title: Coral bleaching under thermal stress: putative involvement of host/symbiont recognition mechanisms.
abstract: BACKGROUND
  Coral bleaching can be defined as the loss of symbiotic zooxanthellae and/or their photosynthetic pigments from their cnidarian host. This major disturbance of reef ecosystems is principally induced by increases in water temperature. Since the beginning of the 1980s and the onset of global climate change, this phenomenon has been occurring at increasing rates and scales, and with increasing severity. Several studies have been undertaken in the last few years to better understand the cellular and molecular mechanisms of coral bleaching but the jigsaw puzzle is far from being complete, especially concerning the early events leading to symbiosis breakdown. The aim of the present study was to find molecular actors involved early in the mechanism leading to symbiosis collapse.
  
  
  RESULTS
  In our experimental procedure, one set of Pocillopora damicornis nubbins was subjected to a gradual increase of water temperature from 28 degrees C to 32 degrees C over 15 days. A second control set kept at constant temperature (28 degrees C). The differentially expressed mRNA between the stressed states (sampled just before the onset of bleaching) and the non stressed states (control) were isolated by Suppression Subtractive Hybridization. Transcription rates of the most interesting genes (considering their putative function) were quantified by Q-RT-PCR, which revealed a significant decrease in transcription of two candidates six days before bleaching. RACE-PCR experiments showed that one of them (PdC-Lectin) contained a C-Type-Lectin domain specific for mannose. Immunolocalisation demonstrated that this host gene mediates molecular interactions between the host and the symbionts suggesting a putative role in zooxanthellae acquisition and/or sequestration. The second gene corresponds to a gene putatively involved in calcification processes (Pdcyst-rich). Its down-regulation could reflect a trade-off mechanism leading to the arrest of the mineralization process under stress.
  
  
  CONCLUSIONS
  Under thermal stress zooxanthellae photosynthesis leads to intense oxidative stress in the two partners. This endogenous stress can lead to the perception of the symbiont as a toxic partner for the host. Consequently, we propose that the bleaching process is due in part to a decrease in zooxanthellae acquisition and/or sequestration. In addition to a new hypothesis in coral bleaching mechanisms, this study provides promising biomarkers for monitoring coral health.
authors: UMR 5244, CNRS EPHE UPVD, Université de Perpignan, 66860 Perpignan Cedex, France. - jeremie.vidal-dupiol@univ-perp.fr
authors: Jeremie
authors: J
authors: Vidal-Dupiol
authors: Mehdi
authors: M
authors: Adjeroud
authors: Emmanuel
authors: E
authors: Roger
authors: Laurent
authors: L
authors: Foure
authors: David
authors: D
authors: Duval
authors: Yves
authors: Y
authors: Mone
authors: Christine
authors: C
authors: Ferrier-Pages
authors: Eric
authors: E
authors: Tambutte
authors: Sylvie
authors: S
authors: Tambutte
authors: Didier
authors: D
authors: Zoccola
authors: Denis
authors: D
authors: Allemand
authors: Guillaume
authors: G
authors: Mitta
mesh_terms: D000819:Animal Communication; D000818:Animals; D037421:Anthozoa / Q000502:physiology*; D003063:Cnidaria / Q000502:physiology*; D018869:Heat-Shock Response / Q000502:physiology*; D010788:Photosynthesis / Q000502:physiology*; D010860:Pigments, Biological / Q000378:metabolism*; D013559:Symbiosis / Q000502:physiology*
chemical_list: D010860:Pigments, Biological