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. 2023 Feb 21;120(8):e2202388120.
doi: 10.1073/pnas.2202388120. Epub 2023 Feb 13.

Increased dominance of heat-tolerant symbionts creates resilient coral reefs in near-term ocean warming

Ana M Palacio-Castro  1   2   3 , Tyler B Smith  4 , Viktor Brandtneris  4 , Grace A Snyder  1 , Ruben van Hooidonk  2   3 , Juan L Maté  5 , Derek Manzello  6 , Peter W Glynn  1 , Peggy Fong  7 , Andrew C Baker  1
Affiliations

Increased dominance of heat-tolerant symbionts creates resilient coral reefs in near-term ocean warming

Ana M Palacio-Castro et al. Proc Natl Acad Sci U S A. .

Abstract

Climate change is radically altering coral reef ecosystems, mainly through increasingly frequent and severe bleaching events. Yet, some reefs have exhibited higher thermal tolerance after bleaching severely the first time. To understand changes in thermal tolerance in the eastern tropical Pacific (ETP), we compiled four decades of temperature, coral cover, coral bleaching, and mortality data, including three mass bleaching events during the 1982 to 1983, 1997 to 1998 and 2015 to 2016 El Niño heatwaves. Higher heat resistance in later bleaching events was detected in the dominant framework-building genus, Pocillopora, while other coral taxa exhibited similar susceptibility across events. Genetic analyses of Pocillopora spp. colonies and their algal symbionts (2014 to 2016) revealed that one of two Pocillopora lineages present in the region (Pocillopora "type 1") increased its association with thermotolerant algal symbionts (Durusdinium glynnii) during the 2015 to 2016 heat stress event. This lineage experienced lower bleaching and mortality compared with Pocillopora "type 3", which did not acquire D. glynnii. Under projected thermal stress, ETP reefs may be able to preserve high coral cover through the 2060s or later, mainly composed of Pocillopora colonies that associate with D. glynnii. However, although the low-diversity, high-cover reefs of the ETP could illustrate a potential functional state for some future reefs, this state may only be temporary unless global greenhouse gas emissions and resultant global warming are curtailed.

Keywords: Durusdinium glynnii; El Niño; Pocillopora; coral bleaching; thermal stress.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Heat stress and Pocillopora cover dynamics at Uva Island during three major El Niño events. (A) DHW during the 1982 to 1983, 1997 to 1998, and 2015 to 2016 El Niño. The yellow dashed line marks the 4 DHW threshold for likelihood of bleaching, and the red dashed line marks eight DHW threshold for likelihood of mortality. Red crosses denote the dates when colony assessments of bleaching and mortality were performed, and the corresponding maximum heat stress levels experienced in the 6-mo prior to the time of the survey (Fig. 2). (B) Pocillopora spp. cover estimated using aggregated data from three datasets. The circles represent the mean values from a dataset for each month-year. The gray line represents the mean Pocillopora cover from all three datasets. The boxplots show the median (second quartile), first and third quartiles, with the bars extending from the minimum to the maximum mean coral cover recorded per dataset. Gray vertical bars demarcate the periods of very strong El Niño events (ONI > 2 °C).
Fig. 2.
Fig. 2.
Proportion of colonies affected (±95% CIs) by heat stress in March 1983, October 1997, March 1998, August 2015, and April 2016. Letters in the panels represent non-overlapping Tukey Honestly Significant Difference (HSD) groups (P < 0.05) for comparisons among years inside each species. Colonies affected included those exhibiting bleaching in >10% of the tissue and/or mortality in >50% of the tissue. No Millepora colonies were found in the studied area by the 1998 sampling. Specific sampling dates and the preceding DHW conditions at Uva Island at the time of the sampling are shown in Fig. 1A. Comparisons among species inside each year are shown in SI Appendix, Fig. S2.
Fig. 3.
Fig. 3.
Pocillopora symbiont community composition and colony condition before and during the 2015 to 2016 El Niño. (A) Temperature and DHW conditions during the sampling periods. The upper plot and left-hand axis show the mean daily temperature (solid lines) and bleaching threshold (dashed lines) for each dataset. Lower plot and right-hand axis show DHW calculated from each data source. The blue vertical lines represent the periods when the coral samples were collected. (B) Symbiont community trajectory (proportion of D. glynnii) in tagged colonies of Pocillopora type 1 (circles) and type 3 (triangles). Color of samples indicates colony health condition as in panel c. Points have been horizontally jittered to allow visualization of all colonies in each time point. Cladocopium quantification is performed at the genus level, but each Pocillopora lineage hosts different Cladocopium types. (C) Distribution of type 1 and type 3 colonies hosting different D. glynnii proportions before the El Niño (August 2014, left-hand panel), during the peak of heat stress (August 2015, center), and after El Niño (April 2016, right-hand panel). Filling colors represent the colony condition.
Fig. 4.
Fig. 4.
Algal symbiont abundance (mean S/H ± 95% confidence intervals) in Pocillopora colonies sampled before and during the 2015 to 2016 El Niño. Only Pocillopora type 3 had significant reductions in total S/H cell ratio with respect to its values in 2014 (P < 0.05). Colors in each panel represent the dominant symbiont genus (>50% of the community) at each sampling point. Numbers (n) in the panels represent the number of colonies used in the cell ratio estimations.
Fig. 5.
Fig. 5.
Conceptual model of Pocillopora spp. cover and D. glynnii prevalence under repeated heat stress (DHW). (A) Observed and hypothesized changes in Pocillopora spp. cover and D. glynnii prevalence during the 1982 to 1983, 1997 to 1998, and 2015 to 2016 El Niño heatwaves at Uva Island reef. The orange and green dots represent the proportion of Pocillopora colonies hosting D. glynnii at Uva Island based on colony sampling. The blue dots represent the increases in D. glynnii that could be attributed to differential mortality of Cladocopium-dominated colonies based on the Pocillopora cover loss recorded after the 1997 to 1998 and 2016 to 2016 heatwaves. The remaining D. glynnii increases not explained by mortality are attributed to symbiont shuffling. (B) Hypothetical coral cover trajectories under the SSP 5-8.5 scenario (black), as well as the SSP 5-8.5 scenario + an increment in bleaching threshold in reefs composed of Durusdinium-dominated Pocillopora by 1.25 °C (dark gray) and 1.5 °C (light gray). The yellow and red crosses demarcate the approximate year when Uva Island reef would start experiencing four and eight DHW, respectively, under the three different scenarios. Warming trends from 2020 to 2050 are likely to be punctuated by climatological anomalies, such as El Niño events, that could lead to additional bleaching/mortality and further increases in D. glynnii dominance.
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References

    1. Carpenter K. E., et al. , One-third of reef-building corals face elevated extinction risk from climate change and local impacts. Science 321, 560–563 (2008). - PubMed
    1. Lough J. M., Anderson K. D., Hughes T. P., Increasing thermal stress for tropical coral reefs: 1871–2017. Sci. Rep. 8, 6079 (2018). - PMC - PubMed
    1. Hughes T. P., et al. , Global warming and recurrent mass bleaching of corals. Nature 543, 373–377 (2017), 10.1038/nature21707. - DOI - PubMed
    1. Hughes T. P., et al. , Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science 359, 80–83 (2018). - PubMed
    1. Skirving W. J., et al. , The relentless march of mass coral bleaching: A global perspective of changing heat stress. Coral Reefs 38, 547–557 (2019).

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