Potential Costs of Acclimatization to a Warmer Climate: Growth of a Reef Coral with Heat Tolerant vs. Sensitive Symbiont Types

Abstract

One of the principle ways in which reef building corals are likely to cope with a warmer climate is by changing to more thermally tolerant endosymbiotic algae (zooxanthellae) genotypes. It is highly likely that hosting a more heat-tolerant algal genotype will be accompanied by tradeoffs in the physiology of the coral. To better understand one of these tradeoffs, growth was investigated in the Indo-Pacific reef-building coral Acropora millepora in both the laboratory and the field. In the Keppel Islands in the southern Great Barrier Reef this species naturally harbors nrDNA ITS1 thermally sensitive type C2 or thermally tolerant type D zooxanthellae of the genus Symbiodinium and can change dominant type following bleaching. We show that under controlled conditions, corals with type D symbionts grow 29% slower than those with type C2 symbionts. In the field, type D colonies grew 38% slower than C2 colonies. These results demonstrate the magnitude of trade-offs likely to be experienced by this species as they acclimatize to warmer conditions by changing to more thermally tolerant type D zooxanthellae. Irrespective of symbiont genotype, corals were affected to an even greater degree by the stress of a bleaching event which reduced growth by more than 50% for up to 18 months compared to pre-bleaching rates. The processes of symbiont change and acute thermal stress are likely to act in concert on coral growth as reefs acclimatize to more stressful warmer conditions, further compromising their regeneration capacity following climate change.

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

“…It is well established that distinct zooxanthellae phenotypes have different metabolic demands (Fitt, 1985). It is interesting to note that corals harbouring "stress" tolerant clade D1 zooxanthellae have reduced skeletal growth rates (Jones and Berkelmans, 2010); which has been interpreted as meaning that D1 is a suboptimal symbiont, despite its stress tolerance (Jones and Berkelmans, 2010). The new biomineralisation model suggests caution with this inference, since the lower skeletal growth rates may actually indicate that the coral host is undertaking less "work" to maintain the benefits of the established symbiosis.…”

Investigating the causes and consequences of symbiont shuffling in a multi-partner reef coral symbiosis under environmental change

“…Partner-switching following a bleaching event may occur by recovery with different symbionts that are better suited to the prevailing conditions [4,11], an idea termed the Adaptive Bleaching Hypothesis (ABH; 'adaptive' referring to a beneficial trait that can be positively selected for; [12]). In particular, recovery with heat-tolerant symbionts can increase resistance to future thermal bleaching [8], but at a potential energetic cost [13][14][15][16]. Investigations of the ABH have revealed that, although corals sometimes change their symbionts [8,9,[17][18][19][20][21], this does not always occur: sometimes corals recover with the same symbiont community they had prior to stress [22][23][24][25][26].…”

Differential coral bleaching—Contrasting the activity and response of enzymatic antioxidants in symbiotic partners under thermal stress

“…1). However, the relative importance of different symbionts to the holobiont's resilience is difficult to assess due to complex interactions and trade-offs (Fitt et al, 2001;Abrego et al, 2008;Fitt et al, 2009;Jones and Berkelmans, 2010). Moreover, while some intracellular sites and effects of thermal damage in the host and symbiont have been identified, there is no consensus about the causal chain of events and the supposed 'weak link' in the symbiosis.…”