Does periphyton turn less palatable under grazing pressure?

Periphyton acts as an important primary producer in stream food webs with bottom-up grazing pressure and is also subject to effects of top-down grazing pressure. However, the underlying mechanisms of these interactions remain unclear. In this study we conducted a mesocosm experiment to explore the periphyton response to grazing pressure by the freshwater snail Bellamya aeruginosa in relation to food quality indicated by polyunsaturated fatty acid (PUFA) biomarkers, including eicosapentaenoic acid (20:5n3) and the 22C fatty acid docosahexaenoic acid (22:6n3), which are essential for cell growth and reproduction and cannot be synthesized by most consumers of periphyton. Results indicated that periphyton grazing pressure led to a decrease in Bacillariophyta, which contain high-quality PUFAs such as eicsapentaenoic acid and docosahexaenoic acid, and an increase in Cyanophyta and Chlorophyta, which are rich in 18C PUFAs such as linoleic acid (18:2n6) and alpha-linolenic acid (18:3n3). We observed upregulation of genes that participate in lipid metabolism promoting unsaturated fatty acid biosynthesis, alpha-linolenic acid metabolism, and glycerophospholipid metabolism, which are related to the carbohydrate and energy metabolism maintaining the energy stability of periphyton. These results demonstrate that the food quality of periphyton decreased under grazing pressure and also elucidate the compositional, chemical, and molecular perspectives of the interactive bottom-up and top-down effects on structuring stream food webs.