Limits to the thermal tolerance of corals adapted to a highly fluctuating, naturally extreme temperature environment

Abstract

Naturally extreme temperature environments can provide important insights into the processes underlying coral thermal tolerance. We determined the bleaching resistance of Acropora aspera and Dipsastraea sp. from both intertidal and subtidal environments of the naturally extreme Kimberley region in northwest Australia. Here tides of up to 10 m can cause aerial exposure of corals and temperatures as high as 37 °C that fluctuate daily by up to 7 °C. Control corals were maintained at ambient nearshore temperatures which varied diurnally by 4-5 °C, while treatment corals were exposed to similar diurnal variations and heat stress corresponding to ~20 degree heating days. All corals hosted Symbiodinium clade C independent of treatment or origin. Detailed physiological measurements showed that these corals were nevertheless highly sensitive to daily average temperatures exceeding their maximum monthly mean of ~31 °C by 1 °C for only a few days. Generally, Acropora was much more susceptible to bleaching than Dipsastraea and experienced up to 75% mortality, whereas all Dipsastraea survived. Furthermore, subtidal corals, which originated from a more thermally stable environment compared to intertidal corals, were more susceptible to bleaching. This demonstrates that while highly fluctuating temperatures enhance coral resilience to thermal stress, they do not provide immunity to extreme heat stress events.

Photosynthetic responses of corals Mussismilia harttii (Verrill, 1867) from turbid waters to changes in temperature and presence/absence of light

“…Their experiments showed significantly different results depending on the time period over which they were carried out: especially were the density of Symbiodinium and F v / F m responses significantly different only in their Trial 1 -when natural seasonal Symbiodinium densities were higher (PUTNAM; EDMUNDS, 2011). Corals growing in environments with large daily temperature variability (up to 7.0 °C) and temperature extremes of up to 37.0 °C were highly susceptible to coral bleaching when exposed to heat stress (SCHOEPF et al, 2015). Corals were highly sensitive to daily average temperatures exceeding their maximum monthly mean by 1 °C for only a few days (SCHOEPF et al, 2015).…”

“…Corals growing in environments with large daily temperature variability (up to 7.0 °C) and temperature extremes of up to 37.0 °C were highly susceptible to coral bleaching when exposed to heat stress (SCHOEPF et al, 2015). Corals were highly sensitive to daily average temperatures exceeding their maximum monthly mean by 1 °C for only a few days (SCHOEPF et al, 2015).…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Greater fluctuations in environmental conditions at highlatitude coral reefs have been hypothesized to increase the environmental tolerance of high-latitude species as observed in tropical corals (Oliver and Palumbi, 2011b;Schoepf et al, 2015), thus potentially increasing their ability to cope with climate change. In high-latitude corals, phenotypic plasticity may support their survival, through diverse coral symbiont communities (Wicks et al, 2010b), enhanced symbiont tolerance to extreme (low) temperatures (Wicks et al, 2010a), enhanced heterotrophic plasticity (Bessell-Browne et al, 2014) and evidence of shifted thermal optima for calcification at cooler temperatures (Ross et al, 2015).…”

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

“…Certainly, the extreme temperature profiles of the HV pool are not unique to Ofu; corals are found in a variety of extreme environments and are exposed to temperatures that would cause bleaching in their conspecifics from other areas (Coles and Riegl, 2013;Kline et al, 2015;Richards et al, 2015;Camp et al, 2017). Wide variation in thermal tolerance and genetic divergence has been reported across latitudes and at large-spatial scales (Middlebrook et al, 2008;Howells et al, 2013;Dixon et al, 2015;Thomas et al, 2017), and it is becoming increasingly clear that locally adapted thermally tolerant pockets of corals exist at fine-spatial scales within a variety of coral reef systems (Goreau and Macfarlane, 1990;Barshis et al, 2010;Castillo et al, 2012;Kenkel et al, 2013bKenkel et al, , 2015Schoepf et al, 2015). For example, Porites astreiodes colonies from the thermally variable inshore patch reef environment of south Florida have greater thermal tolerance than offshore populations less than 10 km away (Kenkel et al, 2013a).…”

“…This functional variation has also been reported at extremely fine-scales; intertidal corals of the remote Kimberley region in Western Australia experience aerial exposure and large daily swings in temperature and populations of Acropora aspera and Dipsastraea sp. have greater thermal tolerance than their subtidal conspecific counterparts 10's of meters away (Schoepf et al, 2015); however the genetic mechanisms remain unknown. Expanding our portfolio of known "thermally resilient" populations will help fine tune our understanding of molecular mechanisms governing thermal tolerance, thereby creating a more robust and potentially predictive understanding.…”