Establishing the coral Galaxea fascicularis as a model for symbiosis research

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PuntinGiulia-2026-06-29.pdf (9.21 MB)

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2026-04

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DOI:
https://doi.org/10.22029/jlupub-21058

Abstract

Coral reefs, among the planet’s most diverse and productive ecosystems, are under increasing threat from climate change and other anthropogenic stressors. Central to coral reef survival is the health and resilience of the coral holobiont—a complex assemblage comprising the coral animal, its symbiotic microalgae (Symbiodiniaceae), and a dynamic microbial community that includes bacteria, archaea, fungi, and viruses. Despite remarkable research progress, key functional roles of holobiont members remain poorly understood, particularly the mechanisms governing coral health under environmental stress and the factors determining susceptibility to holobiont breakdown, which leads to coral bleaching, disease, and mortality. To close these knowledge gaps, this thesis aimed to establish a convenient yet representative new model organism to research coral holobiont dynamics through the experimental manipulation of host-microbial associations. As such, Galaxea fascicularis (Linnaeus, 1767) was identified as an ideal candidate, serving as the “missing link” between established (non-”true coral”) cnidarian models (such as Hydra, Aiptasia, etc.) and actual reef-building corals, and was evaluated for its suitability as a model system. For this purpose, this work: 1) reviewed the state of model organism-based coral research, identifying the scope of G. fascicularis and helping to refine the direction of the experimental chapters of this thesis (Chapter 2); 2) tested the suitability of G. fascicularis to laboratory studies by evaluating its tractability in terms of long-term aquarium maintenance, reproduction in captivity, and symbiosis manipulation (Chapter 3); 3) characterized the bacterial microbiome’s response to chemically-induced bleaching in long-term aquarium reared colonies (Chapter 4); and 4) discussed and outlined future research directions to further enhance the utility of G. fascicularis to address remaining gaps in the understanding of coral holobiont functioning (Chapter 5). The findings support the practical advantages of using G. fascicularis as a model organism. Key achievements include the species demonstrated compatibility with long-term rearing and reproduction in captivity, including compatibility with simplified experimental systems, physiological stress experiments, and symbiosis manipulation. Protocols for producing aposymbiotic fragments through menthol bleaching were refined, enabling reliable production of aposymbiotic fragments for use in experiments, including subsequent re-establishment of symbiosis with novel Symbiodiniaceae strains. Additionally, an initial evaluation of the bacterial microbiome’s response to bleaching revealed insights into how rearing conditions, host identity, and symbiont type influence microbial community composition. Future directions are discussed (Chapter 5) and include refining methodologies for maintaining bleached corals, expanding the range of symbiont strains used for experimental manipulations, and establishing clonal host lines to minimize genetic variability. Further research should also systematically explore holobiont dynamics under varied environmental and rearing conditions to enhance the ecological relevance and reproducibility of findings. These results contribute to a growing body of research aimed at mitigating coral reef decline and highlight the importance of a holobiont-centered approach to coral conservation, where a deeper understanding of microbial roles could inform strategies to enhance coral resilience.

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Anthology

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https://github.com/sPuntinG/Galaxea_Coral_Model
 https://github.com/sPuntinG/Galaxea_Microbiome_PCI/releases/tag/v1.1

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