Fungal isolates from Thalassia testudinum show bioactivity against the pathogen, Labyrinthula sp.

In the coasts of Florida, the most abundant and ecologically important seagrass is Thalassia testudinum. T. testudinum has undergone several die-offs in the past century, including in the 1930's and late 1980's, which were linked to seagrass wasting disease. This disease is caused in part by the opportunistic pathogens in the genus Labyrinthula, which are usually extensively present in T. testudinum in the field, but only causing mild disease. Little is known about the mechanisms used by seagrasses to resist Labyrinthula infections. Previous studies have shown that seagrass leaves produce metabolites that inhibit Labyrinthula growth. Furthermore, seagrass fungal endophytes also produce antipathogenic metabolites, but these were only tested against human pathogens, and their activity against Labyrinthula sp. is unknown. In this study we aimed to identify whether the fungal communities of the leaves of T. testudinum can potentially aid in pathogen defense against Labyrinthula. Through Illumina amplicon sequencing of the leaf biofilm and endosphere samples, we identified several fungi in the endosphere that are known to produce antipathogenic secondary metabolites. We also cultured and tested the activity of organic extracts from fungal endophytes of healthy T. testudinum leaves against a pathogenic strain of Labyrinthula using disk diffusion assays. We determined that 2 isolates, Trichoderma sp. and Diaporthe sp., strongly inhibited Labyrinthula growth, and 20 isolates moderately inhibited Labyrinthula growth. We identified the likely bioactive compounds of the Trichoderma isolate as peptaibols, and of the Diaporthe isolate as cytosporone B by LC-HR/MS. We confirmed the activity of cytosporone B against Labyrinthula in a separate disk diffusion assay with the commercially purchased compound. Finally, we determined that the most bioactive fungi are likely present in low abundance in the field.