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RESEARCH ARTICLE
Contents Vol 60(6)

A new conceptual model for the warm-water breakdown of the coral–algae endosymbiosis

Scott A. Wooldridge
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Australian Institute of Marine Science, PMB #3, Townsville MC, Qld 4810, Australia.

Marine and Freshwater Research 60(6) 483-496 https://doi.org/10.1071/MF08251
Submitted: 2 September 2008  Accepted: 28 November 2008   Published: 19 June 2009

Abstract

The symbiosis between reef-building corals and their algae endosymbionts is sensitive to temperature stress, which makes coral reefs vulnerable to climate change. However, a precise understanding of the capacity for the symbiosis to adapt to climate change is currently restricted by the lack of coherent explanation for the set of cellular events leading to its warm-water breakdown (= coral bleaching). Here, a new coral bleaching model is proposed in which the triggering event is a disruption to the ‘dark’ photosynthetic reactions of the algae endosymbionts, primarily due to a limited availability of CO2 substrate around the Rubisco enzyme (ribulose-1,5-bisphosphate carboxylase). Paradoxically, this CO2-limiting condition may be enhanced by the modern increase in atmospheric CO2 partial pressure (pCO2). Importantly, the model delivers a new standpoint from which to explain: (i) upper thermal bleaching thresholds; and (ii) the mechanism underpinning endosymbiont shuffling. Overall, the model leaves little doubt as to the diminished stability and functioning (i.e. resilience) of the coral–algae endosymbiosis due to the rising pCO2 and warming trend in the upper ocean surface layer. It is concluded that whole-colony bleaching is the destructive endpoint to a suite of cellular processes that operate near continuously in modern symbiotic corals.

Additional keywords: cell-specific density,CO2 limitation, coral bleaching, mitotic index.


Acknowledgements

This work was supported by the Australian Government’s Marine and Tropical Science Research Facility (MTSRF). The final manuscript benefited from the suggestions of several anonymous reviewers. I acknowledge the helpful editorial assistance provided by Prof. A. Boulton.


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