A hypothesis linking sub-optimal seawater <I>p</I>CO<sub>2</sub> conditions for cnidarian-<I>Symbiodinium</I> symbioses with the exceedence of the interglacial threshold (>260 ppmv)

Abstract

Abstract. Most scleractinian corals and many other cnidarians host intracellular photosynthetic dinoflagellate symbionts ("zooxanthellae"). The zooxanthellae contribute to host metabolism and skeletogenesis to such an extent that this symbiosis is well recognised for its contribution in creating the coral reef ecosystem. The stable functioning of cnidarian symbioses is however dependent upon the host's ability to maintain demographic control of its algal partner. In this review, I explain how the modern envelope of seawater conditions found within many coral reef ecosystems (characterised by elevated temperatures, rising pCO 2 , and enriched nutrient levels) are antagonistic toward the dominant host processes that restrict excessive symbiont proliferation. Moreover, I outline a new hypothesis and initial evidence base, which support the suggestion that the additional "excess" zooxanthellae fraction permitted by seawater pCO 2 levels beyond 260 ppmv significantly increases the propensity for symbiosis breakdown ("bleaching") in response to temperature and irradiance extremes. The relevance of this biological threshold is discussed in terms of historical reef extinction events, glacial-interglacial climate cycles and the modern decline of coral reef ecosystems.

A new conceptual model of coral biomineralisation: hypoxia as the physiological driver of skeletal extension

“…Moreover, provided nutrients are not limiting, the zooxanthellae may respond to pH reductions (i.e. increase pCO 2 ) by increasing their background density within the host, particularly in the cooler months (RodolfoMetalpa et al, 2010;Wooldridge, 2012). The new biomineralisation model predicts that the increased respiratory cost of these outcomes will act to enhance skeletal extension rates (but not necessarily skeletal density or overall calcification rates) for as long as sufficient host energy reserves exist.…”

A new conceptual model of coral biomineralisation: hypoxia as the physiological driver of skeletal extension

“…Moreover, provided nutrients are not limiting, the zooxanthellae may respond to pH reductions (i.e. increase pCO 2 ) by increasing their background density within the host, particularly in the cooler months (Rodolfo-Metalpa et al, 2010;Wooldridge, 2012). The new biomineralisation model predicts that the increased respiratory cost of these outcomes will act to enhance skeletal extension rates (but not necessarily skeletal density or overall calcification rates) for as long as sufficient host energy reserves exist.…”

“…Notwithstanding these areas of uncertainty, the interpretive capacity provided by the new model is noteworthy, since it provides a fresh standpoint for utilising skeletal ultrastructure chronologies as instructors of past, present and future challenges to the viability of the coral-algae endosymbiosis. This new standpoint warns that the precipitous (globalwide) declines in the extension rate of coral skeletons (see e.g., De'ath et al, 2009;Tanzil et al, 2009;Bak et al, 2009) is symptomatic of an emerging envelope of environmental conditions -characterised by elevated SST, rising pCO 2 and enriched inorganic nutrient levels -that is unfavourable to the near-future persistence of the coral-algae endosymbiosis (sensu Wooldridge 2009aWooldridge , 2009bWooldridge , 2010Wooldridge , 2012. Future testing and refinement of the ideas presented within this paper offers considerable hope for developing further insights into tackling the climate-induced demise of coral-algae symbioses and the reefs they construct.…”

Coral hosts actively accumulate inorganic carbon for Symbiodiniaceae to maintain symbiotic relationships

“…Coral reefs are recognized as having the highest levels of primary productivity (1500-5000 gC m -2 a -1 ) in nature 1 , while growing in shallow oligotrophic waters 2,3 . The high productivity of coral reefs is widely attributed to their symbiotic relationship with the microalgal zooxanthellae 4,5,6 .…”