title: Symbiophagy as a cellular mechanism for coral bleaching.
abstract: Coral bleaching is a major contributor to the global declines of coral reefs. This phenomenon is characterized by the loss of symbiotic algae, their pigments or both. Despite wide scientific interest, the mechanisms by which bleaching occurs are still poorly understood. Here we report that the removal of the symbiont during light and temperature stress is achieved using the host's cellular autophagic-associated machinery. Host cellular and subcellular morphologies showed increased vacuolization and appearance of autophagic membranes surrounding a variety of organelles and surrounding the symbiotic algae. Markers of autophagy (Rab 7 and LAS) corroborate these observations. Results showed that during stress the symbiont vacuolar membrane is transformed from a conduit of nutrient exchange to a digestive organelle resulting in the consumption of the symbiont, a process we term symbiophagy. We posit that during a stress event, the mechanism maintaining symbiosis is destabilized and symbiophagy is activated, ultimately resulting in the phenomenon of bleaching. Symbiophagy may have evolved from a more general primordial innate intracellular protective pathway termed xenophagy.
authors.affiliation: Pacific Biosciences Research Center, Kewalo Marine Laboratory, University of Hawaii at Manoa, Honolulu, Hawaii 96821, USA.
authors.forename: Craig A
authors.initials: CA
authors.lastname: Downs
authors.forename: Esti
authors.initials: E
authors.lastname: Kramarsky-Winter
authors.forename: Jon
authors.initials: J
authors.lastname: Martinez
authors.forename: Ariel
authors.initials: A
authors.lastname: Kushmaro
authors.forename: Cheryl M
authors.initials: CM
authors.lastname: Woodley
authors.forename: Yossi
authors.initials: Y
authors.lastname: Loya
authors.forename: Gary K
authors.initials: GK
authors.lastname: Ostrander
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000166:cytology* / Q000378:metabolism* / Q000528:radiation effects / Q000648:ultrastructure; D001343:Autophagy / Q000528:radiation effects; D015415:Biomarkers / Q000378:metabolism; D004707:Endoderm / Q000528:radiation effects / Q000648:ultrastructure; D056890:Eukaryota / Q000528:radiation effects / Q000648:ultrastructure; D008027:Light; D013559:Symbiosis / Q000528:radiation effects; D013696:Temperature
chemical_list: D015415:Biomarkers