Endophytic Fungi Associated with Seaweeds as Potential Producers of Antimicrobial Compounds

The urgent need for new molecules capable of targeting multidrug-resistant microorganisms has redirected bioprospecting efforts toward less explored marine environments and their associated communities, such as marine mycobiota. This study aimed to evaluate the antimicrobial potential of endophytic fungi isolated from various seaweed species and infer the mechanisms of action and chemical profile of most potent fungal extracts. Simultaneously, it intended to contribute to a better understanding of ecological requirements of each fungal species. Thirty-one ascomycetous fungi were recovered from the seaweed hosts, with a higher representation of the genera Aspergillus and Penicillium. Only two fungal species revealed notable antimicrobial abilities: Penicillium rubens synthesised extracts with pronounced antibacterial effects against Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus, while extracts of Penicillium brevicompactum inhibited the growth of Candida albicans. Although similar activities have been described in a few previous studies, this is the first report of a markedly inhibitory effect against a pathogenic yeast demonstrated by P. brevicompactum. Moreover, the mechanisms of action underlying these activities were explored for the first time in this study. Specifically, the antimicrobial activities demonstrated by fungal extracts were likely mediated by membrane potential imbalance and DNA damage, associated with the presence of L-norvaline and formylphosphonate. Despite the overall consistency of the results, slight to pronounced differences were also observed in the antibacterial activities and mechanisms of action of the extracts from P. rubens strains. These findings support the hypothesis that marine fungal species possess, and express distinct metabolic profiles shaped by micro- and/or macro-environmental factors.