Abstract
Aquatic hyphomycetes are key microbial decomposers in freshwater that are capable of producing extracellular enzymes targeting complex molecules of leaf litter, thus, being crucial to nutrient cycling in these ecosystems. These fungi are also able to assimilate nutrients (e.g., nitrogen) from stream water, immobilizing these nutrients in the decomposing leaf litter and increasing its nutritional value for higher trophic levels. Evaluating the aquatic hyphomycete functional genetic diversity is, thus, pivotal to understanding the potential impacts of biodiversity loss on nutrient cycling in freshwater. In this work, the inter- and intraspecific taxonomic (ITS1-5.8S-ITS2 region) and functional (nitrate reductase gene) diversity of 40 aquatic hyphomycete strains, belonging to 23 species, was evaluated. A positive correlation was found between the taxonomic and nitrate reductase gene divergences. Interestingly, some cases challenged this trend: Dactylella cylindrospora (Orbiliomycetes) and Thelonectria rubi (Sordariomycetes), which were phylogenetically identical but highly divergent regarding the nitrate reductase gene; and Collembolispora barbata (incertae sedis) and Tetracladium apiense (Leotiomycetes), which exhibited moderate taxonomic divergence but no divergence in the nitrate reductase gene. Additionally, Tricladium chaetocladium (Leotiomycetes) strains were phylogenetically identical but displayed a degree of nitrate reductase gene divergence above the average for the interspecific level. Overall, both inter- and intraspecific functional diversity were observed among aquatic hyphomycetes.
Highlights
Small forested streams make up the majority of river basins’ water courses in temperate regions [1]
Our data showed a correlation between taxonomic (ITS1-5.8S-ITS2 region) and functional divergence, supporting the hypothesis that fungal species that were phylogenetically closer exhibited higher functional gene relatedness
The results presented in the nitrate reductase phylogenetic tree reflected this close phylogenetic relatedness
Summary
Small forested streams make up the majority of river basins’ water courses in temperate regions [1]. The main sources of energy and carbon (C) in these ecosystems are allochthonous inputs of non-living organic C, leaf litter from riparian vegetation [2,3]. The decomposition of plant litter is mostly driven by microorganisms and invertebrates [4]. A polyphyletic group known as aquatic hyphomycetes stands out as the dominant intermediate in organic matter decomposition and nutrient recycling in headwater stream ecosystems [6,7,8]. Aquatic hyphomycetes owe their competitive advantage to the ability to produce and secrete a vast array of extracellular degradative enzymes that break down plant cell wall polysaccharides, such as cellulose, hemicellulose, pectin and lignin [9,10]
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