Βιοχημικές και μεταβολικές προσαρμογές των δίθυρων Mytilus galloprovincialis και Modiolus barbatus στην αύξηση της θερμοκρασίας του περιβάλλοντος

Abstract

Environmental temperature has played an important role during the evolution of species, as well as in their geographical distribution. Thermal biology studies the effects of temperature at every level of biological organization in various time-scales. Its study helps the determination of the thermal limits of organisms in terms of socalled global warming. The aim of the present study was to determine the metabolic and biochemical responses of bivalves with different vertical distribution in Thermaikos Gulf to the elevated sea water temperature, as well as to their exposure to air. For this purpose Mytilus galloprovincialis (exposed to air during the tidal) and Modiolus barbatus (never exposed to air) were used. Especially, we studied the rate of mortality in both species, Hsp70 and Hsp90 expression, phosphorylation of MAPKs p-38 and JNK, which seem to be implicated in Hsps’ expression, enzymatic activity of glycolytic enzymes pyruvate kinase, aldolase, phosphofructokinase, hexokinase, phospho-glycerol-kinase and phosphoenol-pyruvate carboxykinase and the levels of Scope for Growth (SFG) The results of the present study show that M. galloprovincialis have an increased mortality at sea water temperatures over 26#C. However, also lower temperatures seem to affect the duration of valve open and closure. PK’s low enzymatic activity in tissues of M.galloprovinci#lis acclimated at 24#C, indicates a low glycolytic rate, characteristic of bivalves’ anaerobic metabolism. At 26oC and 28oC PK’s enzymatic activity showed a biphasic change pattern. Hsp70 and Hsp90 are induced after long term acclimation at temperatures over 24oC. Over this temperature, phosphorylationl evels of p-38 and JNK are also increased. Clearance rate of M.galloprovincialis (the most important parameter for the determination of SFG) is significantly decreased over 25oC. Its negative value at 26oC and 28oC indicates a mobilization of energy reserves in order the energetic cost of Hsps expression to be covered. It seems that mussels M. galloprovinci#lis live at temperatures near or above their critical temperatures at their habitat during the summer. For M. barbatus the threshold temperature for the induction of Hsps is about 22-23oC, while for M. galloprovincialis it is greater than 25oC. Interenstingly, in reverse to the findings for M. galloprovincialis, this temperature for M. barbatus is not similar to the one, at which increased mortality is found, but lower by about 2oC. Tissues of M. barbatus also present higher glycolytic capacity in regard to the tissues of M. galloprovincialis. These findings suggest an earlier limitation of aerobic capacity, but simultaneously greater ability for pathetic survival for M. barbatus than for M. galloprovincialis. Both species were found not to be able to survive for long periods of time when exposed at air temperatures over 34 oC. Mortality and Hsps’ expression are elevated in the first few hours of exposure. It also seems that shift from normoxic to suboxic conditions during the tidal cycle causes an important change in the organisms’ metabolic type. Based on models for the prediction of future climate changes, it seems that mussels will face air temperatures higher at least by 5-6oC than today. Knowing that today M. galloprovincialis lives at temperatures near the leathal for the species during the summer, we could conclude that such an increased air temperature would probably cause a significantly increased mortality and reduction of mussels’ population in the Thermaikos Gulf. 232 #