Potential antifouling compounds of marine-derived fungi from Hong Kong

Marine biofouling, undesirable growth of marine invertebrates or seaweeds on artificial structures immersed in seawater, is an extremely widespread phenomenon and causes serious operational problems and huge economic loss in the marine industry. The role of bacteria in marine biofilm in modulating the settlement of marine invertebrate larvae has been extensively investigated. However, the effect of marine-derived fungi in the biofilm on the larval settlement of marine invertebrates and the antifouling activity of marine-derived fungi has never been studied. The major goals of this thesis research were to study the extent and possible function of marine-derived fungi on the settlement of invertebrate larvae and to isolate antifouling compounds from marine-derived fungi. Similar to the bacterial community, fungal comunities in the biofilm changed significantly during the development process of the biofilm. However, the fungal community in the biofilm showed clearer seasonal changes than the bacterial community, indicating that the fungal community was more sensitive to the temporal changes in environmental conditions. About 60% of the fungal isolates isolated in this study showed various antibacterial activity against at least one target bacterial species, while 33% of tested fungal isolates showed an inhibitive effect to the larval settlement of Balanus amphitrite, six strains were toxic to barnacle larvae, and only 2 strains induced larval settlement of B. amphitrite. Three fungal isolates, Arthrinium c.f. saccharicola, Cladosporium sp. and Ampelomyces sp., were selected for antifouling compound isolation using bioassay-guided fractionation. In total, 9 bioactive compounds were isolated, including bis(2-ethylhexyl)phthalate (BEHP), 2-phenylethanol, cyclo-(Leu-Pro), cyclo-(Phe-Pro), p-hydroxyphenylethanol, p-hydroxybenzaldehyde, cyclo-(Leu-Hyp), pandangolide 1 and 3-chloro-2,5-dihydroxybenzyl alcohol (CHBA). Among these compounds, CHBA showed the most promising antibacterial and antilarval settlement activity and weak toxicity to barnacle cyprids. These results indicated that marine-derived fungi could be a promising source of natural antifouling compounds. The effects of culture conditions and competitive cultivation with bacteria on mycelial growth, metabolite profile, and antibacterial activity of one of the fungal isolates A. c.f. saccharicola were also investigated. The results showed clear differences in the optimal conditions for achieving maximal mycelial growth, bioactivity and metabolite production of the fungus