The effect of temperature stress on coral–Symbiodinium associations containing distinct symbiont types

Abstract

Several studies have demonstrated that the temperature tolerance of scleractinian reef-building corals is controlled, in part, by hosting physiologically distinct symbiotic algae. We investigated the thermal tolerance of coral-algal associations within seven common species of reef-building corals hosting distinct Symbiodinium subclades collected from Heron Island during experimentally induced bleaching conditions. During experimental heating, photosynthetic fitness was assessed by the dark-adapted yield of PSII (F v /F m ), and excitation pressure across PSII (Q m ) of each coral-algal association using pulse amplitude modulation fluorometry. The onset of bleaching was determined by the measurement of Symbiodinium cell density. Using the ribosomal internal transcribed spacer 2 (ITS-2) region, we showed that Symbiodinium type-coral host associations were temporally and spatially conserved in a high proportion of the colonies sampled within each species. Generally, the species Acropora millepora, Platygyra daedalea, Acropora aspera and Acropora formosa contained Symbiodinium ITS-2 type C3, whereas the species Montipora digitata, Porites cylindrica and Porites lutea contained Symbiodinium type C15. Bleaching susceptibility showed some association with Symbiodinium type, but further research is required to confirm this. Corals hosting C3 Symbiodinium displayed higher reductions in F v /F m during heating compared to their C15 counterparts, irrespective of host species. However, a corresponding reduction in Symbiodinium density was not observed. Nonetheless, A. aspera and A. formosa showed significant reductions in Symbiodinium density relative to controls. This correlated with large increases in Q m and decreases in F v /F m in heated explants. Our results suggest a range of bleaching susceptibilities for the coral species investigated, with A. aspera and A. formosa showing the greatest susceptibility to bleaching and M. digitata showing the lowest bleaching susceptibility. The data provide strong evidence for distinct differences in temperature tolerance between C3 and C15 Symbiodinium types when in-hospite; however, future studies addressing the confounding effect of host species would help to confirm this.

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

“…Our hypothesis was, following the OTB, that (a) antioxidant defences in Symbiodinium will increase in response to elevated temperature and declining photosynthetic performance, followed by (b) an increase in antioxidant defences of the coral host with (c) the occurrence of bleaching in the susceptible coral at high temperatures as a result of (a) and (b). For the tolerant coral M. digitata (with Symbiodinium type C15), previous work has shown that photoprotective measures might be a key element of thermal tolerance (Fisher et al, 2012), so we hypothesised that no significant antioxidant response would occur in either the symbiont or host, consistent with the OTB that identifies the symbiont as the origin of excessive ROS production under thermal stress.…”

“…Data were recorded daily at noon and 30 min after sunset, with the fibre optic positioned at the same spot on the same explant at each time point. Maximum midday excitation pressure on photosystem II (Q m ), hereafter referred to as light pressure, was calculated as Q m = 1 − [(ΔF/F′ m at noon)/(F v /F m at dusk on Day 0)] (as defined by Fisher et al, 2012).…”

“…However, for advanced bleaching states (Day 9), increases in Q m might also be related to a brighter light environment for the remaining symbionts, as a result of a decrease in light absorption and increase in light scattering by the coral skeleton (Enríquez et al, 2005). In contrast to A. millepora, Montipora digitata was thermally robust and moderate reductions in F v /F m and increases in Q m suggest that the coral was within an acceptable range for its protective or photoacclimatory mechanisms, e.g., thermal dissipation of light energy (Gorbunov et al, 2001;Fisher et al, 2012).…”

“…The goal of this study was to contrast the response of major enzymatic antioxidants (ascorbate peroxidase, catalase, catalase peroxidase, and superoxide dismutase) in both partners under thermal stress in a bleaching susceptible (Acropora millepora) and tolerant coral (Montipora digitata), and correlate these processes to the phenotypic bleaching response. Both corals are known to harbour different symbiont types (ITS2-types C3 vs. C15) and previous work has established the varying thermal susceptibility for a number of Indo-Pacific corals harbouring these types at the chosen study site (Fisher et al, 2012). Our hypothesis was, following the OTB, that (a) antioxidant defences in Symbiodinium will increase in response to elevated temperature and declining photosynthetic performance, followed by (b) an increase in antioxidant defences of the coral host with (c) the occurrence of bleaching in the susceptible coral at high temperatures as a result of (a) and (b).…”

Mechanisms of Thermal Tolerance in Reef-Building Corals across a Fine-Grained Environmental Mosaic: Lessons from Ofu, American Samoa

“…Following the 12-months of reciprocal transplantation, MV pool corals transplanted to the HV pool showed an increase in thermal tolerance without a shift in symbiont clade . It is important to note, however, that these studies focused on clade-level differences between pools, which remains too course a resolution for studies of thermal tolerance in corals, as sub-cladal differences in thermal tolerance have been widely documented (Tchernov et al, 2004;Jones et al, 2008;Sampayo et al, 2008;Fisher et al, 2012;Hume et al, 2015).…”