Microbiome and pollutants in the freshwater sponges Ephydatia muelleri (Lieberkühn, 1856) and Spongilla lacustris (Linnaeus, 1758) from the sub-Arctic Pasvik river (Northern Fennoscandia).

Despite the ecosystem functions offered by sponges in freshwater habitats, fragmentary studies have targeted their microbiome and the bioaccumulation of legacy and emerging organic micropollutants, making it difficult to test hypotheses about sponge-microbe specificity and response to environmental factors and stressors. The sponge species Ephydatia muelleri and Spongilla lacustris, coexisting in two sites of the Pasvik River (northern Fennoscandia), were analyzed for persistent organic pollutant (POPs) and chemicals of emerging concern (CECs), along with quali-quantitative microbiological features. River water and sediment were similarly treated to establish if the obtained data were site- or sponge-specific. CECs mainly occurred in abiotic matrices, with trimethoprim and ciprofloxacin prevailing in water and sediment, respectively. Only ciprofloxacin and diclofenac were detected in sponges, with higher concentrations generally determined in S. lacustris than E. muelleri. Overall, POP concentrations were in the order polycyclic aromatic hydrocarbons > chlorobenzenes > polychlorobiphenyls > polychloronaphthalenes, with higher values in sponges with respect to abiotic matrices. Generally, POPs occurred at higher concentrations in S. lacustris than E. muelleri. Enzyme activity measurements displayed diverse trends across samples and sites, with E. muelleri displaying higher glycolytic activity than S. lacustris. Prokaryotic abundance in sponges generally exceeded that found in abiotic matrices. Proteobacteria, Planctomycetota, Actinobacteriota, Verrucomicrobiota, and Cyanobacteria predominated in sponge samples, with slight differences between sponge species and sampling sites, whereas Desulfobacterota and Acidobacterota were retrieved mostly in sediment samples. The sponge-associated bacterial communities appeared to be differently affected by pollutant concentration at the site level. Overall, this study highlights the ecological role of freshwater sponges, shedding light on their microbial associations, pollutant bioaccumulation, and potential as bioindicators of aquatic ecosystem health. The findings emphasize the importance of considering both microbial diversity and contaminant accumulation for a holistic understanding of the roles played by freshwater sponges in human-impacted environments.