Upper-mesophotic and shallow reef corals exhibit similar thermal tolerance, sensitivity and optima

Abstract

The physiology of ectotherms living in marine environments is strongly influenced by their local thermal experience. Scleractinian corals living near their thermal optimums are increasingly vulnerable to bleaching and mortality as oceanic heat waves increase globally. Mesophotic coral ecosystems (MCEs) below 30 m depths are characteristically cooler than adjacent shallow water reefs, which according to theory should result in differential metabolic responses to temperature between depths. How local temperatures influence physiological responses in mesophotic corals is poorly understood. We compared thermal sensitivities of four coral species between a shallow (5-10 m) and upper-mesophotic (30-35 m) reef in Bermuda. Thermal performance curves (TPC) were measured in laboratory mesocosms for four common coral species (Diploria labyrinthiformis, Orbicella franksi, Montastraea cavernosa and Porites astreoides) across a wide range of temperatures (19-36). Our results indicate that the maximum rate of gross photosynthetic (GP) performance (GP-P max ) and the mean overall photosynthetic rates (GP-lnc) varied significantly among species. In contrast, thermal sensitivity (P max , T opt , lnc, E, Eh, or Th) did not vary between depths for conspecifics except for deactivation energy (GP-Eh) in D. labyrinthiformis. Additionally, gross respiration (R) did not differ among species or between depths for any thermal metric. Similar metabolic responses between depths suggest that local adaptation and/or acclimatization to different thermal conditions is likely not occurring. Instead, upper-mesophotic corals in Bermuda do not have lower bleaching thresholds than shallow water conspecifics, but similar thermal sensitivities supporting the potential for MCEs to function as a thermal refuge.

Missing the Reef for the Corals: Unexpected Trends Between Coral Reef Condition and the Environment at the Ecosystem Scale

“…Some datasets required calculation of benthic cover from raw data (i.e., photoquadrat point counts, line-point transect counts, and line transects). Observations shallower than 30 m were retained, because that is the depth threshold typically invoked to separate shallow reefs from deeper, mesophotic reefs (e.g., Gould et al, 2021). The most recent year's observation was retained when multitemporal observations existed for a given geographic site.…”

Assessment of temperature optimum signatures of corals at both latitudinal extremes of the Red Sea

“…Just like thermal thresholds, above which corals are predicted to undergo bleaching, T opt can vary between species within regions (e.g. Gould et al ., 2021 ; Jurriaans and Hoogenboom, 2019 ) and across regions for similar species (e.g. Sawall et al ., 2014 ; Ulstrup et al ., 2006 ).…”

“… Castillo et al ., 2012 ; Oliver and Palumbi, 2011 ), TPCs are used to assess the range of survivable temperatures of an organism and characterize its response to temperature variability within this range ( Padfield et al ., 2021 ; Sinclair et al ., 2016 ). As such, TPCs can assist in predicting the evolution in species richness and diversity and the functional impacts of elevation of seawater temperatures ( Aichelman et al ., 2019 ; Gould et al ., 2021 ; Padfield et al ., 2021 ; Silbiger et al ., 2019 ). Recent studies implementing the TPC approach succeeded in quantifying differences in temperature acclimatization of various coral species between environments with different temperature regimes (e.g.…”

“…Recent studies implementing the TPC approach succeeded in quantifying differences in temperature acclimatization of various coral species between environments with different temperature regimes (e.g. Aichelman et al ., 2019 ; Gould et al ., 2021 ; Jurriaans and Hoogenboom, 2019 ; Silbiger et al ., 2019 ; Rodolfo-Metalpa et al ., 2014 ). For example, the Caribbean reef-building coral Orbicella franski displayed higher metrics derived from TPCs [ T opt , activation energy Eh , rate at a standardized temperature b(T c ) ] in the warmer waters of Panama compared to populations of the same species acclimatized to the cooler waters of Bermuda ( Silbiger et al ., 2019 ).…”

Plasticity of Porites astreoides Early Life History Stages Suggests Mesophotic Coral Ecosystems Act as Refugia in Bermuda

“…Additionally, variable bleaching responses and different host gene expression patterns between inshore and offshore populations exposed to heat stress suggest that P. astreoides may be able to acclimatize or adapt to thermal stress (Kenkel et al, 2013a,b;Wong et al, 2021). Across a depth gradient, this species was also found to have similar thermal tolerance ranges, despite variable thermal histories, suggesting high levels of plasticity to temperature (Gould et al, 2021). Consequently, P. astreoides is expected to increase in relative abundance compared to other Atlantic scleractinian species (Green et al, 2008), and is therefore an ideal study species as it will likely be an important contributor to the future community composition of Caribbean and Western Atlantic reefs.…”