Fungal Biodiversity Mediates the Effects of Drying on Freshwater Ecosystem Functioning

Kelp Culture Enhances Coastal Biogeochemical Cycles by Maintaining Bacterioplankton Richness and Regulating Its Interactions

As an important carbon sink, seaweed cultivation plays a vital role in controlling global climate change. However, most studies have been focused on the seaweed itself, and knowledge of bacterioplankton dynamics in seaweed cultivation activities is still limited. Here, a total of 80 water samples were obtained from a coastal kelp cultivation area and adjacent non-culture area in the seedling and mature stages. The bacterioplankton communities were analyzed using high-throughput sequencing of bacterial 16S rRNA genes, and the microbial genes involving biogeochemical cycles were measured by a high-throughput quantitative PCR (qPCR) chip. Seasonal variations in alpha diversity indices of bacterioplankton were found, and kelp cultivation mitigated this decline in biodiversity from the seedling to the mature stage. Further beta diversity and core taxa analyses revealed that the maintenance of biodiversity was due to kelp cultivation favoring the survival of rare bacteria. Comparisons of gene abundances between coastal water with and without kelp cultivation showed a more powerful capacity of biogeochemical cycles induced by kelp cultivation. More importantly, a positive relationship between bacterial richness and biogeochemical cycling functions was observed in samples with kelp cultivation. Finally, a co-occurrence network and pathway model indicated that the higher bacterioplankton biodiversity in kelp culture areas compared to non-mariculture regions could balance the microbial interactions to regulate biogeochemical cycles and thus enhance the ecosystem functions of kelp cultivation coasts. The findings of this study allow us to better understand the effects of kelp cultivation on coastal ecosystems and provide novel insights into the relationship between biodiversity and ecosystem functions. IMPORTANCE In this study, we tried to address the effects of seaweed cultivation on the microbial biogeochemical cycles and the underlying relationships between biodiversity and ecosystem functions. We revealed clear enhancement of biogeochemical cycles in the seaweed cultivation areas compared to the non-mariculture coasts at both the beginning and ending of the culture cycle. Moreover, the enhanced biogeochemical cycling functions in the culture areas were found to contribute to the richness and interspecies interactions of bacterioplankton communities. The findings of this study allow us to better understand the effects of seaweed cultivation on coastal ecosystems and provide novel insights into the relationship between biodiversity and ecosystem functions.

Extreme weather events threaten biodiversity and functions of river ecosystems: evidence from a meta-analysis

Both gradual and extreme weather changes trigger complex ecological responses in river ecosystems. It is still unclear to what extent trend or event effects alter biodiversity and functioning in river ecosystems, adding considerable uncertainty to predictions of their future dynamics. Using a comprehensive database of 71 published studies, we show that event - but not trend - effects associated with extreme changes in water flow and temperature substantially reduce species richness. Furthermore, event effects - particularly those affecting hydrological dynamics - on biodiversity and primary productivity were twice as high as impacts due to gradual changes. The synthesis of the available evidence reveals that event effects induce regime shifts in river ecosystems, particularly affecting organisms such as invertebrates. Among extreme weather events, dryness associated with flow interruption caused the largest effects on biota and ecosystem functions in rivers. Effects on ecosystem functions (primary production, organic matter decomposition and respiration) were asymmetric, with only primary production exhibiting a negative response to extreme weather events. Our meta-analysis highlights the disproportionate impact of event effects on river biodiversity and ecosystem functions, with implications for the long-term conservation and management of river ecosystems. However, few studies were available from tropical areas, and our conclusions therefore remain largely limited to temperate river systems. Further efforts need to be directed to assemble evidence of extreme events on river biodiversity and functioning.

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.

Flow Intermittency Affects Leaf Decomposition and Benthic Consumer Communities of Alpine Streams: A Case Study along the Po River

Streams and rivers are becoming increasingly intermittent in Alpine regions due to the global climate change and related increases of local water abstractions, making it fundamental to investigate the occurrence of supraseasonal drying events and their correlated effects. We aimed to investigate leaf litter decomposition, the C:N ratio of the litter, and changes in associated macroinvertebrate communities in three reaches of the Po River: One upstream, consistently perennial, a perennial mid-reach with high hydrological variability, and an intermittent downstream reach. We placed leaf litter bags of two leaf types—chestnut and oak; both showed comparable decomposition rates, but the remaining litter mass was different and was attributed to the C:N ratio and palatability. Furthermore, (1) in perennial reaches, leaf litter decomposed faster than in the intermittent ones; (2) in intermittent reaches, the C:N ratio showed a decreasing trend in both leaf types, indicating that drying affected the nitrogen consumption, therefore the conditioning phase; (3) associated macroinvertebrate communities were richer and more stable in perennial reaches, where a higher richness and abundance of EPT taxa and shredders was observed. Our results suggest that the variations in the hydrology of mountain streams caused by global climate change could significantly impact on functional processes and biodiversity of benthic communities.