Microbial communities associated with the freshwater sponge Spongilla lacustris from the Arctic Pasvik river, Norway

Freshwater sponges are affiliated to the order Spongillina within the class Demospongiae and are able to tolerate extreme physical and chemical conditions, namely desiccation and cold temperatures. Freshwater sponges play important ecological roles in aquatic ecosystems often supported by microbial symbionts with important functions in nutrient dynamics, and by unicellular algal symbionts. Despite sponge microbiomes play pivotal roles in their habitats, the characterization of bacterial populations associated with freshwater sponges is scant in comparison with those of marine sponges, especially in Arctic areas. The diversity and abundance of the microbial communities associated with S. lacustris, living in tight contact with soft substrates in slow-flowing areas of the Pasvik River (Northern Fennoscandia) were investigated. First data were also provided on the microbial enzymatic activity rates, prokaryotic abundance, biomass and heterotrophic bacterial viable counts. Persistent pollutants (i.e., polychlorinated biphenyls, heavy metals, polycyclic aromatic hydrocarbons, and organochlorine pesticides) were determined in sponge mesohyl tissues. Extraction of total DNA and sequencing of the 16S rRNA genes was performed on sponge, water and sediment samples, to investigate sponge transient and core microbiome and the taxonomic sharing level. Results provided insights into the phylogenetic bacterial diversity of this sponge species, confirming Proteobacteria as the dominant associated phylum. It also revealed the co-dominance (with Alpha- and Gammaproteobacteria) of Acidobacteria and Chloroflexi. Our results suggested that S. lacustris harbours its own distinct bacterial community which differ from those occurring in seawater and sediment, supporting the common belief that sponges selectively acquire the microorganisms from the surrounding environment. Enzyme activity rates confirmed that LAP was the most abundant enzyme, followed by AP and GLU, comparable to those recorded in sediments for LAP and GLU. In our study cell morphologies were characterised by a numerical prevalence of rods and cocci. Different widespread classes of contaminants were searched and retrieved in S. lacustris mesohyl. To our knowledge, the present study is the first addressing the diversity and composition of the bacterial communities associated with S. lacustris. Our results extend the so far restricted knowledge of bacterial community in freshwater sponges.