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1. Analysis of a mechanistic model of corals in association with multiple symbionts: within-host competition and recovery from bleaching

   https://doi.org/10.1093/conphys/coac066  

   Brown, Alexandra Lynne; Pfab, Ferdinand; Baxter, Ethan C; Detmer, A Raine; Moeller, Holly V; Nisbet, Roger M; Cunning, Ross (January 2022, Conservation Physiology)

   Cooke, Steven (Ed.)

   Abstract Coral reefs are increasingly experiencing stressful conditions, such as high temperatures, that cause corals to undergo bleaching, a process where they lose their photosynthetic algal symbionts. Bleaching threatens both corals’ survival and the health of the reef ecosystems they create. One possible mechanism for corals to resist bleaching is through association with stress-tolerant symbionts, which are resistant to bleaching but may be worse partners in mild conditions. Some corals have been found to associate with multiple symbiont species simultaneously, which potentially gives them access to the benefits of both stress-sensitive and -tolerant symbionts. However, within-host competition between symbionts may lead to competitive exclusion of one partner, and the consequences of associating with multiple partners simultaneously are not well understood. We modify a mechanistic model of coral-algal symbiosis to investigate the effect of environmental conditions on within-host competitive dynamics between stress-sensitive and -tolerant symbionts and the effect of access to a tolerant symbiont on the dynamics of recovery from bleaching. We found that the addition of a tolerant symbiont can increase host survival and recovery from bleaching in high-light conditions. Competitive exclusion of the tolerant symbiont occurred slowly at intermediate light levels. Interestingly, there were some cases of post-bleaching competitive exclusion after the tolerant symbiont had helped the host recover. 

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2. What makes a winner? Symbiont and host dynamics determine Caribbean octocoral resilience to bleaching.

   https://doi.org/10.1126/sciadv.adj6788  

   Coffroth, Mary Alice; Buccella, Louis A.; Eaton, Katherine M.; Lasker, Howard R.; Gooding, Alyssa T.; Franklin, Harleena (November 2023, Science Advances)

   Unlike reef-building, scleractinian corals, Caribbean soft corals (octocorals) have not suffered marked declines in abundance associated with anthropogenic ocean warming. Both octocorals and reef-building scleractinians depend on a nutritional symbiosis with single-celled algae living within their tissues. In both groups, increased ocean temperatures can induce symbiont loss (bleaching) and coral death. Multiple heat waves from 2014 to 2016 resulted in widespread damage to reef ecosystems and provided an opportunity to examine the bleaching response of three Caribbean octocoral species. Symbiont densities declined during the heat waves but recovered quickly, and colony mortality was low. The dominant symbiont genotypes within a host generally did not change, and all colonies hosted symbiont species in the genusBreviolum.Their association with thermally tolerant symbionts likely contributes to the octocoral holobiont’s resistance to mortality and the resilience of their symbiont populations. The resistance and resilience of Caribbean octocorals offer clues for the future of coral reefs. 

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3. Marine heatwaves depress metabolic activity and impair cellular acid–base homeostasis in reef‐building corals regardless of bleaching susceptibility

   https://doi.org/10.1111/gcb.15622  

   Innis, Teegan; Allen‐Waller, Luella; Brown, Kristen T.; Sparagon, Wesley; Carlson, Christopher; Kruse, Elisa; Huffmyer, Ariana S.; Nelson, Craig E.; Putnam, Hollie M.; Barott, Katie L. (April 2021, Global Change Biology)

