Specificity trumps flexibility-location-based stable associations between Symbiodiniaceae genera and Platygyra verweyi (Scleractinia; Merulinidae)

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.

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

In the past decade, the frequency of mass coral bleaching events has increased due to seawater temperature anomalies persisting for longer periods. Coral survival from temperature anomalies has been based on how each species in each location responds to stress, which is unique to individual species and may be due to the way stressful experiences accumulate through time in the form of ecological and physiological memory. A deeper understanding of ecological and physiological memory in corals is necessary to understand their survival strategies into the future. Laboratory experiments can help us simulate seawater temperatures experienced by corals in the past and compare their responses to those of the present and future. In this study, we sampled corals with different life history traits from one location perturbed by seawater temperature incursions (variable site) and from a second, relatively undisturbed location (stable site). We sampled across two seasons to observe the responses to bleaching threshold temperatures in the past (1998-29 °C), present (2018-31 °C), and future (2050-33 °C). Corals were healthy at 29 °C and 31 °C, but a fast-growing, temperature-susceptible coral species experienced high mortality at 33 °C compared to a slow-growing, temperature-resistant coral species. Moreover, corals from the variable site and during the spring season fared better under temperature stress. The results of this study provide insight into the possible role of life-history traits on coral's response to seasons and locations in terms of memory to long-term and short-term thermal anomalies and climate change.

The seasonal investigation of Symbiodiniaceae in broadcast spawning, Acropora humilis and brooding, Pocillopora cf. damicornis corals

The density and diversity of Symbiodiniaceae associated with corals can be influenced by seasonal changes . This study provided the first annual investigation of Symbiodiniaceae density and diversity associated with Acropora humilis and Pocillopora cf. damicornis corals in the Gulf of Thailand using both zooxanthellae cell count and next-generation sequencing (ITS-1, ITS-2 regions) techniques, respectively. The results from this study indicated that zooxanthellae cell densities in both coral species differ significantly. The number of zooxanthellae was negatively correlated with the physical environment variable (light intensity). The diversity within A. humilis consisted of two genera, Cladocopium (Cspc_C3: 56.39%, C3w: 33.62%, C93type1: 4.42% and Cspf: 3.59%) and a small amount of Durusdinium (D1: 1.03%) whereas P. cf. damicornis was found to be 100% associated with Durusdinium (D1: 95.58%, D6: 1.01% and D10: 2.7%) suggesting that each coral species may select their appropriate genus/species of Symbiodiniaceae in response to local environmental stressors. The results of this study provided some information on the coral-Symbiodiniaceae relationship between seasons, which may be applied to predict the potential adaptation of corals in localized reef environments.

Homogenization of Endosymbiont Communities Hosted by Equatorial Corals during the 2016 Mass Bleaching Event

Thermal stress drives the bleaching of reef corals, during which the endosymbiotic relationship between Symbiodiniaceae microalgae and the host breaks down. The endosymbiont communities are known to shift in response to environmental disturbances, but how they respond within and between colonies during and following bleaching events remains unclear. In 2016, a major global-scale bleaching event hit countless tropical reefs. Here, we investigate the relative abundances of Cladocopium LaJeunesse & H.J.Jeong, 2018 and Durusdinium LaJeunesse, 2018 within and among Pachyseris speciosa colonies in equatorial Singapore that are known to host both these Symbiodiniaceae clades. Bleached and unbleached tissues from bleaching colonies, as well as healthy colonies, during and following the bleaching event were sampled and analyzed for comparison. The nuclear ribosomal internal transcribed spacer (ITS) regions were separately amplified and quantified using a SYBR Green-based quantitative polymerase chain reaction (qPCR) method and Illumina high-throughput sequencing. We found Cladocopium to be highly abundant relative to Durusdinium. The relative abundance of Durusdinium, known to be thermally tolerant, was highest in post-bleaching healthy colonies, while bleached and unbleached tissues from bleaching colonies as well as tissue from healthy colonies during the event had depressed proportions of Durusdinium. Given the importance of Durusdinium for thermal tolerance and stress response, it is surprising that bleached tissue showed limited change over healthy tissue during the bleaching event. Moreover, colonies were invariably dominated by Cladocopium during bleaching, but a minority of colonies were Durusdinium-dominant during non-bleaching times. The detailed characterization of Symbiodiniaceae in specific colonies during stress and recovery will provide insights into this crucial symbiosis, with implications for their responses during major bleaching events.