Coral Bleaching: the role of the host

A review of: Baird AH, Bhagooli R, Ralph PJ, Takahashi S (2008) Coral Bleaching: the role of the host. Trends in Ecology and Evolution, 24 (1): 16-20.

Coral reefs are under threat from rising sea surface temperatures and a strategy that might permit corals to adapt rapidly is to switch to more thermally tolerant symbiotic partners, as discussed in the review titled: Can coral change their zooxanthellae to overcome thermal stress? (Berkelmans et al 2006). The role of the host in coral bleaching has not been fully elucidated and so the authors discuss the evidence and propose several possible response mechanisms.

Photoinhibition results from an accumulation of oxidative stress in photosystem II of the symbiont which can vary between different Symbiodinium clades. However, the holobiont’s response is not always related to the thermal tolerance of the symbiont. For example, Symbiodinium clades C1 and C3 occur in many different host species with divergent responses to increases in temperature.

The authors propose hypothetical strategies which might reduce bleaching damage, such as the production of a range of fluorescent pigments (FPs), observed in 97% of coral species in shallow waters of the Great Barrier Reef. FPs are able to absorb, scatter and dissipate high energy solar radiation through fluorescence, consequently reducing photoinhibition and severity of bleaching. Low densities of FPs are found in bleaching-susceptible taxa such as the pocilloporids and acroporids. Conversely, less-susceptible taxa like poritids and faviids have relatively high densities of FPs and suffer less mortality following a bleaching event. The role of FPs in bleaching reduction is compelling but remains to be tested experimentally.

Mycosporine-like amino acids (MAA) absorb and dissipate UV radiation as heat without forming toxic intermediates. MAAs are synthesised via the shikimic acid pathway, not found in animal cells and presumed to be derived from the symbionts or from heterotrophic feeding. MAAs are more abundant and diverse in host tissues than in freshly isolated symbionts, leading to the speculation that the host stimulates the symbionts to produce a greater diversity of MAAs or the host is able to modify MAAs translocated from the symbionts. Whatever the mechanism involved, it is evident that the host is a major influence on the complement and distribution of MAAs.

Symbiotic hosts have many different types of anti-oxidant enzymes not found in non-symbiotic animals, which act in combination with the antioxidant defences of the symbiont. This is discussed in the review titled: Pairing up Pays off: Symbiosis confers tolerance to environmental stresses (Richier et al 2005).

Heat shock proteins protect from heat and light stress by acting as molecular chaperones, found in higher concentrations in light-acclimatised tissues of certain coral species. Moreover, species which can increase heterotrophic feeding are able to survive experimental bleaching compared to species that cannot, presumably because the host can compensate for the loss of energy resulting from reduced symbiont densities. Heterotrophic feeding may also reduce the host demands on the symbiont, thereby allowing the symbiont to allocate more energy to its own antioxidant defences.

The bleaching phenomenon is seen as the breakdown in communication between the host and symbiont and the consequence of bleaching cannot be fully resolved when considering either in isolation. Corals have been in symbioses with photosynthetic organisms for up to 200 million years and living together presents the holobiont with unique challenges in determining response to stresses. The paper provided possible defence mechanisms in response to environmental factors but did not discuss the role of archaea and bacterial populations which are integral members of the holobiont. The complex interactions between the host and resident microorganisms need to be considered in order to acquire a more accurate perspective.

Additional Reference:

Berkelmans, R., van Oppen, M.J.H. (2006) The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change, Proceedings of the Royal Society, 273: 2305-2312. (Reviewed by Jennifer Mace 24^th October 2011)

Richier S, Furla P, Plantivaux A, Merle PL, Allemand D (2004) Symbiosis-induced adaptation to oxidative stress. Journal of Experimental Biology, 208: 277-285. (Reviewed by Mario Lewis 2^nd November 2011)