Fungal communities in groundwater springs along the volcanic zone of Iceland

Aquatic fungi are a largely unexplored group of organisms with a still unknown diversity of ecological niches. Groundwater biomes comprise vast but poorly explored habitats. In this study, we sampled the Icelandic groundwater, a unique system that has been separated and isolated into distinct basins for millions of years due to volcanic activity. The aim was to explore the fungal diversity of the basins and its connection with the surface waters. We screened the DNA-based fungal diversity of 11 Icelandic groundwater springs and their emerging surface waters by ITS metabarcoding, revealing that the species turnover was indeed distinct for each groundwater basin. Furthermore, the groundwater taxa, which include many psychrophilic yeast-like and ascomycete fungi, seem to serve as a constant inoculum for the surface water. Nevertheless, the groundwater springs had a lower sequence proportion of early diverging, primary aquatic fungal lineages (10%) and completely unknown lineages (13%) than the surface waters (41% and 22%, respectively), likely explained by the lower diversity of suitable hosts for these presumed parasitic lineages. Our results highlight the importance of overlooked organism groups in the resolution of overarching research questions in ecology, nutrient circulation, and global change biology at large.