Drying shapes the ecological niche of aquatic fungi with implications on ecosystem functioning

Abstract

Fungi are among the most abundant and diverse organisms on Earth and play pivotal roles in global carbon processing,nutrient cycling and foodwebs. Despite their abundant and functional importance, little is known about the patterns andmechanisms governing their community composition in intermittent rivers and ephemeral streams, which are the mostcommon fluvial ecosystems globally. Thus far, it is known that aquatic fungi have evolved various life-history strategiesand functional adaptations to cope with drying.Nevertheless, some of these adaptations have ametabolic cost and tradeoffsbetween growth, reproduction and dispersion thatmay affect ecosystem functioning. Thus, understanding their ecologicalstrategies along a gradient of drying is crucial to assess how species will respond to global change and to identifymeaningful taxa tomaintain ecosystemfunctions. By combining in situ hydrological information with a niche-based approach,we analysed the role of drying in explaining the spatial segregation of fungal species, and we determined theirspecialization and affinity over a gradient of drying. In addition, we estimated whether species niches are good predictorsof two key ecosystem processes: organic matter decomposition and fungal biomass accrual. Overall, we found thatannual drying duration and frequency were the most influential variables upon species niche differentiation across the15 studied streams. Our cluster analysis identified four drying niche-based groups with contrasting distributions and responsesover the drying gradient: drying-sensitive, partly tolerant to drying, generalist, and drying-resistant specialist. Inaddition, we found that species belonging to the drying specialist group showed a weak contribution to both ecosystemprocesses, suggesting trade-offs between drying resistance strategies and the energy invested in growth. Taken together,our results suggest that increased water scarcity may jeopardise the capacity of aquatic fungi to guarantee ecosystemfunctioning and to maintain biogeochemical cycles despite their ability to cope with drying.