Giulia Puntin

Coral bleaching, the breakdown of the symbiosis between the coral host and endosymbiotic microalgae, is the main cause of widespread coral reef degradation. Current visual assessment methods for coral health, including color reference cards, are constrained by subjective human color perception and limited resolution. Red, green, blue (RGB) color channel analysis through digital photography provides a rapid, noninvasive, and standardized approach for estimating key physiological parameters. While image analysis is extensively used in plant studies, the relationship between coral color and physiological parameters during bleaching requires further attention. In this study, we analyzed the correlation between Symbiodiniaceae cell density and total chlorophyll content across three coral species ( Acropora muricata , Pocillopora verrucosa , and Stylophora pistillata ) with 19 color indices currently established as predictors of chlorophyll content in plants. We exposed corals to three bleaching treatments (short‐term and long‐term heat stress and menthol bleaching) and correlated physiological parameters with color indices to identify the index with the strongest linear correlation. We found that the red channel had the strongest and most consistent linear correlation with Symbiodiniaceae cell density and chlorophyll content across species ( R ² values up to 0.97), enabling a direct interpretation of relative changes in channel intensity values as changes in tissue parameters. Additionally, training a model with 10–12 samples allows for accurate predictions of Symbiodiniaceae densities, achieving over 95% accuracy, compared to models trained on larger datasets. This research improves image analysis as a reliable, non‐invasive tool for monitoring coral health using RGB data.

Coral biodiversity can enhance community productivity; however, the direct effects of inter- and intraspecific neighbouring coral colonies on productivity remain poorly understood due to the complex interplay of biotic and abiotic factors. Here, we set up a fully controlled incubation experiment, where we quantified the effects of species identity and composition on the photosynthesis, respiration, and calcification as proxies for productivity of nine stony coral species from three families. Baseline productivity varied strongly between species. Several corals responded to neighbouring colonies without direct contact, suggesting that they may detect nearby individuals and potentially adjust their physiological activity. Regardless of whether species increased or decreased productivity, the responses were consistently more pronounced and positive towards conspecific than heterospecific neighbours, indicating potential kin selection effects between members of the same species. Notably, species with lower productivity in monoculture tended to exceed expectations in polyculture, while species that were highly productive in monoculture often underperformed in multispecies assemblages. Our results highlight that contact-free interactions in marine animals may shape biodiversity-productivity effects otherwise known from plant communities. Therefore, neighbour-specific interactions should be considered when designing coral reef restoration approaches, as they may influence the success of different species and assemblages.

The mutualistic symbiosis between stony corals and unicellular algae of the family Symbiodiniaceae forms the base of coral reef ecosystems. However, anthropogenic stressors, such as rising seawater temperatures, cause a breakdown of the coral-algal symbiosis, so-called coral bleaching, which leads to mass mortalities and a rapid loss of coral reefs. To functionally disassemble the coral-algal symbiosis, corals have been artificially rendered aposymbiotic using temperature stress, DCMU, or menthol. As menthol has proven to be an efficient and gentle bleaching agent, we tested four menthol treatments with six commonly investigated stony coral species. The overarching aim was to establish a broadly efficient bleaching protocol as a guide for future investigations. Menthol-induced bleaching was traced with chlorophyll fluorescence and tissue color analyses over time and confirmed as final symbiont cell density two weeks after the last day of menthol treatment. Here we found that the coral species varied greatly in their menthol bleaching tolerance, underlining the importance of establishing bespoke bleaching approaches. Acropora muricata and Stylophora pistillata were efficiently bleached within two days of menthol exposure with symbiont cell reductions of 94 % to 98 %. For Galaxea fascicularis, Montipora digitata and Porites cylindrica, six days of menthol exposure proved most successful in reducing symbiont density by 92 to 97 %. While these coral species suffered no mortality, fragments of Pocillopora verrucosa died or suffered severe necrosis in most of the protocols, making the tested menthol treatments unsuitable for this species. We demonstrate that repeated menthol treatment at low concentrations renders most coral species aposymbiotic within few days without visual or physiological damage. Our study, therefore, provides a guideline for efficient and customized application of menthol bleaching treatments for future coral symbiosis research.

