Eco-physiological Responses of Aquatic Fungi to Three Global Change Stressors Highlight the Importance of Intraspecific Trait Variability

Drying Shapes Aquatic Fungal Community Assembly by Reducing Functional Diversity

Aquatic fungi are highly diverse organisms that play a critical role in global biogeochemical cycles. Yet it remains unclear which assembly processes determine their co-occurrence and assembly patterns over gradients of drying intensity, which is a common stressor in fluvial networks. Although aquatic fungi possess drying-specific adaptations, little is known about how functional similarity influences co-occurrence probability and which functional traits are sorted by drying. Using field data from 15 streams, we investigated how co-occurrence patterns and assembly processes responded to drying intensity. To do so, we determined fungal co-occurrence patterns, functional traits that best explain species co-occurrence likelihood, and community assembly mechanisms explaining changes in functional diversity over the drying gradient. Our results identified 24 species pairs with positive co-occurrence probabilities and 16 species pairs with negative associations. The co-occurrence probability was correlated with species differences in conidia shape and fungal endophytic capacity. Functional diversity reduction over the drying gradient is generally associated with non-random abiotic filtering. However, the assembly processes changed over the drying gradient, with random assembly prevailing at low drying intensity and abiotic filtering gaining more importance as drying intensifies. Collectively, our results can help anticipate the impacts of global change on fungal communities and ecosystem functioning.

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

Fungi are among the most abundant and diverse organisms on Earth and play pivotal roles in global carbon processing, nutrient cycling and food webs. Despite their abundant and functional importance, little is known about the patterns and mechanisms governing their community composition in intermittent rivers and ephemeral streams, which are the most common fluvial ecosystems globally. Thus far, it is known that aquatic fungi have evolved various life-history strategies and functional adaptations to cope with drying. Nevertheless, some of these adaptations have a metabolic cost and trade-offs between growth, reproduction and dispersion that may affect ecosystem functioning. Thus, understanding their ecological strategies along a gradient of drying is crucial to assess how species will respond to global change and to identify meaningful taxa to maintain ecosystem functions. 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 their specialization and affinity over a gradient of drying. In addition, we estimated whether species niches are good predictors of two key ecosystem processes: organic matter decomposition and fungal biomass accrual. Overall, we found that annual drying duration and frequency were the most influential variables upon species niche differentiation across the 15 studied streams. Our cluster analysis identified four drying niche-based groups with contrasting distributions and responses over the drying gradient: drying-sensitive, partly tolerant to drying, generalist, and drying-resistant specialist. In addition, we found that species belonging to the drying specialist group showed a weak contribution to both ecosystem processes, 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 ecosystem functioning and to maintain biogeochemical cycles despite their ability to cope with drying.

Multiple drying aspects shape dissolved organic matter composition in intermittent streams

Water availability is a fundamental driver of biogeochemical processing in highly dynamic ecosystems such as intermittent rivers and ephemeral streams (IRES), which are recognized as the most common fluvial ecosystem globally. Because of their global extent, IRES have a remarkable contribution to organic matter processing, which is expected to intensify as climate change and water extraction expand IRES extension. Nevertheless, the effect of the complexity of the drying process on river biogeochemistry remains unclear. This study investigated how drying aspects affect the dissolved organic carbon (DOC) concentration and composition in 35 streams along a wide flow-intermittence gradient in the NE Iberian Peninsula. To do that, four drying aspects: annual drying duration, annual frequency, duration of the last drying event, and time since the last drying event were characterized. Results showed that DOC concentration and the contribution of humic-like compounds were positively associated with intensifying drying conditions. In addition, protein-like compounds decreased over the drying gradient. More specifically, changes in DOC concentration were driven mainly by annual drying duration, whereas annual drying frequency and the duration of the last drying event jointly explained dissolved organic matter composition. These results suggest that the quantity and composition of dissolved organic matter in streams respond differently to the temporal aspects of the drying process. Our study can help to better anticipate changes in organic matter in the context of climate change.