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In the context of global change, symbiotic cnidarians are largely affected by seawater temperature elevation leading to symbiosis breakdown. This process, also called bleaching, is triggered by the dysfunction of the symbiont photosystems causing an oxidative stress and cell death to both symbiont and host cells. In our study, we wanted to elucidate the intrinsic capacity of isolated animal cells to deal with thermal stress in the absence of symbiont. In that aim, we have characterized an animal primary cell culture form regenerating tentacles of the temperate sea anemone Anemonia viridis. We first compared the potential of whole tissue tentacle or separated epidermal or gastrodermal monolayers as tissue sources to settle animal cell cultures. Interestingly, only isolated cells extracted from whole tentacles allowed establishing a viable and proliferative primary cell culture throughout 31 days. The analysis of the expression of tissue-specific and pluripotency markers defined cultivated cells as differentiated cells with gastrodermal origin. The characterization of the animal primary cell culture allowed us to submit the obtained gastrodermal cells to hyperthermal stress (+ 5 and + 8 °C) during 1 and 7 days. Though cell viability was not affected at both hyperthermal stress conditions, cell growth drastically decreased. In addition, only a + 8 °C hyperthermia induced a transient increase of antioxidant defences at 1 day but no ubiquitin or carbonylation protein damages. These results demonstrated an intrinsic resistance of cnidarian gastrodermal cells to hyperthermal stress and then confirmed the role of symbionts in the hyperthermia sensitivity leading to bleaching.

Seawater chemistry for long-term in situ exposureDuring the one week fieldwork, seawater pH (NBS scale) was measured each day (n=7) using a pH-meter and an electrode (Metrohm pH mobile). Seawater samples were filtered with a Whatman GF/F, treated with 0.05 ml of 50 % HgCl 2 (Merck, Analar) and stored in the dark at 4°C pending analysis. Three replicate were analysed at 25°C. Titration of TA standards provided by A.G. Dickson was within 0.5 µmol kg -1 of the nominal value. The other parameters of the carbonate system (pCO 2 , CO 3 2-, HCO 3 -, and Ω ara ) were calculated from pH, mean TA, temperature, pressure and mean salinity using the free-access CO 2 SYS (Pierrot et al. 2006) package. Data were in the range reported by Suggett et al. (2012).Experimental CO 2 re-circulating seawater system

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