Learning from the past is not enough to survive present and future bleaching threshold temperatures

A gauge of coral physiology: re-examining temporal changes in Endozoicomonas abundance correlated with natural coral bleaching

Bacteria contribute to many physiological functions of coral holobionts, including responses to bleaching. The bacterial genus, Endozoicomonas, dominates the microbial flora of many coral species and its abundance appears to be correlated with coral bleaching. However, evidences for decoupling of bleaching and Endozoicomonas abundance changes have also been reported. In 2020, a severe bleaching event was recorded at reefs in Taiwan, providing a unique opportunity to re-examine bleaching-Endozoicomonas association using multiple stony corals in natural environments. In this study, we monitored tissue color and microbiome changes in three coral species (Montipora sp., Porites sp., and Stylophora pistillata) in Kenting National Park, following the bleaching event. All tagged Montipora sp. and Porites sp. recovered from bleaching within 1 year, while high mortality occurred in S. pistillata. Microbiome analysis found no correlation of Endozoicomonas relative abundance and bleaching severity during the sampling period, but found a stronger correlation when the month in which bleaching occurred was excluded. Moreover, Endozoicomonas abundance increased during recovery months in Montipora sp. and Porites sp., whereas in S. pistillata it was nearly depleted. These results suggest that Endozoicomonas abundance may represent a gauge of coral health and reflect recovery of some corals from stress. Interestingly, even though different Endozoicomonas strains predominated in the three corals, these Endozoicomonas strains were also shared among coral taxa. Meanwhile, several Endozoicomonas strains showed secondary emergence during coral recovery, suggesting possible symbiont switching in Endozoicomonas. These findings indicate that it may be possible to introduce Endozoicomonas to non-native coral hosts as a coral probiotic.

Species-specific coral microbiome assemblages support host bleaching resistance during an extreme marine heatwave

Scleractinian assemblages are threatened by marine heat waves with coral survivorship depending on host genetics and microbiome composition. We documented an extreme marine heat wave in Fiji and the response of corals in two thermally stressed reef flats. Through high-throughput amplicon sequencing of 16S and ITS rDNA phylogenetic markers, we assessed coral microbiomes (Symbiodiniaceae, prokaryotes, fungi, and Apicomplexa) of paired bleached and unbleached colonies of four common coral species representative of dominant genera in the South Pacific. While all coral species exhibited one or more pathways to bleaching resistance, harboring assemblages composed primarily of thermally tolerant photosymbionts did not always result in host bleaching resistance. Montipora and Pocillopora species, which associate with diverse Symbiodiniaceae and vertically transmit their photosymbionts, fared better than Acropora, which acquire their photosymbionts from the environment, and Porites, which associate with a narrow photosymbiont assemblage. Prokaryotic and fungal beta diversity did not differ between bleached and unbleached conspecifics, however, the relative abundance of the fungus Malassezia globosa was significantly greater in unbleached colonies of Montipora digitata. Each coral species harbored distinct assemblages of Symbiodiniaceae, prokaryotes, and Apicomplexa, but not fungi, reiterating the importance of host genetics in structuring components of its microbiome. Terrestrial fungal and prokaryotic taxa were detected at low abundance across coral microbiomes, indicating that allochthonous microbial inputs occur, but that coral microbiomes remain dominated by marine microbial taxa. Our study offers valuable insights into the microbiome assemblages associated with coral tolerance to extreme water temperatures.

Baseline dynamics of Symbiodiniaceae genera and photochemical efficiency in corals from reefs with different thermal histories

Ocean warming and marine heatwaves induced by climate change are impacting coral reefs globally, leading to coral bleaching and mortality. Yet, coral resistance and resilience to warming are not uniform across reef sites and corals can show inter- and intraspecific variability. To understand changes in coral health and to elucidate mechanisms of coral thermal tolerance, baseline data on the dynamics of coral holobiont performance under non-stressed conditions are needed. We monitored the seasonal dynamics of algal symbionts (family Symbiodiniaceae) hosted by corals from a chronically warmed and thermally variable reef compared to a thermally stable reef in southern Taiwan over 15 months. We assessed the genera and photochemical efficiency of Symbiodiniaceae in three coral species: Acropora nana, Pocillopora acuta, and Porites lutea. Both Durusdinium and Cladocopium were present in all coral species at both reef sites across all seasons, but general trends in their detection (based on qPCR cycle) varied between sites and among species. Photochemical efficiency (i.e., maximum quantum yield; F_v/F_m) was relatively similar between reef sites but differed consistently among species; no clear evidence of seasonal trends in F_v/F_m was found. Quantifying natural Symbiodiniaceae dynamics can help facilitate a more comprehensive interpretation of thermal tolerance response as well as plasticity potential of the coral holobiont.