Chemical resistance of gorgonian corals against fungal infections

The frequency and impact of diseases affecting corals throughout the Caribb ean have been increasing but little is known about the factors promoting th e emergence and outbreak of disease. A disease caused by a fungal pathogen [Aspergillus sydowii (Thom et Church)] which affects Caribbean sea fall cor als provided an opportunity to examine the efficacy of coral crude extracts in disease resistance. Minimum inhibitory concentration (MIC) assays showe d that of the 20 common gorgonian species in the Florida Keys, extracts fro m 15 species had MICs <15 mg ml(-1) against A. sydowii pathogenic to sea fa ns. Extracts from several species in two gorgonian genera (Pseudoplexaura a nd Pseudopterogorgia) were among the most active, with MICs <10 mg ml(-1) G orgonia ventalina L., one of two sea fan species known to be hosts to A. sy dowii in the field, had an MIC <10 mg ml(-1) suggesting that complete disea se resistance requires more active extracts. For the antifungal compounds t o be effective in situ, they must also occur in sufficiently high concentra tions in living coral tissue. For example, Pseudopterogorgia americana (Gme lin) had comparatively potent extracts but did not have sufficient concentr ations 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 concen tration are considered together (i.e. potency x concentration), several oth er gorgonian corals emerge as likely hosts to A. sydowii. Crude extracts fr om the most active gorgonian species were also effective against two geogra phic variants of A. sydowii pathogenic to sea fans, a non-pathogenic terres trial 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)). Alth ough the potency in these assays did not attain a clinically significant le vel, the potency is comparable to a known antifungal agent, hygromycin B, w hich had an MIC less than or equal to7.5 mg ml(-1) in our assays, highlight ing the potential of these gorgonian corals for bioprospecting.