Adaptation de la faune souterraine aux basses températures : mécanismes et enjeux écologiques
Résumé
Subterranean fauna adaptations to low temperatures: mechanisms and ecological aspects.
Subterranean ecosystems are generally characterised by reduced yearly thermal variations (± 1°C along the year). Thus, hypogean organisms should present stenotherm profiles (reduced tolerance to thermal variations), as they never endure variation of the temperature during their life cycle. Yet, several biospeologists observed a surprising survival to negatives temperatures in the subterranean crustacean Niphargus rhenorhodanensis. In order to determine the N.
rhenorhodanensis thermobiological scale as well as the adaptive mechanisms allowing its cold tolerance, we measured its responses to low temperatures and we compared them to those of another hypogean crustacean (Niphargus virei) and to those of a morphologically close surface-dwelling crustacean (Gammarus fossarum). N. rhenorhodanensis showed larger survival times at -2°C than the two other organisms.
Performance experiments (oxygen consumption, locomotory and ventilatory activities) when exposed to different temperatures (from
-2 to 28°C) pointed out that N. rhenorhodanensis exhibited a eurytherm profile (maximising its performance on a broader range of temperature). Finally, during cold exposure, N. rhenorhodanensis shows i) a cryoprotectant (trehalose, free amino acid) body accumulations, ii) a decrease in free water body content (water that is able to participate in ice formation), and iii) a inoculative freezing tolerance. At the opposite, inoculative freezing is lethal in the two other organisms, with limited cryoprotectant body content increases, and no variation in free water body contents. With the support of recent phylogeographical studies, we hypothesise that cold hardiness pointed out in N. rhenorhodanensis might result form its life history during quaternary glaciations.
Subterranean ecosystems are generally characterised by reduced yearly thermal variations (± 1°C along the year). Thus, hypogean organisms should present stenotherm profiles (reduced tolerance to thermal variations), as they never endure variation of the temperature during their life cycle. Yet, several biospeologists observed a surprising survival to negatives temperatures in the subterranean crustacean Niphargus rhenorhodanensis. In order to determine the N.
rhenorhodanensis thermobiological scale as well as the adaptive mechanisms allowing its cold tolerance, we measured its responses to low temperatures and we compared them to those of another hypogean crustacean (Niphargus virei) and to those of a morphologically close surface-dwelling crustacean (Gammarus fossarum). N. rhenorhodanensis showed larger survival times at -2°C than the two other organisms.
Performance experiments (oxygen consumption, locomotory and ventilatory activities) when exposed to different temperatures (from
-2 to 28°C) pointed out that N. rhenorhodanensis exhibited a eurytherm profile (maximising its performance on a broader range of temperature). Finally, during cold exposure, N. rhenorhodanensis shows i) a cryoprotectant (trehalose, free amino acid) body accumulations, ii) a decrease in free water body content (water that is able to participate in ice formation), and iii) a inoculative freezing tolerance. At the opposite, inoculative freezing is lethal in the two other organisms, with limited cryoprotectant body content increases, and no variation in free water body contents. With the support of recent phylogeographical studies, we hypothesise that cold hardiness pointed out in N. rhenorhodanensis might result form its life history during quaternary glaciations.
De par leur structure, les écosystèmes souterrains présentent des variations thermiques extrêmement faibles (± 1°C sur l'année). De ce fait, les animaux hypogés devraient présenter des caractéristiques de sténothermes (faible tolérance aux variations thermiques), puisqu'ils ne subissent aucune variation de la température durant leur cycle de vie. Pourtant, plusieurs biospéologistes ont constaté que le crustacé aquatique souterrain Niphargus rhenorhodanensis pouvait supporter des températures négatives. Afin de déterminer l'échelle thermobiologique de N. rhenorhodanensis ainsi que les mécanismes adaptatifs lui permettant de tolérer les basses températures, nous avons comparé ses réponses à celles d'un autre amphipode souterrain (N. virei), ainsi qu'à celles d'un crustacé morphologiquement proche vivant dans les cours d'eau épigé (Gammarus fossarum). N. rhenorhodanensis montre des temps de survies à -2°C significativement supérieurs à ceux des deux autres organismes. Les mesures de performance (métabolisme, activité locomotrice, ventilation) en fonction de la température (sur une gamme allant de -2°C à 28°C) démontrent que N. rhenorhodanensis présente les caractéristiques d'un organisme eurytherme : il maximise sa performance sur large plage de température. Enfin, lors d'une exposition au froid, N. rhenorhodanensis accumule des substances cryoprotectrices dans ses tissus (tréhalose, acides aminés libres), diminue sa teneur en eau libre (eau corporelle disponible pour la congélation), et tolère la congélation lorsqu'elle est initié par contact avec la glace. A l'inverse, chez les deux autres organismes, la congélation est mortelle, les teneurs en substances cryoprotectrices n'augmentent que très peu et l'eau libre ne varie pas. Avec l'appui de récents travaux phylogéographiques, nous émettons l'hypothèse que la présence de telles adaptations chez N.
rhenorhodanensis serait liée à son histoire de vie durant les dernières ères glaciaires.
rhenorhodanensis serait liée à son histoire de vie durant les dernières ères glaciaires.
Domaines
Ecologie, Environnement
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