Cobalt induces the set-up of new structural networks in river biofilms: Impairment of autotrophic-heterotrophic coupling.

Biofilms are integral to the biogeochemical cycles of aquatic ecosystems, primarily through complex interactions among microorganism that mediate the transformation and movement of key elements. In a previous study, we reported that Cyanobacteria within biofilms grown in outdoor mesocosms exhibited sensitivity to cobalt (Co) when exposed to increasing Co concentrations (background concentrations, 0.1, 0.5, and 1 μM). Under exposure to Co, biofilms were found to adopt alternative carbon fixation pathways via non-photosynthetic prokaryotes, suggesting a disruption in phototrophic activity and raising questions about broader autotroph-heterotroph interactions that drive biofilm functions. In the present study, we extended this investigation to assess the impact of Co on microeukaryotes (primarily microalgae) and their interactions with prokaryotes, thereby encompassing the entire biofilm community. Eukaryotic abundance and diversity were quantified using qPCR and DNA metabarcoding, while microbial interactions were inferred through co-occurrence network analysis based on operational taxonomic units (OTUs). Our findings indicate that Co exposure significantly altered the composition of the microalgal and meiofaunal communities, with Bacillariophycea exhibiting pronounced sensitivity. At 1 μM Co, microbial networks were characterized by reduced OTU richness and fewer interactions, yet displayed stronger structural centrality around a limited number of taxa. Notably, in control conditions, 40 % of keystone taxa were affiliated with microalgae, whereas at 1 μM Co, keystone taxa were predominantly prokaryotic. These results indicate that Co disrupts autotroph-heterotroph coupling, driving a shift toward prokaryotic dominance in microbial interactions. The study highlights adaptive strategies employed by biofilms to mitigate metal-induced stress and maintain functional integrity in contaminated environments.