Dynamic symbioses reveal pathways to coral survival through prolonged heatwaves

Abstract

Prospects for coral persistence through increasingly frequent and extended heatwaves seem bleak. Coral recovery from bleaching is only known to occur after temperatures return to normal, and mitigation of local stressors does not appear to augment coral survival. Capitalizing on a natural experiment in the equatorial Pacific, we track individual coral colonies at sites spanning a gradient of local anthropogenic disturbance through a tropical heatwave of unprecedented duration. Unexpectedly, some corals survived the event by recovering from bleaching while still at elevated temperatures. These corals initially had heat-sensitive algal symbiont communities, endured bleaching, and then recovered through proliferation of heat-tolerant symbionts. This pathway to survival only occurred in the absence of strong local stressors. In contrast, corals in highly disturbed areas were already dominated by heat-tolerant symbionts, and despite initially resisting bleaching, these corals had no survival advantage in one species and 3.3 times lower survival in the other. These unanticipated connections between disturbance, coral symbioses and heat stress resilience reveal multiple pathways to coral survival through future prolonged heatwaves.

Marine heatwaves depress metabolic activity and impair cellular acid–base homeostasis in reef‐building corals regardless of bleaching susceptibility

“…The tradeoff with greater bleaching resistance in M. capitata for slower recovery from bleaching relative to P. compressa indicates that this strategy is only advantageous when heat stress events are far enough apart for complete recovery to occur. The downsides of this type of tradeoff are apparent when heat stress is particularly prolonged (e.g., 10 months), when corals initially more resistant to bleaching no longer had a survival advantage (Claar et al, 2020). This would indicate that the duration and magnitude of heat stress, as well as that of the subsequent recovery periods, control the tradeoff between resistance and recovery, and may be reversing the relative advantages between species that occur during isolated heat stress events, as has been observed in other reef systems (Grottoli et al, 2014).…”

Transcriptomic plasticity and symbiont shuffling underpin Pocillopora acclimatization across heat-stress regimes in the Pacific Ocean

“…For example, whereas coral genotype is fixed within a colony, photosymbiont community composition can shift over time thereby altering the prevalence of the dominant genotype of the algal partner. Such shifts in the photosymbiont community can occur over short time scales, in response to acute thermal challenges for example (Claar et al 2020), or over longer term periods as a result of competitive exclusion between photosymbiont lineages within a colony (Howells et al 2020). Both mechanisms underpinning symbiont shift can indicate acclimatization of the coral holobiont to its local (thermal) environment.…”

“…In this case, the high DNAJ expression we observed in SVD 5 colonies relative to SVD 4 colonies present on warmer reefs may signal that the SVD 5 holobionts are already at or near their upper heat tolerance limits and may therefore only be able to rely on symbiont shuffling to resist future warming. Evidence from other coral species, however, suggests that symbiont shuffling may only be effective as a "last ditch" strategy to weather acute extreme heating events and that this strategy, while increasing survival in the short-term, may negatively impact resilience in colonies under future warming events (Claar et al 2020).…”

Extending the natural adaptive capacity of coral holobionts

“…Such replacement seems to be underway in the Caribbean, with the spread of the heat-tolerant, potentially invasive Durusdinium trenchii 115,153 . Among Pacific reefs with biannual or annual repeat bleaching, symbiont communities have also already been observed to shift towards dominance of heat-tolerant Symbiodiniaceae 154,155 , although it is unknown whether such shifts persist across generations. However, even the most resilient symbionts are expected to provide no more than 2 °C of additional thermal tolerance to the coral holobiont, a threshold that will likely be exceeded in the tropics within the next 100 years 156 .…”