Gas Powered Reefs: Exploring the Nature and Variability of the Coral Volatilome

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Biogenic volatile organic compounds (BVOCs) are a diverse class of chemicals well studied in terrestrial ecosystems, where they play important ecological and physiological roles, while influencing atmospheric processes and climate. As the health of tropical coral reefs rapidly deteriorates, BVOCs can potentially mitigate stress events directly by functioning as antioxidants and indirectly through atmospheric chemistry. Yet, the composition of the coral volatilome (total BVOCs) and its potential importance for coral reef functioning is understudied. I investigated the BVOC capacity of corals systematically – from microbial symbiont to coral holobiont. I examined Symbiodiniaceae, the photosynthetic microbes that provide nutrients to corals. Across six species, I detected 82 BVOCs and using a targeted thermal stress experiment on two key species, Cladocopium goreaui and Durusdinium trenchii, I identified significant changes in specific BVOCs, with the majority of these significantly increasing following stress. Given that Symbiodiniaceae are associated with complex and abundant bacterial communities, I described the microbiome of 18 species of Symbiodiniaceae. Three bacterial genera were consistently present across all Symbiodiniaceae species: Labrenzia, Marinobacter and an unclassified Chromatiaceae. I then characterised the volatilome of Labrenzia sp. 21p and Marinobacter adhaerens HP15 and detected 35 BVOCs between them. The composition of the Labrenzia sp. volatilome significantly changed following incubation in Symbiodiniaceae exudate and additional changes were detected in the quantities of individual BVOCs in both bacterial species. This indicates the ability of Symbiodiniaceae-associated bacteria to alter their BVOC production in the presence of Symbiodiniaceae-derived chemicals. I finally characterised the volatilomes of two common, heat-sensitive, reef-building corals (Acropora intermedia and Pocillopora damicornis) during a simulated heat stress event. I detected 88 BVOCs from these holobionts however, the BVOC richness of both species decreased following stress (by 41% in A. intermedia and 62% in 𝘗. damicornis) and the abundance of multiple BVOCs significantly decreased. As such, this study revealed that thermal stress influences the coral holobiont by decreasing the richness and abundance of the volatilome. From microbe to holobiont, I consistently detected a wide range of BVOCs, many of which are implicated in stress response, signalling, antimicrobial defence or potentially impacting local climate. I demonstrated that corals and therefore potentially coral reefs, are significant sources of numerous BVOCs that should receive increased attention to determine their biological functions. This work highlights the diversity of BVOCs produced by corals and their constituents, and provides important new knowledge for the successful management and conservation of these threatened ecosystems.
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