Acclimatization of massive reef-building corals to consecutive heatwaves

Abstract

Reef-building corals typically live close to the upper limits of their thermal tolerance and even small increases in summer water temperatures can lead to bleaching and mortality. Projections of coral reef futures based on forecasts of ocean temperatures indicate that by the end of this century, corals will experience their current thermal thresholds annually, which would lead to the widespread devastation of coral reef ecosystems. Here, we use skeletal cores of long-lived Porites corals collected from 14 reefs across the northern Great Barrier Reef, the Coral Sea, and New Caledonia to evaluate changes in their sensitivity to heat stress since 1815. High-density ‘stress bands’—indicative of past bleaching—first appear during a strong pre-industrial El Niño event in 1877 but become significantly more frequent in the late twentieth and early twenty-first centuries in accordance with rising temperatures from anthropogenic global warming. However, the proportion of cores with stress bands declines following successive bleaching events in the twenty-first century despite increasing exposure to heat stress. Our findings demonstrate an increase in the thermal tolerance of reef-building corals and offer a glimmer of hope that at least some coral species can acclimatize fast enough to keep pace with global warming.

Long‐Term Impacts of the 1997–1998 Bleaching Event on the Growth and Resilience of Massive Porites Corals From the Central Red Sea

“…Geochemistry, Geophysics, Geosystems events, resulting in an increase in thermal tolerance (T. M. DeCarlo et al, 2019;Maynard et al, 2008). This could happen through either physiological adaptation, such as symbiont shuffling (Baker, 2003), bacterial restructuring (Ziegler et al, 2017), or regulation in gene expression (Kenkel & Matz, 2017)-in particular front loading (Barshis et al, 2013) whereby corals alter the relative levels of expression of housekeeping genes governing normal physiological maintenance and environmentally responsive genes that help protect the coral against unfavorable conditions (Dixon et al, 2018).…”

“…Although in many places El Niño and La Niña result in opposing temperature trends (e.g., colder than average temperatures in response to La Niña and warmer than average temperatures in response to El Niño), there are regions where both La Niña and El Niño can result in thermal anomalies of the same sign (positive or negative). Despite recent progress (T. M. DeCarlo et al, 2019;Gintert et al, 2018;Guest et al, 2012;Palmer, 2018;Pratchett et al, 2013;Putnam et al, 2017) not enough is known about the possible long-term adaptation or acclimatization of corals to thermal stress on the timescales relevant to global warming and how such adaptive responses could potentially improve the resistance of coral calcification, and thus the persistence of the coral reef framework, to climate change. This event resulted in mass coral bleaching followed by a drastic shift in coral community composition (Furby et al, 2013).…”

“…Bleaching events not only result in the mass mortality of corals but also negatively affect the rates of calcification of the surviving individuals, threatening the future persistence of these unique ecosystems (Cantin & Lough, 2014;D'Olivo et al, 2013). Despite recent progress (T. M. DeCarlo et al, 2019;Gintert et al, 2018;Guest et al, 2012;Palmer, 2018;Pratchett et al, 2013;Putnam et al, 2017) not enough is known about the possible long-term adaptation or acclimatization of corals to thermal stress on the timescales relevant to global warming and how such adaptive responses could potentially improve the resistance of coral calcification, and thus the persistence of the coral reef framework, to climate change. Consequently, it is imperative that we develop a better understanding of how coral reefs will respond to periods of severe thermal stress as these events become more frequent and intense.…”

Can Herbivore Management Increase the Persistence of Indo-Pacific Coral Reefs?

“…Although the potential for management to be effective in this regard is often framed in terms of resilience, which implies a return to predisturbance states, a more realistic management target might be "coral reef persistence, " i.e., extending the timeframe over which there are reefs capable of providing ecosystem services, and which are sufficiently intact to recover. Essentially, increased persistence would buy time for the larger problems to be dealt with, for coral acclimation or adaption to occur (Couch et al, 2017;Coles et al, 2018;DeCarlo et al, 2019), or for other solutions to be realized (Kennedy et al, 2013;Anthony et al, 2017).…”

Coral bleaching susceptibility is predictive of subsequent mortality within but not between coral species

“…These data indicate that environmental history of individuals and populations plays a significant role in determining coral responses to stress, and is likely driven by a combination of both acclimatization (Bay et al , 2013; Bay and Palumbi, 2015; Kenkel and Matz, 2016) and adaptation (Barshis et al , 2013, 2018; Palumbi et al , 2014; Matz et al , 2018). On a global scale, there is recent evidence that the temperature threshold for coral bleaching has risen in correspondence with global warming, suggesting widespread acclimatization and/or adaptation (Coles et al , 2018; DeCarlo et al , 2019). Alternatively, differences in the composition of coral communities between reefs can also influence bleaching extent due to species-specific differences in thermal tolerance (Rowan et al , 1997; Berkelmans and Van Oppen, 2006; Sampayo et al , 2008; Swain et al , 2016; Edmunds, 2018).…”