Chemical cues affecting susceptibility of gorgonian corals to fungal infection
Hicks, Melissa Kathryn
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Coral diseases have become more prevalent and destructive over the past 20 years, possibly due to an increase in stressful environmental factors that may weaken corals defenses against disease. Aspergillosis is a disease caused by the fungus Aspergillus sydowii, which apparently infects only two species of gorgonian corals in the Caribbean Ocean (Gorgonia ventalina and G. flabellum). We hypothesized that the differential resistance to infection is caused by differences in chemical defenses among gorgonians. Freeze-dried gorgonian powders and extracts deterred fungal growth, but potencies varied among gorgonian species and among fungi. Extracts and powders generated from G. ventalina all strongly inhibited fungal growth. Since G. ventalina was predicted to have weak antifungal chemical defenses compared to gorgonians not known to suffer from aspergillosis, we concluded that gorgonian susceptibility to fungal infection is determined by factors other than, or in addition to, chemical defenses. In order to investigate specific gorgonian antifungal strategies, we attempted to use bioassay-guided fractionation to isolate antifungal compounds from four gorgonians: Gorgonia ventalina, Briareum asbestinum, Eunicea succinea, and Pseudopterogorgia americana. We succeeded in isolating two antifungal compounds, diastereomers of 9,11-seco-24-hydroxydinosterol, from the gorgonian Pseudopterogorgia americana. This compound was previously identified by other groups, but this study is the first to establish its antifungal activity. At natural concentration, one diastereomer of 9,11-seco-24-hydroxydinosterol inhibited the growth of three different fungi, suggesting that at least this diastereomer may possess broad-spectrum antifungal activity. The results from our survey of gorgonian chemical defenses indicate that susceptibility to aspergillosis cannot be explained by chemical growth inhibition alone. Further areas of investigation include induction of gorgonian chemical defenses, examination of growth-inhibiting mechanisms of antifungal metabolites, and identification of non-chemical factors affecting gorgonians vulnerability to fungal infection.