Cyanobacteria alter lipid metabolism in zooplankton via exudates of obesogens

Abstract

Lipid metabolism of zooplankton plays an important role in aquatic food web, however, is threatened by abiotic and biotic factors. Recently blooming cyanobacteria providing low-quality food for zooplankton, have been found to be a potential source of lipid metabolism disorder and reproductive disturbance in aquatic animals, though mechanisms of operation are unclear. Here we assessed effects of cyanobacterial exudates on lipid metabolism and reproduction in Daphnia magna. Microcystis aeruginosa exudates (MaE, 2 × 10⁴ cells/mL and 4 × 10⁵cells/mL) induced increased lipid droplets and altered lipid components in exposed Daphnia. MaE activated ecdysone and juvenile hormone signaling pathways by increasing hormone content and activities of ecdysone receptor and steroid receptor coactivator, which stimulated sterol regulatory element binding protein to increase lipid accumulation. MaE also increased expression of ECR, HR3, Neverland and RXR genes in the ecdysone pathway, Met and SRC genes in the juvenile hormone pathway, and SREBP-1 and DGAT-1 genes in the triacylglycerol (TAG) synthesis pathway. The increase in lipid production promoted both reproduction and growth of Daphnia. Glycerol tristearate (GTS, a TAG lipid species) in MaE solutions was positively correlated with cholesterol, TAG, and reproductive hormones in exposed Daphnia. Disordered lipid metabolism of zooplankton caused by cyanobacteria exudates is consistent with obesogen hypothesis (Baillie-Hamilton, 2002) and poses a risk to aquatic ecosystems.