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Abstract Corals have complex symbiotic associations that can be influenced by the environment. We compare symbiotic dinoflagellate (family: Symbiodiniaceae) associations and the microbiome of five scleractinian coral species from three different reef habitats in Palau, Micronesia. Although pH and temperature corresponded with specific host‐Symbiodiniaceae associations common to the nearshore and offshore habitats, bacterial community dissimilarity analyses indicated minimal influence of these factors on microbial community membership for the coralsCoelastrea aspera,Psammocora digitata, andPachyseris rugosa. However, coral colonies sampled close to human development exhibited greater differences in microbial community diversity compared to the nearshore habitat for the coral speciesCoelastrea aspera,Montipora foliosa, andPocillopora acuta, and the offshore habitat forCoelastrea aspera, while also showing less consistency in Symbiodiniaceae associations. These findings indicate the influence that habitat location has on the bacterial and Symbiodiniaceae communities comprising the coral holobiont and provide important considerations for the conservation of coral reef communities, especially for island nations with increasing human populations and development. 

Abstract Coral reefs are declining worldwide, yet some coral populations are better adapted to withstand reductions in pH and the rising frequency of marine heatwaves. The nearshore reef habitats of Palau, Micronesia are a proxy for a future of warmer, more acidic oceans. Coral populations in these habitats can resist, and recover from, episodes of thermal stress better than offshore conspecifics. To explore the physiological basis of this tolerance, we compared tissue biomass (ash-free dry weight cm⁻²), energy reserves (i.e., protein, total lipid, carbohydrate content), and several important lipid classes in six coral species living in both offshore and nearshore environments. In contrast to expectations, a trend emerged of many nearshore colonies exhibiting lower biomass and energy reserves than colonies from offshore sites, which may be explained by the increased metabolic demand of living in a warmer, acidic, environment. Despite hosting different dinoflagellate symbiont species and having access to contrasting prey abundances, total lipid and lipid class compositions were similar in colonies from each habitat. Ultimately, while the regulation of colony biomass and energy reserves may be influenced by factors, including the identity of the resident symbiont, kind of food consumed, and host genetic attributes, these independent processes converged to a similar homeostatic set point under different environmental conditions. 

Abstract Active chlorophyllafluorometry is a well‐established tool for noninvasively diagnosing coral functional state, but has not yet been developed as a rapid phenotyping (functional screening) platform as for agriculture and forestry. Here, we present a proof‐of‐concept using Light‐Induced Fluorescence Transient‐Fast Repetition Rate fluorometry (LIFT‐FRRf) to identify coral photobiological‐based phenotypes in the context of rapidly scaling coral propagation practices on the northern Great Barrier Reef. For example, resolving light niche plasticity to inform transplantation, and identifying functionally diverse colonies to maximize stock selection. We first used optically diverse laboratory‐reared corals and coral endosymbiont (Symbiodiniaceae) isolates to develop a phenotyping approach integrating FRRf instantaneous kinetic parameters (light harvesting, electron turnover rates) and light‐dependent parameters (dynamic “quenching” terms, saturating light intensity [E_K]). Subsequent field‐based LIFT‐FRRf phenotyping of coral from a selective (2‐4 m depth) reef habitat revealed that widely topographically dispersed platingAcroporataxa exhibited broad light niche plasticity (E_Kvariance) underpinned by multiple phenotypes that were predominantly differentiated by minimum electron turnover capacity; fluorometer configurations that cannot resolve kinetic parameters will thus likely have more limited capacity to resolve phenotypes. As such, platingAcroporahave broad propagation potential in terms of multiple functional variants for stock and across diverse light environments (growth, transplantation). In contrast, coral taxa (Pocillopora verrucosa,Echinopora lamellosa) with relatively restricted topographic dispersion exhibited less light niche plasticity and only single phenotypes, thereby imposing more constraints for propagation. We discuss the core technical, operational, and conceptual steps required to develop more sophisticated coral phenotyping platforms. 

Abstract Reef‐building corals in the genusPoritesare one of the most important constituents of Indo‐Pacific reefs. Many species within this genus tolerate abnormally warm water and exhibit high specificity for particular kinds of endosymbiotic dinoflagellates that cope with thermal stress better than those living in other corals. Still, during extreme ocean heating, somePoritesexhibit differences in their stress tolerance. While corals have different physiological qualities, it remains unknown whether the stability and performance of these mutualisms is influenced by the physiology and genetic relatedness of their symbionts. We investigated two ubiquitous Pacific reef corals,Porites rusandPorites cylindrica, from warmer inshore and cooler offshore reef systems in Palau. While these corals harbored a similar kind of symbiont in the genusCladocopium(within the ITS2C15 subclade), rapidly evolving genetic markers revealed evolutionarily diverged lineages corresponding to eachPoritesspecies living in each reef habitat. Furthermore, these closely relatedCladocopiumlineages were differentiated by their densities in host tissues, cell volume, chlorophyll concentration, gross photosynthesis, and photoprotective pathways. When assessed using several physiological proxies, these previously undifferentiated symbionts contrasted in their tolerance to thermal stress. Symbionts withinP.cylindricawere relatively unaffected by exposure to 32℃ for 14 days, whereasP.ruscolonies lost substantial numbers of photochemically compromised symbionts. Heating reduced the ability of the offshore symbiont associated withP.rusto translocate carbon to the coral. By contrast, high temperatures enhanced symbiont carbon assimilation and delivery to the coral skeleton of inshoreP.cylindrica. This study indicates that large physiological differences exist even among closely related symbionts, with significant implications for thermal susceptibility among reef‐buildingPorites. 

Symbiotic mutualisms are essential to ecosystems and numerous species across the tree of life. For reef-building corals, the benefits of their association with endosymbiotic dinoflagellates differ within and across taxa, and nutrient exchange between these partners is influenced by environmental conditions. Furthermore, it is widely assumed that corals associated with symbionts in the genusDurusdiniumtolerate high thermal stress at the expense of lower nutrient exchange to support coral growth. We traced both inorganic carbon (H¹³CO₃^–) and nitrate (¹⁵NO₃^–) uptake by divergent symbiont species and quantified nutrient transfer to the host coral under normal temperatures as well as in colonies exposed to high thermal stress. Colonies representative of diverse coral taxa associated withDurusdinium trenchiiorCladocopiumspp. exhibited similar nutrient exchange under ambient conditions. By contrast, heat-exposed colonies withD. trenchiiexperienced less physiological stress than conspecifics withCladocopiumspp. while high carbon assimilation and nutrient transfer to the host was maintained. This discovery differs from the prevailing notion that these mutualisms inevitably suffer trade-offs in physiological performance. These findings emphasize that many host–symbiont combinations adapted to high-temperature equatorial environments are high-functioning mutualisms; and why their increased prevalence is likely to be important to the future productivity and stability of coral reef ecosystems.