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210 CHAPITRE 5. ADAPTATIONS RESPIRATOIRES DE S. MESATLANTICA response during acclimation. Interestingly, urate does not have a physiological effect on Hc affinity either since Hc was insensitive to urate even if a strong urate response to acclimation was observed. These two organic ions which are classic modulators in decapod crustaceans do not have a marked role in the respiratory response in S. mesatlantica according to our data. This is in accordance with some results from Cyanagraea praedator and Bythograea thermydron for which a lactate response occurs but no or a very small lactate effect on Hc affinity is observed (Chausson, 2001). A stronger effect of Mg 2+ on Hc affinity was observed for S. mesatlantica than for C. maenas. Mg 2+ content increased in stressful conditions but the calculated effect for a change from 20 mM to 25 mM Mg 2+ is -0.023 for log(P 50 ), while an increase from the average value of 4.2 mM to 5.6 mM lactate (1.4 mM being the standard deviation in our measures) results in a change of -0.027 for log(P 50 ) and of -0.1 for a 11.9 mM final lactate concentration (the maximum value observed). Thus the intensity of the Mg 2+ is not negligible compared to the lactate effect for example, and provides another interpretation for the increase in magnesium content observed at high temperature and in hypoxia which would increase Hc affinity for oxygen and improve loading at the gills in the stressful conditions. Acclimation to our conditions did not result in a significant modulation of Hc affinity by the usual organic modulators : Hc affinity is always high. Another effector is the hemolymph pH which could not be measured on board during the cruise ; an acid-base response of the hemolymph of Segonzacia mesatlantica during environmental stresses is not known yet but is likely to occur during hyperventilation or exercise (avoidance of hydrothermal fluid). The high affinity without organic modulation is adaptive for hypoxic media. Such a high affinity results in a 95 % saturation of the pigment even at an oxygen arterial partial pressure of 3.9 Torr (without lactate at 15°C, pH 7.8) or 2.6 Torr (with 4mM lactate at 15°C, pH 7.8), which in a very efficient pigment for O 2 extraction even in severe hypoxia. Thus the main issue when exposed to stressful conditions may not be to improve extraction but to be able to unload the bound O 2 near the active tissues, which can be facilitated by a strong Bohr effect. The potential effect of pressure also has to be considered ; pressure can modify chemical equilibria depending on the reaction volumes (Gross et Jaenicke, 1994). Structural studies of oxygenated and deoxygenated Hc subunits from Limulus polyphemus have suggested the rotation of one of its three domain by 8° upon oxygenation (Magnus et al., 1994). Work on complexes have led to "turning wheel" and camera iris aperture models for describing the movement of the two trimer layer in Hc hexamer upon oxygenation (Haas et al., 1993, Decker et al., 1996) ; Hc from the spider Eurypelma californicum is less compact in the oxygenated form (Decker et al., 1996) and the distance between the two oxygenated hexamers in a dodecamer of Homarus americanus Hc is shortened in the presence of L-lactate (Hartmann et al., 2001). Based on these data, pressure could result in a decrease of Hc affinity (the deoxygenated form being the more compact for Eurypelma californicum) and an increase
5.4. MANUSCRIT : RESPIRATORY ADAPTATIONS OF S. MESATLANTICA 211 of the lactate sensitivity (the lactate-bearing form being more compact for Homarus americanus) compared to data collected at sea-surface pressure. 5.4.5 Conclusion Segonzacia mesatlantica Hc has some common features present in hydrothermal crustacean Hc : an intrinsic high affinity for O 2 , a strong Bohr effect and a high hemolymphatic concentration resulting in a high capacitance. No phenotypic plasticity at the subunit level was involved in the respiratory response to the conditions tested here but complex proportions varied according to temperature, with an increased hexamer proportion at high temperature. The specific properties of each complex remain to be determined to physiologically interpret this observation. Hemolymph composition responses included a dramatic increase in urate levels in repressurized crabs, regardless of the experienced conditions, a higher Hc level at low temperature and at high oxygenation, and higher magnesium levels at high temperature and in moderate hypoxia. No lactate response was observed. Magnesium can act at the metabolic level by decreasing the activity of the crab but also increases Hc affinity for O 2 . Interestingly, S. mesatlantica Hc was sensitive to lactate but not to urate. This results in a physiological uncoupling between the usual organic effectors of Hc and Hc properties since no effector exhibits simultaneously a concentration response to stress and an effect on Hc affinity. This is also the case in other hydrothermal crustaceans for lactate at least. The common adaptive strategies for hydrothermal crustaceans can be summarized as 1) a constant high affinity of Hc for O 2 , but not or weakly modulated by organic cofactors and temperature, for efficient loading at the gills in the whole range of conditions they experience and 2) a strong Bohr effect to facilitate the unloading near the metabolizing tissues. The absence of an excessively strong increase of affinity by L-lactate or urate in hypoxia is adaptive since it permits to keep unloading O 2 at the tissues. Changes in complex proportions and thiosulfate levels are also potential functional modulators for which further investigations are needed. To reach an integrated understanding of the respiratory response in S. mesatlantica and in other hydrothermal crustaceans, it would be of interest to measure physiological variables such as ventilation, cardiac frequency and output, arterial and venous pH and oxygen partial pressure, and oxygen consumption under various conditions including severe hypoxia and in the presence of sulphides. These experiments should be made under pressure and the physiological rest status of the individuals should be checked before applying the stress, since the urate data in our study suggest that repressurization itself can act as a stressor. Acknowledgments : P. Legendre got involved in this research as recipient of a CNRS Associate Research Director-
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THESE DE DOCTORAT DE L’UNIVERSITE
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i Résumé / Abstract Réponse adap
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iv SOMMAIRE 4.2 Objectifs de l’é
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(a) FIG. 1.6 - Fonctionnement d’u
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134 CHAPITRE 4. PLASTICITÉ PHÉNOT
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Page 243 and 244: Bibliographie Adair, G. S. The hemo
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TABLE DES MATIÈRES 261 4.3.5 Spect
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TABLE DES MATIÈRES 263 Table des m
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