Chemical resistance of gorgonian corals against fungal infections

Abstract

The frequency and impact of diseases affecting corals throughout the Caribbean have been increasing but little is known about the factors promoting the emergence and outbreak of disease. A disease caused by a fungal pathogen [Aspergillus sydowii (Thom et Church)] which affects Caribbean sea fan corals provided an opportunity to examine the efficacy of coral crude extracts in disease resistance. Minimum inhibitory concentration (MIC) assays showed that of the 20 common gorgonian species in the Florida Keys, extracts from 15 species had MICs < 15 mg ml⁻¹ against A. sydowii pathogenic to sea fans. Extracts from several species in two gorgonian genera (Pseudoplexaura and Pseudopterogorgia) were among the most active, with MICs < 10 mg ml⁻¹. Gorgonia ventalina L., one of two sea fan species known to be hosts to A. sydowii in the field, had an MIC < 10 mg ml⁻¹, suggesting that complete disease resistance requires more active extracts. For the antifungal compounds to be effective in situ, they must also occur in sufficiently high concentrations in living coral tissue. For example, Pseudopterogorgia americana (Gmelin) had comparatively potent extracts but did not have sufficient concentrations in the tissue to be effective. Conversely, Plexaura homomalla Esper extracts were less potent but occurred in high enough concentrations in the tissue to be effective against A. sydowii. When potency and extract concentration are considered together (i.e. potency × concentration), several other gorgonian corals emerge as likely hosts to A. sydowii. Crude extracts from the most active gorgonian species were also effective against two geographic variants of A. sydowii pathogenic to sea fans, a non-pathogenic terrestrial strain of A. sydowii, and three strains of A. flavus Link known to be human, plant, and insect pathogens (MIC range, 7.5 to > 15 mg ml⁻¹). Although the potency in these assays did not attain a clinically significant level, the potency is comparable to a known antifungal agent, hygromycin B, which had an MIC ≤7.5 mg ml⁻¹ in our assays, highlighting the potential of these gorgonian corals for bioprospecting.