Coral host physiology and symbiont dynamics associated with differential recovery from mass bleaching in an extreme, macro-tidal reef environment in northwest Australia

Coral heat tolerance under variable temperatures: Effects of different variability regimes and past environmental history vs. current exposure

“…This is particularly highlighted by the fact that exposure to heat stress equivalent to ~ 27 DHDs in corals maintained at constant daily temperatures elicited only minor negative responses as they did not suffer significant color loss or mortality and maintained F v / F m and calcification rates (but they had 45% lower P / R ratios). While we observed a similar negative effect of temperature variability on heat tolerance of intertidal A. aspera in a previous lab experiment (Schoepf et al 2019), it contradicts our findings of enhanced bleaching resilience for this coral population in the field (Schoepf et al 2015; Le Nohaïc et al 2017; Jung et al 2021) (Table S7). So how can we reconcile these seemingly contradictory findings, and why does high‐frequency temperature variability not always promote heat tolerance, as also observed in some other studies (Putnam and Edmunds 2011; Klepac and Barshis 2020)?…”

“…For example, it has been shown that transplantation from highly variable to moderately variable temperature environments can lead to reduced heat tolerance in Acropora hyacinthus over 12–27 months (Palumbi et al 2014). The lack of an observed effect of preconditioning suggests that intertidal A. aspera corals from the Kimberley region are long‐term acclimatized or locally adapted to highly variable temperatures, which has led to enhanced bleaching resilience compared to conspecifics from the subtidal where daily temperature variability is much more moderate (Schoepf et al 2020; Jung et al 2021) (Table S7).…”

“…Five visibly healthy coral colonies (labeled A, B, C, D, E) were collected in April 2016 from the intertidal environment at Shell Island (16°28′45.8″S, 123°2′41.3″E), where they regularly experience prolonged aerial exposure during spring low tides (up to hours), high light levels (up to ~ 2000 μ mol m −2 s −1 ) and extreme daily temperature fluctuations of up to 7°C, with short‐term maxima of up to 38°C (Dandan et al 2015; Schoepf et al 2015, 2020). In contrast to subtidal conspecifics, intertidal A. aspera have a naturally higher heat tolerance (Dandan et al 2015; Schoepf et al 2015) and survive/recover better from bleaching events (Le Nohaïc et al 2017; Schoepf et al 2020; Jung et al 2021). Both intertidal and subtidal corals are dominated by symbionts from the genus Cladocopium (previously clade C; LaJeunesse et al 2018) (Schoepf et al 2015, 2020); however, symbiont community composition nevertheless differs between intertidal and subtidal corals and may be linked to their differential bleaching resilience (Jung et al 2021).…”

“…In contrast to subtidal conspecifics, intertidal A. aspera have a naturally higher heat tolerance (Dandan et al 2015; Schoepf et al 2015) and survive/recover better from bleaching events (Le Nohaïc et al 2017; Schoepf et al 2020; Jung et al 2021). Both intertidal and subtidal corals are dominated by symbionts from the genus Cladocopium (previously clade C; LaJeunesse et al 2018) (Schoepf et al 2015, 2020); however, symbiont community composition nevertheless differs between intertidal and subtidal corals and may be linked to their differential bleaching resilience (Jung et al 2021). Monthly average temperatures at Shell Island range from ~ 25°C to 31°C, and the bleaching threshold was experimentally established to be ~ 32°C, approximately 1°C above local MMM sea surface temperatures (SST) (Schoepf et al 2015).…”

“…The corals originated from the macrotidal Kimberley region in NW Australia where the world's largest tropical tides (up to 12 m) expose corals on shallow, intertidal reefs to short‐term maximum temperatures of up to 38°C, daily temperature variability of up to 8°C, and frequent aerial exposure at low tide (Dandan et al 2015; Schoepf et al 2015). Long‐term acclimatization and/or adaptation to these extreme conditions have resulted in enhanced heat tolerance, survival, and recovery capacity post bleaching (Schoepf et al 2015; Le Nohaïc et al 2017; Jung et al 2021). However, these corals were nevertheless unable to increase their heat tolerance after 6 months of acclimation to 1°C warmer temperatures in the lab; furthermore, under these warming levels, daily temperature variability actually lowered their ability to cope with a heat stress event compared to corals exposed to constant daily temperatures (Schoepf et al 2019).…”