Abstract
Interest in algal endophytic fungi continues to grow as their ecological importance and potential applications in medicine, aquaculture, and environmental biotechnology become increasingly evident. This study used a culture-based approach to characterize the diversity and community structure of endophytic fungi associated with Sargassum thunbergii at five coastal sites on Jeju Island and to examine their relationships with marine environmental factors. Based on isolated and identified strains, we compared among-site community differences and links to environmental variables. In total, 235 isolates representing 94 species across 54 genera were obtained, with Ascomycota comprising 97.9% of all isolates. Community composition differed markedly by site, with high prevalence of several genera, including Didymella and Neocosmospora. α-diversity was higher at western sites (Gosan, Hyeopjae) and lower at northeastern sites (Gimnyeong, Onpyeong), and β-diversity analyses also separated communities among sites. Ordination and constrained analyses identified mean monthly water temperature and dissolved oxygen as the environmental factors most closely associated with community variation. Notably, Neocosmospora solani occurred at all sites, whereas many species were restricted to single locations. These findings are consistent with the idea that the physicochemical characteristics of Jeju's coastal waters may contribute to regional differences in S. thunbergii endophyte communities. This work provides a baseline for understanding alga-fungus interactions around Jeju and highlights the potential of endophyte diversity as a sensitive indicator of coastal environmental change.