Breakdown of the coral-algae symbiosis: towards formalising a linkage between warm-water bleaching thresholds and the growth rate of the intracellular zooxanthellae

[摘要] Impairment of the photosynthetic machinery of the algal endosymbiont("zooxanthellae") is the proximal driver of the thermal breakdown of thecoral-algae symbiosis ("coral bleaching"). Yet, the initial site of damage,and early dynamics of the impairment are still not well resolved. In thisperspective essay, I consider further a recent hypothesis which proposes anenergetic disruption to the carbon-concentrating mechanisms (CCMs) of thecoral host, and the resultant onset of CO₂-limitation within thephotosynthetic "dark reactions" as a unifying cellular mechanism. Thehypothesis identifies the enhanced retention of photosynthetic carbon forzooxanthellae (re)growth following an initial irradiance-driven expulsionevent as a strong contributing cause of the energetic disruption. If true,then it implies that the onset of the bleaching syndrome and setting ofupper thermal bleaching limits are emergent attributes of the coralsymbiosis that are ultimately underpinned by the characteristic growthprofile of the intracellular zooxanthellae; which is known to depend notjust on temperature, but also external (seawater) nutrient availability andzooxanthellae genotype. Here, I review this proposed bleaching linkage at avariety of observational scales, and find it to be parsimonious with theavailable evidence. Future experiments are suggested that can more formallytest the linkage. If correct, the new cellular model delivers a valuable newperspective to consider the future prospects of the coral symbiosis in anera of rapid environmental change, including: (i) the underpinning mechanics(and biological significance) of observed changes in resident zooxanthellaegenotypes, and (ii) the now crucial importance of reef water quality inco-determining thermal bleaching resistance.

[学科分类] 地球化学与岩石