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Photochemical efficiency ( Fv /Fm) was compared between a common symbiotic bioeroding sponge, Cliona cf. orientalis, and a common reef-builder, Acropora palifera using pulse amplitude modulated fluorescence (PAM). The study was conducted on Sesoko Island, Okinawa, where reefs were severely damaged during previous bleaching episodes. Sponge and coral dinoflagellate symbionts were treated with heat and light in a tank experiment, both in hospite (=still within their host) and isolated from their hosts. We found significant differences for photochemical efficiency of holobionts (=host and symbiont together) compared to the isolate symbionts and over time. All symbionts suffered in isolation and displayed stronger reactions to the treatments, and there was evidence for increasing damage despite returning to control conditions. However, because of large variability of the bi-symbiont coral samples and restrictions of the experimental design, our main results remained inconclusive, with no significant differences between sponge and coral samples and between the different stress treatments. Judging the results based on the uniform trends in the subsets of data, the G-clade sponge symbionts appeared to be more stress tolerant than the C- and D-clade coral symbionts, with no treatment effects in hospite and less damage in isolation compared to the coral symbionts, but this is an unconfirmed assumption. Isolated sponge symbionts were very resistant against heat stress, but may have suffered from light stress. In hospite, the latter risk can be countered by the sponge’s 3-dimensional morphology, the endolithic life style that affords shading, and by behavioural adaptation, i.e. the ability to move symbionts away from the source of stress. Overall, C. cf. orientalis symbionts displayed a more stable photochemical efficiency during and after stress than those of A. palifera. Results of this study suggest that with climate change C. cf. orientalis might have a better survival potential than A. palifera, but further investigations are necessary. [ABSTRACT FROM AUTHOR]

Background: Endozoicomonas bacteria symbiosis with various marine organisms is hypothesized as a potential indicator of health in corals. Although many amplicon analyses using 16S rRNA gene have suggested the diversity of Endozoicomonas species, genome analysis has been limited due to contamination of host-derived sequences and difficulties in culture and metagenomic analysis. Therefore, the evolutionary and functional potential of individual Endozoicomonas species symbiotic with the same coral species remains unresolved. Results: In this study, we applied a novel single-cell genomics technique using droplet microfluidics to obtain single-cell amplified genomes (SAGs) for uncultured coral-associated Endozoicomonas spp. We obtained seven novel Endozoicomonas genomes and quantitative bacterial composition from Acropora tenuis corals at four sites in Japan. Our quantitative 16S rRNA gene and comparative genomic analysis revealed that these Endozoicomonas spp. belong to different lineages (Clade A and Clade B), with widely varying abundance among individual corals. Furthermore, each Endozoicomonas species possessed various eukaryotic-like genes in clade-specific genes. It was suggested that these eukaryotic-like genes might have a potential ability of different functions in each clade, such as infection of the host coral or suppression of host immune pathways. These Endozoicomonas species may have adopted different host adaptation strategies despite living symbiotically on the same coral. Conclusions: This study suggests that coral-associated Endozoicomonas spp. on the same species of coral have different evolutional strategies and functional potentials in each species and emphasizes the need to analyze the genome of each uncultured strain in future coral-Endozoicomonas relationships studies. BqfFwXzdNKfPeBMVktWjTu Video Abstract [ABSTRACT FROM AUTHOR]

High temperatures disrupt coral–algal symbioses in multiple ways, with negative impacts on the physiology of the coral host, the algal symbiont, and the combined holobiont. Most heat stress studies on hard corals have understandably focused on species trends based on the combined observation of multiple individuals to account for phenotypic plasticity among colonies. As the "average coral" continues to decline while sea temperatures rise, the outlier colonies that exhibit neutral or positive responses to heat stress are coming to represent larger proportions of marginal coral populations. These colonies are those most likely to guide the future trajectory of reef ecosystems, but their dynamics are often obscured by aggregate analyses. To directly measure and analyze intraspecific variation in heat stress responses within a natural coral population, we performed aquarium experiments on sixteen colonies of the structurally important branching coral Montipora digitata from Okinawa Island, Japan. We resolved host and symbiont genotypes, exposed replicate coral fragments to ambient or elevated temperature, and monitored stress-driven differences in host calcification, symbiont photochemistry, and colony mortality. Over the 6-month experiment, six colonies appeared to tolerate stress (exhibiting no major physiological changes), seven were sensitive to stress (exhibiting reduced growth), and three expired. Both host and symbiont genotype contributed to this variation. These results demonstrate the degree to which unique M. digitata holobionts may differentially respond to thermal stress in warming oceans and highlight the important role of intraspecific variation in shaping future reef assemblages. [ABSTRACT FROM AUTHOR]