   Abstract Ocean warming is causing global coral bleaching events to increase in frequency, resulting in widespread coral mortality and disrupting the function of coral reef ecosystems. However, even during mass bleaching events, many corals resist bleaching despite exposure to abnormally high temperatures. While the physiological effects of bleaching have been well documented, the consequences of heat stress for bleaching‐resistant individuals are not well understood. In addition, much remains to be learned about how heat stress affects cellular‐level processes that may be overlooked at the organismal level, yet are crucial for coral performance in the short term and ecological success over the long term. Here we compared the physiological and cellular responses of bleaching‐resistant and bleaching‐susceptible corals throughout the 2019 marine heatwave in Hawai'i, a repeat bleaching event that occurred 4 years after the previous regional event. Relative bleaching susceptibility within species was consistent between the two bleaching events, yet corals of both resistant and susceptible phenotypes exhibited pronounced metabolic depression during the heatwave. At the cellular level, bleaching‐susceptible corals had lower intracellular pH than bleaching‐resistant corals at the peak of bleaching for both symbiont‐hosting and symbiont‐free cells, indicating greater disruption of acid–base homeostasis in bleaching‐susceptible individuals. Notably, cells from both phenotypes were unable to compensate for experimentally induced cellular acidosis, indicating that acid–base regulation was significantly impaired at the cellular level even in bleaching‐resistant corals and in cells containing symbionts. Thermal disturbances may thus have substantial ecological consequences, as even small reallocations in energy budgets to maintain homeostasis during stress can negatively affect fitness. These results suggest concern is warranted for corals coping with ocean acidification alongside ocean warming, as the feedback between temperature stress and acid–base regulation may further exacerbate the physiological effects of climate change. 

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4. Repeat bleaching of a central Pacific coral reef overthe past six decades (1960–2016)

   https://doi.org/10.1038/s42003-018-01  

   Hannah C. Barkley1, 6 (October 2018, Communications biology)

   The oceans are warming and coral reefs are bleaching with increased frequency and severity,fueling concerns for their survival through this century. Yet in the central equatorial Pacific,some of the world’s most productive reefs regularly experience extreme heat associated with El Niño. Here we use skeletal signatures preserved in long-lived corals on Jarvis Island to evaluate the coral community response to multiple successive heatwaves since 1960. By tracking skeletal stress band formation through the 2015-16 El Nino, which killed 95% of Jarvis corals, we validate their utility as proxies of bleaching severity and show that 2015-16was not the first catastrophic bleaching event on Jarvis. Since 1960, eight severe (>30%bleaching) and two moderate (<30% bleaching) events occurred, each coinciding with ElNiño. While the frequency and severity of bleaching on Jarvis did not increase over this time period, 2015–16 was unprecedented in magnitude. The trajectory of recovery of this historically resilient ecosystem will provide critical insights into the potential for coral reef resilience in a warming world. 

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5. Symbiont shuffling induces differential DNA methylation responses to thermal stress in the coral Montastraea cavernosa

   https://doi.org/10.1111/mec.16246  

   Rodriguez‐Casariego, Javier A.; Cunning, Ross; Baker, Andrew C.; Eirin‐Lopez, Jose M. (November 2021, Molecular Ecology)

   Abstract Algal symbiont shuffling in favour of more thermotolerant species has been shown to enhance coral resistance to heat‐stress. Yet, the mechanistic underpinnings and long‐term implications of these changes are poorly understood. This work studied the modifications in coral DNA methylation, an epigenetic mechanism involved in coral acclimatization, in response to symbiont manipulation and subsequent heat stress exposure. Symbiont composition was manipulated in the great star coralMontastraea cavernosathrough controlled thermal bleaching and recovery, producing paired ramets of three genets dominated by either their native symbionts (genusCladocopium) or the thermotolerant species (Durusdinium trenchi). Single‐base genome‐wide analyses showed significant modifications in DNA methylation concentrated in intergenic regions, introns and transposable elements. Remarkably, DNA methylation changes in response to heat stress were dependent on the dominant symbiont, with twice as many differentially methylated regions found in heat‐stressed corals hosting different symbionts (Cladocopiumvs.D.trenchii) compared to all other comparisons. Interestingly, while differential gene body methylation was not correlated with gene expression, an enrichment in differentially methylated regions was evident in repetitive genome regions. Overall, these results suggest that changes in algal symbionts favouring heat tolerant associations are accompanied by changes in DNA methylation in the coral host. The implications of these results for coral adaptation, along with future avenues of research based on current knowledge gaps, are discussed in the present work. 

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