Coral bleaching, the breakdown of the symbiosis between the coral host and endosymbiotic microalgae, is the main cause of widespread coral reef degradation. Current methods for assessing coral health based on visual appearance, such as the use of color reference cards, are limited by subjective human color perception and low resolution. Digital photography with RGB (Red, Green, Blue) color channel analyses offers a fast, non-invasive, and standardized alternative to estimate physiological parameters. However, the link between coral color and physiological parameters during bleaching may vary depending on the type of stressor. While such approaches are extensively used in plant studies, their application in estimating Symbiodiniaceae cell density and chlorophyll content in corals requires further attention. In this study, we analyzed the correlation between Symbiodiniaceae cell density and chlorophyll content across three coral species ( Acropora muricata , Pocillopora verrucosa, and Stylophora pistillata ) with 19 color indices derived from the RGB channels currently established as predictors of chlorophyll content in plants. Corals were exposed to three bleaching conditions (acute short-term and chronic long-term heat stress and menthol bleaching) to identify the best color indices for assessing coral health through image analysis. We found that the Red index had the strongest linear correlation with symbiont cell density and chlorophyll content across species (R ² up to 0.97), so that relative changes in this color index can be directly interpreted as corresponding changes in tissue parameters. To train a model that predicts symbiont densities of a distinct sample set using the Red index, we found 10 to 12 samples to be sufficient to achieve an accuracy of > 95 % of the models trained on the full datasets. This research contributes to improved image analysis as a reliable and non-invasive tool for monitoring, by providing guidelines for a systematic use of RGB data to interpret coral health.

Dinoflagellates of the family Symbiodiniaceae are main symbionts of diverse marine animals. A large diversity of Symbiodiniaceae also occur beyond the bounds of their multicellular hosts, occupying environmental niches on coral reefs. The link between spatial diversity at ecosystem scale to microhabitats of Symbiodiniaceae within the coral holobiont are largely unknown. Using ITS2 amplicon sequencing, we compared Symbiodiniaceae communities across four environments (seawater, near-reef and distant sediments, and turf algae mats) and two coral microhabitats (tissue and mucus) on a coral reef in the Red Sea. Analysis of ITS2 sequences revealed that coral and environmental habitats were both dominated by the genera Symbiodinium , Cladocopium , and Durusdinium , but environmental habitats additionally harbored Fugacium , Gerakladium , and Halluxium . Each environmental habitat harbored a distinct Symbiodiniaceae community, with 14-27 % exclusive ITS2 sequences. Nonetheless, 17 ITS2 sequences were shared among all habitat types and were variants defining nearly half of the ITS2 type profiles used to further resolve Symbiodiniaceae identity of coral-based communities. Tissues and surface mucus layers of 49 coral colonies from 17 genera had largely identical Symbiodiniaceae communities. Together with the large difference between environmental Symbiodiniaceae communities and those in the mucus, our results indicate a clear barrier between host-associated and environmental Symbiodiniaceae communities marked by only few shared complete type profiles under normal conditions. It remains to be determined how Symbiodiniaceae community dynamics between coral microhabitats and environmental reservoirs change during coral bleaching events. Monitoring coral colonies after mucus sampling confirmed its suitability for repeated long-term monitoring of coral-associated Symbiodiniaceae communities.

Coral reefs support the livelihood of half a billion people but are at high risk of collapse due to the vulnerability of corals to climate change and local anthropogenic stressors. While understanding coral functioning is essential to guide conservation efforts, research is challenged by the complex nature of corals. They exist as metaorganisms (holobionts), constituted by the association between the (coral) animal host, its obligate endosymbiotic algae (Symbiodiniaceae), and other microorganisms comprising bacteria, viruses, archaea, fungi and other protists. Researchers therefore increasingly turn to model organisms to unravel holobiont complexity, dynamics, and how these determine the health and fitness of corals. The coral Galaxea fascicularis is an emerging model organism for coral symbiosis research with demonstrated suitability to aquarium rearing and reproduction, and to manipulation of the host-Symbiodiniaceae symbiosis. However, little is known about the response of the G. fascicularis microbiome to menthol bleaching—the experimental removal of the Symbiodiniaceae which represents the first step in coral-algal symbiosis manipulation. For this, we characterized the bacterial microbiome of symbiotic and menthol-bleached G. fascicularis originating from the Red Sea and South China Sea (Hong Kong) that were long-term aquarium-reared in separate facilities. We found that the coral-associated microbiomes were composed of relatively few bacterial taxa (10-78 ASVs). Symbiotic polyps (clonal replicates) from the same colony had similar microbiomes, which were distinct from those of other colonies despite co-culturing in shared aquaria. A pattern of seemingly differential response of the bacterial microbiome to menthol bleaching between the two facilities emerged, warranting further investigation into the role of rearing conditions. Nevertheless, the changes in community composition overall appeared to be stochastic suggesting a dysbiotic state. Considering the importance of bleaching treatment of captive corals for symbiosis research, our results—although preliminary—contribute fundamental knowledge for the development of the Galaxea model for coral symbiosis research.

