Biodegradation of natural and synthetic steroid hormones by river biofilms: Impacts on bacterial community structure

Steroid hormones released into the environment by human activities are increasingly found in aquatic environments. Even at low concentrations, they can disrupt natural systems. One of the most affected components is river biofilms, thin layers of microorganisms that grow on submerged surfaces. They play an important role in nutrient cycling, pollutant degradation, and act as indicators of water quality. This study evaluated the ability of natural biofilms from two sites to remove four steroid hormones: natural estrogenic compounds estrone (E1) and 17β-estradiol (E2), synthetic estrogenic compound 17α-ethinylestradiol (EE2), and progesterone (PRO), a natural progestogen. Microcosm assays demonstrated the efficient removal of E1, E2, and EE2, with maximum removal rates of 97.4 (4 days), 93.8 (3 days), and 88.2% (6 days), respectively. PRO was also removed, but with lower efficiency (48.7% in 3 days). The predominant mechanism used by biofilms for hormone removal was biodegradation. Hormone exposure affected biofilms differently depending on their origin. Biofilms collected upstream showed reduced diversity and richness, indicating sensitivity. In contrast, those collected near a wastewater treatment plant had higher diversity and evenness, suggesting resilience and possible adaptation to micropollutants. The predominant phyla were Proteobacteria and Firmicutes, with a reduction in Alloprevotella and an increase in Paenibacillus following exposure. These results highlight the dual role of river biofilms as natural barriers and biological indicators of contamination by steroid hormones, as well as reinforce their relevance in natural attenuation and the potential for bioremediation in urban rivers.