malaccana)was higher than that of northern periwinkle(L.brevicula).The lysosomal membrane stability of E.malaccana and L.brevicula may be significantly associated with thermal tolerance.Overall,high-intertidal organisms have specific thermal adaptations to extreme and rapidly changing temperature environment.High-intertidal gastropods have divergent cardiac responses to different warming rates,in comparison to mid-intertidal gastropod.The thermophilic snail E.malaccana employs a variety of mechanisms to adapt to the extremely high temperatures of its habitat.Maintaining membrane stability under thermal stress could help high-intertidal organisms to tolerate higher thermal stress.These findings are useful for understanding and predicting the effects of global warming on the physiology of intertidal organisms in the future. |
