Does periphyton turn less palatable under grazing pressure?

Abstract

Abstract Periphyton acts as an important primary producer in stream food webs with bottom-up and also subject to top-down effects from grazing pressure. Yet the underlying mechanism remains unclear. Here we conducted a mesocosm experiment to explore response of food quality indicated by biomarkers polyunsaturated fatty acids (PUFAs), including eicosapentaenoic acid (EPA, 20: 5n3) and 22C fatty acids (FAs) docosahexaenoic acid (DHA, 22: 6n3), essential for cell growth and reproduction and most consumers cannot synthesize, of periphyton to Bellamya aeruginosa grazing pressure. Results indicated that grazing pressure led to a decrease in Bacillariophyta which contain high-quality PUFAs such as EPA and DHA and an increase in Cyanophyta and Chlorophyta which are rich in 18C PUFAs such as linoleic acid (LIN, 18: 2n6) and alpha-linolenic acid (ALA, 18: 3n3) in periphyton. We observed up-regulation of genes that participate in lipid metabolism promoting biosynthesis of unsaturated FAs, ALA metabolism and glycerophospholipid metabolism and those relate to carbohydrate and energy metabolism maintaining energy stability of periphyton. Therefore, food quality of periphyton decreased under grazing pressure, and the results also provide the compositional, chemical, and molecular perspectives of the interactive bottom-up and top-down effects on structuring stream food webs.