Coral biodiversity has an enhancing but saturating effect on community productivity, however, the direct effects of neighbouring coral colonies on productivity remain poorly understood due to the complex interplay of biotic and abiotic factors. We set up a fully controlled aquarium experiment, in which we quantified the effects of species identity and composition on the productivity of nine stony coral species from three families. Baseline productivity and the response to neighbouring organisms strongly differed between species. Regardless of whether species increased or decreased productivity, the responses were consistently more pronounced and positive towards conspecific than heterospecific neighbours, indicating kin selection effects between closely related species. Species productivity in monoculture and productivity in polyculture were inversely correlated, with inherently less productive species overperforming in polyculture and vice versa. Our results highlight that contact-free interactions in marine animals shape biodiversity-productivity effects otherwise known from plant communities.

A bstract: Coral reefs support the livelihood of half a billion people but are at high risk of collapse due to the vulnerability of corals to climate change and local anthropogenic stressors. While understanding coral functioning is essential to guide conservation efforts, research is challenged by the complex nature of corals. They exist as metaorganisms (holobionts), constituted by the association between the (coral) animal host, its obligate endosymbiotic algae (Symbiodiniaceae), and other microorganisms comprising bacteria, viruses, archaea, fungi and other protists. Researchers therefore increasingly turn to model organisms to unravel holobiont complexity, dynamics, and how these determine the health and fitness of corals. The coral Galaxea fascicularis is an emerging model organism for coral symbiosis research with demonstrated suitability to aquarium rearing and reproduction, and to manipulation of the host-Symbiodiniaceae symbiosis. However, little is known about the response of the G. fascicularis microbiome to menthol bleaching—the experimental removal of the Symbiodiniaceae which represents the first step in coral-algal symbiosis manipulation. For this, we characterized the bacterial microbiome of symbiotic and menthol-bleached G. fascicularis originating from the Red Sea and South China Sea (Hong Kong) that were long-term aquarium-reared in separate facilities. We found that the coral-associated microbiomes were composed of relatively few bacterial taxa (10-78 ASVs). Symbiotic polyps (clonal replicates) from the same colony had similar microbiomes, which were distinct from those of other colonies despite co-culturing in shared aquaria. A pattern of seemingly differential response of the bacterial microbiome to menthol bleaching between the two facilities emerged, warranting further investigation into the role of rearing conditions. Nevertheless, the changes in community composition overall appeared to be stochastic suggesting a dysbiotic state. Considering the importance of bleaching treatment of captive corals for symbiosis research, our results—although preliminary—contribute fundamental knowledge for the development of the Galaxea model for coral symbiosis research.

Stony corals build the framework of coral reefs, ecosystems of immense ecological and economic importance. The existence of these ecosystems is threatened by climate change and other anthropogenic stressors that manifest in microbial dysbiosis such as coral bleaching and disease, often leading to coral mortality.

Reef-building corals owe their evolutionary success to their symbiosis with unicellular algae (Symbiodiniaceae). However, increasingly frequent heat waves lead to coral mass-bleaching events and pose a serious threat to the survival of reef ecosystems. Despite significant efforts, a mechanistic understanding of coral–algal symbiosis functioning, what leads to its breakdown and what can prevent it, remains incomplete. The main obstacles are low amenability of corals to experimental handling and, owing to its obligatory nature, the difficulties of manipulating the coral–algal association. Indeed, many studies on the symbiotic partnership are conducted on other cnidarian model organisms and their results may therefore not be fully transferable to tropical reef-building corals. Here, we identify the tropical stony coral species Galaxea fascicularis as a novel candidate coral model system. Individual polyps of this species can be separated, enabling highly replicated genotype studies, and are well suited to experimental investigation of the symbiosis as they can be easily and effectively rid of their algal symbionts (bleached). We show that bleached adult individuals can reestablish symbiosis with non-native symbionts, and we report the completion of the gametogenic cycle ex situ, with the successful spawning in aquaria over multiple years. These achievements help overcome several of the major limitations to direct research on corals and highlight the potential of G. fascicularis as an important new model system for investigations of symbiosis functioning and manipulation.