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

Biofilms play a crucial role in biogeochemical cycles, making them essential to the functionality and stability of aquatic ecosystems. Their functioning is mainly driven by interactions between microorganisms that ensure the cycling of major elements. Evaluating the impact of stressors, such as metals, on these interactions is challenging. This study examined the effects of cobalt (Co) on the microeukaryotic community and their relationships with prokaryotes within biofilms grown in the presence of several Co concentrations (background concentrations, 0.1, 0.5, and 1 μM) for 28 days and 35 days after the end of Co injection. Previous work has suggested that Cyanobacteria were highly sensitive to Co leading to a reduction of the photosynthetic potential of the biofilm. In this study, the major primary producers, namely Bacillariophyceae were also found to be sensitive to Co. The direct consequence of this sensitivity was an impairment of the autotroph-heterotroph coupling and the dominance of prokaryotic taxa in microbe-microbe interactions. The biofilms co-occurrence networks were then smaller, less connected but more centralized at 1 μM Co, even after the 35 days of recovery. Keystones, that were half affiliated with microalgae in the absence of Co, became predominantly prokaryotes, controlling resource production and element cycling in Co-stressed biofilms. These changes in co-occurrence network organization and microbial community composition demonstrated a cascade of effects related to contamination in rivers. Our results further highlighted the strategy biofilms adopt to counteract the impairment of autotroph-heterotroph coupling and to maintain their functions and ecosystemic roles in contaminated environments.