Bacterial communities associated with healthy corals produce antimicrobial compounds that inhibit the colonization and growth of invasive microbes and potential pathogens. To date, however, bacteria-derived antimicrobial molecules have not been identified in reefbuilding corals. Here we report the isolation of an antimicrobial compound produced by Pseudovibrio sp. P12, a common and abundant coral-associated bacterium. This strain was capable of metabolizing dimethylsulfoniopropionate (DMSP), a sulfur molecule produced in high concentrations by reef-building corals and playing a role in structuring their bacterial communities. Bioassay-guided fractionation coupled with nuclear magnetic resonance (NMR) and mass spectrometry (MS), identified the antimicrobial as tropodithietic acid (TDA), a sulfur-containing compound likely derived from DMSP catabolism. TDA was

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Reef-building corals are among the largest producers of dimethylsulfoniopropionate (DMSP), an essential compound in marine biogeochemical cycles. DMSP can be catabolised in coral mucus by a wide diversity of coral-associated bacteria, where it can either be demethylated, leading to the incorporation of sulfur and carbon into bacterial biomass – or cleaved by lyases, releasing the climatically-active gas dimethyl sulfide (DMS). It has been demonstrated that thermal stress increases DMSP concentrations in many coral species, however the effect of increased DMSP availability on coral-associated bacteria has not been explored. Here we performed thermal stress experiments to examine how changes in DMSP availability impact bacterial degradation pathways in the mucus of Acropora millepora. DMSP concentrations increased with temperature, reaching a maximum of 177.3 μM after 10 days of heat stress, which represents the highest concentration of DMSP recorded in any environment to date. Bacterial communities in coral mucus were significantly different from the surrounding seawater, yet they did not vary significantly between temperature or time. However, during thermal stress, when DMSP concentrations increased, a significant increase in the abundance of both the demethylation gene dmdA and the cleavage gene dddP were recorded. Importantly, our results show that for the highest DMSP concentrations recorded (above 30 μM), the cleavage pathway became more abundant than the demethylation pathway. This suggests that under high DMSP concentrations characteristic of heat stress, a larger fraction of the DMSP pool in the coral mucus is likely catabolised through the DMS-producing cleavage pathway.

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Charonia tritonis (Charoniidae), one of the largest marine gastropods and an echinoderm specialist, preys on Crown-of-Thorns starfish (CoTS), a recurring pest that continues to be a leading cause of coral mortality on Indo-Pacific reefs. Widespread historical exploitation has impacted their numbers, with standing populations considered rare throughout their habitat. Their life-stage attributes, i.e., teleplanic larvae, planktotrophic phase spanning years permitting transoceanic dispersal, and recruitment to coral reefs through oceanic influx with intense larval mortality, have likely hindered their recovery. Decline in numbers is hypothesised to account partially for periodic CoTS outbreaks, yet predator-prey dynamics between these two species that might influence this are poorly understood. The C. tritonis excretory secretome elicits a proximity deterrence effect on CoTS, the nature of which is under investigation as a possible tool in CoTS biocontrol scenarios. However, specificity and zone of impact in situ are unknown, and whether the mere presence of C. tritonis and/or predation pressure has any regulatory influence over CoTS populations remains to be established. The fundamental taxonomy and distinctive characteristics, biology and ecology of C. tritonis is summarized, and knowledge gaps relevant to understanding their role within coral reefs identified. Information is provided regarding exploitation of C. tritonis across its habitat, and prospects for conservation interventions, including captive rearing and stock enhancement to repopulate local regions, are discussed. Its predator-prey ecology is also examined and potential to mitigate CoTS considered. Recommendations to direct future research into this predator and for its inclusion in a CoTS integrated pest management strategy to improve coral reef health are offered.

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