Antarctic krill (Euphausia superba) in a warming ocean: thermotolerance and deciphering Hsp70 responses

The Antarctic krill, Euphausia superba, is a Southern Ocean endemic species of proven ecological importance to the region. In the context of predicted global warming, it is particularly important to understand how classic biomarkers of heat stress function in this species. In this respect, Hsp70s are acknowledged as good candidates. However, previous studies of expression kinetics have not been able to demonstrate significant upregulation of these genes in response to heat shocks at 3 °C and 6 °C for 3 and 6 h. The current work complements these previous results and broadens the prospects for the use of Hsp70s as a relevant marker of thermal shock in this krill species. New experiments demonstrate that induction of Hsp70 isoforms was not detected during exposure to heat shock, but increased expression was observed after several hours of recovery. To complete the analysis of the expression kinetics of the different isoforms, experiments were carried out over short time scales (1 and 2 h at 3 °C and 6 °C) as well as at higher temperatures (9 °C, 12 °C, and 15 °C for 3 h), without any significant response. A 6-week monitoring of animals at 3 °C showed that the time factor is decisive in the establishment of the response. CT_max experiments with incremental times of 1 °C per day or 1 °C every 3 days have shown a particularly high resilience of the animals. The demonstration of the abundance of Hsp70s present before thermal stress in various species of krill, as well as in specimens of E. superba of various origins, showed that the delay in the response in expression could be related to the high constitutive levels of Hsp70 available before the stress experiments. The alternative labelling of the two main isoforms of Hsp70 according to the origin of the animals allowed hypotheses to be put forward on the functioning of thermoregulation in Antarctic krill as well as ice krill.