Elevated temperature inhibits recruitment of transferrin-positive vesicles and induces iron-deficiency genes expression in Aiptasia pulchella host-harbored Symbiodinium.

Abstract

Coral bleaching is the consequence of disruption of the mutualistic Cnidaria-dinoflagellate association. Elevated seawater temperatures have been proposed as the most likely cause of coral bleaching whose severity is enhanced by a limitation in the bioavailability of iron. Iron is required by numerous organisms including the zooxanthellae residing inside the symbiosome of cnidarian cells. However, the knowledge of how symbiotic zooxanthellae obtain iron from the host cells and how elevated water temperature affects the association is very limited. Since cellular iron acquisition is known to be mediated through transferrin receptor-mediated endocytosis, a vesicular trafficking pathway specifically regulated by Rab4 and Rab5, we set out to examine the roles of these key proteins in the iron acquisition by the symbiotic Symbiodinium. Thus, we hypothesized that the iron recruitments into symbiotic zooxanthellae-housed symbiosomes may be dependent on rab4/rab5-mediated fusion with vesicles containing iron-bound transferrins and will be retarded under elevated temperature. In this study, we cloned a novel monolobal transferrin (ApTF) gene from the tropical sea anemone Aiptasia pulchella and confirmed that the association of ApTF with A. pulchella Rab4 (ApRab4) or A. pulchella Rab5 (ApRab5) vesicles is inhibited by elevated temperature through immunofluorescence analysis. We confirmed the iron-deficient phenomenon by demonstrating the induced overexpression of iron-deficiency-responsive genes, flavodoxin and high-affinity iron permease 1, and reduced intracellular iron concentration in zooxanthellae under desferrioxamine B (iron chelator) and high temperature treatment. In conclusion, our data are consistent with algal iron deficiency being a contributing factor for the thermal stress-induced bleaching of symbiotic cnidarians.