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.

水生真菌是一种尚未开发的生物，其生态位的多样性仍然未知。地下水生物群落包括广阔但勘探程度不高的栖息地。在这项研究中，我们对冰岛地下水进行了采样，这是一个独特的系统，由于火山活动，该系统已被分离并隔离到不同的盆地中了数百万年。目的是探索盆地的真菌多样性及其与地表水的联系。我们通过ITS元条形码技术筛选了11个冰岛地下水泉及其新兴地表水的基于DNA的真菌多样性，揭示了每个地下水盆地的物种周转量确实不同。此外，包括许多嗜冷酵母样和子囊真菌的地下水类群似乎可以作为地表水的恒定接种物。不过，与地表水（分别为41％和22％）相比，地下水泉的早期分叉，初级水生真菌谱系（10％）和完全未知的谱系（13％）的顺序比例要低，这可能是由于合适的较低多样性这些假定的寄生谱系的宿主。我们的研究结果突出了被忽视的生物体在解决生态学，营养循环和全球变化生物学方面的总体研究问题中的重要性。