Nutrients and Pharmaceuticals Structure Bacterial Core Communities in Urban and Montane Stream Biofilms

Bacteria in stream biofilms play a significant role in stream biogeochemical processes and may be vulnerable to high levels of pollution entering urban streams. To investigate the impact of contaminants on stream biofilm bacteria in their natural environment, we exposed developing biofilms to experimental additions of nutrients (nitrogen, phosphorus, and iron), pharmaceuticals (caffeine and diphenhydramine), a combination of nutrients and pharmaceuticals, or no contaminants at all, using contaminant exposure substrates (CES) in three catchments in northern Utah. The study was conducted at both montane and urban sites to assess how existing pollution influences biofilm response.

We identified bacterial core communities for each contaminant treatment and land-use type (e.g., nutrient addition montane bacterial core, pharmaceutical addition urban bacterial core) by selecting taxa present in at least 75% of the samples within each specific group. The results showed that montane and urban land-use types distinctly shaped bacterial cores, with nutrients and pharmaceuticals exerting subtle but noticeable effects. Nutrients increased the dominance of already abundant copiotrophs, such as Pseudomonadaceae (Gammaproteobacteria) and Comamonadaceae (Betaproteobacteria), within bacterial cores at both montane and urban sites. Conversely, pharmaceuticals promoted species-rich bacterial cores with unique contaminant-degrading taxa within Pseudomonadaceae and Anaerolineaceae (Chloroflexi).

Interestingly, even at urban sites with existing pharmaceutical pollution, the addition of pharmaceuticals further increased bacterial core richness, particularly within DR-16 (Betaproteobacteria), WCHB1-32 (Bacteroidetes), and Leptotrichiaceae (Fusobacteria). In treatments combining nutrients and pharmaceuticals, nutrients exerted a stronger selective force than pharmaceuticals, leading to bacterial cores more similar to those observed with nutrient addition alone, and encouraging the presence of unique Oscillatoriales (Cyanobacteria) taxa in urban streams.

Our findings indicate that the addition of nitrogen, phosphorus, and iron enhances the dominance of already abundant copiotrophs, while the addition of caffeine and diphenhydramine supports the inclusion of unique taxa associated with contaminant degradation in bacterial cores. Furthermore, biofilm bacteria at urban sites remain sensitive to pharmaceuticals commonly found in these waters, highlighting a dynamic interaction between pharmaceutical pollution, bacterial diversity, and contaminant degradation.