Abstract
The photo-physiological characteristics of thermo-tolerantSymbiodiniumtypes have been postulated to have negative effects on the energetics of the reef corals by reducing fitness. To investigate this, two key and inextricably coupled indicators of fitness, lipids and reproduction, were monitored in colonies of the broadcast-spawning coralAcropora milleporaover a two-year period that included a natural bleaching event. In the absence of bleaching ITS1-type clade D predominant colonies had 26% lower stored lipids compared to C2 colonies. At spawning time, this correlated with 28% smaller eggs in type-D colonies. This energetic disparity is expected to have reduced larval duration and settlement-competency periods in type-D compared to type-C2 colonies. More importantly, irrespective of the effect of genotype, the fitness of all corals was adversely affected by the stress of the bleaching event which reduced prespawning lipids by 60% and halved the number of eggs compared to the previous year. Our results extend work that has shown that direct temperature stress and symbiont change are likely to work in concert on corals by demonstrating that the lipids and reproduction of the reef building corals on tropical reefs are likely to be impaired by these processes as our climate warms.
Highlights
Reef-building corals are vulnerable to climate change as they live in a narrow thermal range and are extremely sensitive to variations
The two colonies that changed from C2 to D before the bleaching retained their new D symbiont types throughout and after the bleaching event
Our results show that climate change may diminish the energy stores and fecundity of reef corals in two separate and independent processes
Summary
Reef-building corals are vulnerable to climate change as they live in a narrow thermal range and are extremely sensitive to variations. An increase of only 1-2◦C above the coral’s normal summer maximum, in combination with light, can cause expulsion of the endosymbiotic algae (zooxanthellae) that live within their tissues, resulting in coral bleaching [1]. Such extremes are expected to occur regularly on reefs within the century meaning that corals must adapt or acclimatize at a rate of 0.2◦C every decade if they are to keep pace with even the lowest predicted temperature rise of 2◦C by 2100 [2]. The phenomenon of symbiont change raises the question: what effects will symbiont change have on holobiont (coral + endosymbionts) energetics and what will this mean to the capacity for coral reefs to recover following disturbance?
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