The seagrass mycobiome: What happened to fungal associations as plants moved back into the marine environment?

Background/Question/Methods: Seagrasses are globally distributed marine flowering plants that are foundation species in coastal ecosystems. Seagrasses play essential roles as habitats and hatcheries, in nutrient cycling and in protecting the coastline from erosion. Although many studies have focused on seagrass ecology, only a limited number have investigated their associated fungi. Previously we used culture-independent sequencing of the ribosomal internal transcribed spacer (ITS) region to characterize the diversity of fungi associated with the seagrass, Zostera marina (ZM) from Bodega Bay, CA. We found that there were many fungal taxa for which a taxonomic assignment could not be made living on and inside ZM leaves, roots and rhizomes and that these plant tissues harbored distinct fungal communities. We also found evidence of dark septate endophytes (DSE) living inside the leaves of ZM, particularly Colleotrichum species. DSE are known to colonize terrestrial plants and can be opportunistic pathogens. In an effort to expand knowledge of ZM-associated fungi and marine fungi more generally, we used a culture-dependent approach to generate a ZM fungal collection and then phylogenetically placed the 28S rRNA gene region for each isolate in the context of the diversity of fungal isolates from other seagrass species obtained from the literature.

Results/Conclusions: The majority of ZM-associated fungal isolates were taxonomically classified as belonging to the Ascomycota (n = 103), while the rest of isolates were classified as Basidiomycota (n = 4) and Zygomycota (n = 1). Within the Ascomycota, isolates were further identified as belonging to three taxonomic classes: Eurotiomycetes (n = 62), Dothideomycetes (n = 30), and Sordariomycetes (n = 11). We isolated several Colleotrichum sp. from ZM. We sequenced and assembled a draft genome for one representative Colleotrichum isolate and are working to compare this genome with genomes of Colleotrichum sp. found associated with land plants. Additionally, we observed a positive relationship between the number of tissue types and number of media types a fungal genus was isolated from, which we hypothesize is reflective of some marine fungal genera being habitat generalists. This is further supported by preliminary analyses of culture-independent sequencing of the ITS region of fungi associated with ZM on a global scale, which indicate that many ZM fungal isolates are only found at one location, while others are ubiquitous across locations and sample types. This work highlights a need for further studies focusing on marine fungi and the potential importance of these understudied communities to the larger seagrass ecosystem.