Cellular mechanisms underlying temperature-induced bleaching in the tropical sea anemone Aiptasia pulchella

Temperature-induced bleaching in symbiotic cnidarians is a result of the de tachment and loss of host cells containing symbiotic algae. We tested the h ypothesis that host cell detachment is evoked through a membrane thermotrop ic event causing an increase in intracellular calcium concentration, [Ca2+] (i), which could then cause collapse of the cytoskeleton and perturb cell a dhesion. Electron paramagnetic resonance measurements of plasma membranes f rom the tropical sea anemone Aiptasia pulchella and the Hawaiian coral Poci llopora damicornis labeled with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO ) revealed no membrane thermotropic event. In addition, intracellular imagi ng using Fura-2AM as well as labeling anemones with Ca-45 revealed no signi ficant change in [Ca2+](i). However, bleaching could be evoked at ambient t emperature with 25 mmoll(-1) caffeine without affecting [Ca2+](i). [Ca2+](i ) could be altered with ionomycin in isolated host cells, but ionomycin cou ld not induce bleaching in A. pulchella. As caffeine can affect levels of i ntracellular protein phosphorylation, the ability of other agents that alte r intracellular levels of protein phosphorylation to evoke bleaching was in vestigated. The protein phosphatase inhibitor vanadate could induce bleachi ng in A. pulchella. Two-dimensional gels of P-32-labeled proteins from cold -shocked, caffeine-treated and control anemones show that both temperature shock and caffeine alter the array of phosphorylated host soluble proteins. We conclude that enidarian bleaching is linked to a temperature-induced al teration in protein phosphorylation.