Depth-dependant thermotolerance of the symbiotic Mediterranean gorgonian Eunicella singularis: Evidence from cellular stress markers

Abstract Over the last decades, increases in Mediterranean seawater temperature have caused several mass mortality episodes in gorgonian populations. Along the Northwestern Mediterranean coast, Eunicella singularis , the only symbiotic Mediterranean gorgonian, was particularly affected. The mortalities were observed to be “patchy”, however, suggesting differential resistance within this species. Following the same experimental design used in Ferrier-Pages et al. (2009), E. singularis tips were collected at − 15 and − 35 m depth (Marseilles, France) and then subjected to two steady-step temperature increases under aquarium conditions: the first from 18 °C to 24 °C over 33 days and the second up to 26 °C over 19 days. Consequences of hyperthermia were evaluated by studying three cellular stress markers: the total oxyradical scavenging capacity (TOSC), ubiquitinated proteins and heat-shock protein (Hsp70) levels. This is the first time that these markers have been studied in such a temperate gorgonian. The first temperature increase to 24 °C did not produce any response in stress marker levels. In contrast, the 26 °C step generated a transient stimulation, 24 h after the increase, of antioxidant defenses for both populations. After 2 weeks of exposure to 26 °C, no Hsp70 induction was detected, but an increase in protein damages indicated a lower resistance in shallow populations corroborating the previous observation of differential resistance capacity between deep and shallow populations of E. singularis , with a higher thermotolerance in deep populations. The intensification of mass mortality episodes highlights the urgency to improve our knowledge of thermal resistance mechanisms. Such knowledge will help administrators to establish conservation plans to protect these emblematic gorgonians.