The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change

Abstract

The ability of coral reefs to survive the projected increases in temperature due to global warming will depend largely on the ability of corals to adapt or acclimatize to increased temperature extremes over the next few decades. Many coral species are highly sensitive to temperature stress and the number of stress (bleaching) episodes has increased in recent decades. We investigated the acclimatization potential of Acropora millepora, a common and widespread Indo-Pacific hard coral species, through transplantation and experimental manipulation. We show that adult corals, at least in some circumstances, are capable of acquiring increased thermal tolerance and that the increased tolerance is a direct result of a change in the symbiont type dominating their tissues from Symbiodinium type C to D. Our data suggest that the change in symbiont type in our experiment was due to a shuffling of existing types already present in coral tissues, not through exogenous uptake from the environment. The level of increased tolerance gained by the corals changing their dominant symbiont type to D (the most thermally resistant type known) is around 1-1.5 8C. This is the first study to show that thermal acclimatization is causally related to symbiont type and provides new insight into the ecological advantage of corals harbouring mixed algal populations. While this increase is of huge ecological significance for many coral species, in the absence of other mechanisms of thermal acclimatization/adaptation, it may not be sufficient to survive climate change under predicted sea surface temperature scenarios over the next 100 years. However, it may be enough to 'buy time' while greenhouse reduction measures are put in place.

Variation in photosynthesis and respiration in ­geographically distinct populations of two reef‑building coral species

“…Glynn et al 2001, Berkelmans & van Oppen 2006. We hypothesize that the occurrence of Symbiodinium D in Turbinaria reniformis at Lizard Island is an adaptive response to the higher temperature regime at this site compared with Davies Reef (Ulstrup et al 2006a).…”

“…Partitioning of genetically distinct Symbiodinium communities in conspecific coral populations has been found to correlate with latitude , Rodriguez-Lanetty et al 2001, Savage et al 2002, Ulstrup et al 2006a) as well as with coral bleaching susceptibilities (Glynn et al 2001, Berkelmans & van Oppen 2006, Ulstrup et al 2006a). This suggests that thermal tolerance is controlled by local environmental conditions that corals have adapted to (West & Salm 2003) or that have caused the expression of distinct phenotypes.…”

Corals from the Persian/Arabian Gulf as models for thermotolerant reef-builders: Prevalence of clade C3 Symbiodinium, host fluorescence and ex situ temperature tolerance

“…Therefore, we investigated whether the transfer of P. lobata from Abu Dhabi and Fiji waters to the aquarium resulted in changes to the dominant symbiont, e.g. due to transport stress, changes in envi-ronmental conditions, or due to the exchange of symbionts (Baker et al, 2004;Berkelmans and van Oppen, 2006) with other tank inhabitants. We determined the dominant Symbiodinium associated with P. lobata after 15 months of laboratory culture and compared the composition with those from samples fixed immediately after collection in the field.…”

“…Yet another strategy might be to host symbiont communities dominated by thermotolerant symbionts, either by the uptake of new symbionts after a bleaching event or by changing the relative abundance of symbiont types already present in the host (symbiont shuffling) (Baker, 2001;Baker et al, 2004;Berkelmans and van Oppen, 2006;Buddemeier and Fautin, 1993;Jones et al, 2008;Rowan et al, 1997). However, it is not yet established whether the observed acclimatisation will allow the corals to survive in the long run since only 23% of coral species may be able to change their symbionts (Goulet, 2006(Goulet, , 2007.…”

An assessment of vessel traffic patterns in the Northwestern Hawaiian Islands between 1994 and 2004

“…One primary intent of the NWHI MNM is to serve as a sentinel research site isolated from major local or regionalscale anthropogenic disturbances to examine the effects of global-scale threats to coral reefs such as thermal stress (McWilliams et al, 2005;Berkelmans and van Oppen, 2006), marine diseases (Kim et al, 2005) and seawater acidification (Orr et al, 2005;Guinette et al, 2006). Critical to the role of the MNM as a sentinel research site is the assessment of historic and current levels of anthropogenic activity and their associated impacts on marine ecosystems.…”