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Abstract

Filamentous fungi asymptomatically colonize the inner tissues of macroalgae, yet
their ecological roles remain largely underexplored. Here, we tested if metabolites
produced by fungal endophytes might protect their host against a phylogenetically
broad spectrum of protistan pathogens. Accordingly, the cultivable fungal endophytes
of four brown algal species were isolated and identified based on LSU and SSU
sequencing. The fungal metabolomes were tested for their ability to reduce the infection
by protistan pathogens in the algal model Ectocarpus siliculosus. The most active
metabolomes effective against the oomycetes Eurychasma dicksonii and Anisolpidium
ectocarpii, and the phytomixid Maullinia ectocarpii were further characterized chemically.
Several pyrenocines isolated from Phaeosphaeria sp. AN596H efficiently inhibited the
infection by all abovementioned pathogens. Strikingly, these compounds also inhibited
the infection of nori (Pyropia yezoensis) against its two most devastating oomycete
pathogens, Olpidiopsis pyropiae, and Pythium porphyrae. We thus demonstrate that
fungal endophytes associated with brown algae produce bioactive metabolites which
might confer protection against pathogen infection. These results highlight the potential
of metabolites to finely-tune the outcome of molecular interactions between algae,
their endophytes, and protistan pathogens. This also provide proof-of-concept toward
the applicability of such metabolites in marine aquaculture to control otherwise
untreatable diseases.