Distinct sensitivity of fungal freshwater guilds to water quality

Multiple anthropogenic stressors have been shown to impact animal and plant communities in freshwater ecosystems, but the responses of aquatic fungi remain largely unknown. Stressor effects on fungal communities may, however, result in changes of decomposition of plant litter and, thus, impact nutrient cycling, a key process for ecosystem functioning. We tested the impact of increased chloride and sediment levels, as well as reduced water flow velocity, on eukaryotic freshwater communities, with an emphasis on fungi, in a mesocosm experiment. Each of the three stressors was applied individually and in all combinations in a full-factorial design. Litterbags with non-sterilised tree leaves and sterile ceramic tiles were added to the mesocosms, to analyse the responses of communities in decaying plant material and in biofilms. Fungi preferably occurring in biofilms were supposed to represent indigenous aquatic fungi, while litterbag communities should be predominantly composed of fungi known from terrestrial litter. Community composition was assessed by high-throughput sequencing of amplified barcoding regions. Similarity matrices of operational taxonomic unit (OTU) tables calculated by UCLUST and CD-HIT-OTU-Illumina were significantly correlated. Preferred occurrence in biofilm and litter communities, respectively, was used for the grouping of OTUs into three ecological guilds. Stressor sensitivity varied among the guilds. While non-fungal, in particular autotrophic, OTUs responded to several treatments, two of the fungal guilds, i.e. those exclusively colonising litter and those preferably occurring on the ceramic tiles, showed no response to any applied treatment. Only fungi preferably, but not exclusively, colonising litter significantly responded to chloride addition. Their distribution patterns again correlated significantly with those of non-fungal OTUs, indicating possible interdependencies between both groups. The results indicate that eukaryotic freshwater communities are composed of different guilds, with distinctive sensitivity and tolerance to anthropogenic stressors.