Abstract: Epilithic biofilm is a rich source of polyunsaturated fatty acids (PUFAs) and thus is a high-quality basal food resource in river food webs. Biofilms not only provide food for consumers but are also influenced by consumers. However, our understanding of such feedback is insufficient. To understand the impact of grazing pressure on biofilms, we conducted a 35-day mesocosm experiment under two different conditions, with and without Bellamya aeruginosa, and compared the differences in biofilm using fatty acid and metagenomic sequencing. Under grazing pressure, the proportion of saturated fatty acids (SAFAs) and PUFAs, such as arachidonic acid (ARA), increased by 7.72% and 0.13%, respectively, while the proportion of monounsaturated fatty acids (MUFAs) decreased by 10.14%. Among the microorganisms involved in lipid metabolism, the relative abundance of Proteobacteria and Cyanobacteria decreased, while that of Verrucomicrobia and Actinobacteria increased. At the genus level, the relative abundance of Porphyromonas and Sphingomonas decreased, while Xanthomonas increased. The relative abundance of the fatty acid degradation pathway in biofilm was significantly reduced, while that of the functional gene acetyl-CoA acyltransferase increased significantly in KEGG function prediction. Our findings suggest that biofilm can maintain energy demand and stability by reducing the degradation of fatty acids under grazing pressure.

Key words: grazing pressure, epilithic biofilm, microbial community, fatty acid, metagenomics