Gene expression and activity of digestive enzymes of Daphnia pulex in response to food quality differences

Food quality is an important factor influencing organisms' well-being. In freshwater ecosystems, food quality has been studied extensively for the keystone herbivore genus Daphnia, as they form the critical trophic link between primary producers and higher order consumers such as fish. For Daphnia, the edible fraction of phytoplankton in lakes (consisting mostly of unicellular algae and cyanobacteria) is extraordinarily diverse. To be able to digest different food particles, Daphnia possess a set of digestive enzymes that metabolize carbohydrates, lipids and proteins. Recent studies have found a connection between gene expression and activity of single digestive enzyme types of Daphnia, i.e. lipases and proteases, and transcriptome studies have shown that a variety of genes coding for gut enzymes are differentially expressed in response to different food algae. However, never before has a set of digestive enzymes been studied simultaneously both on the gene expression and the enzyme activity level in Daphnia. Here, we investigated several digestive enzymes of Daphnia pulex in a comparison between a high-quality (green algal) and a low-quality (cyanobacterial) diet. Diet significantly affected the expression of all investigated digestive enzyme genes and enzyme activity was altered between treatments. Furthermore, we found that gene expression and enzyme activity were significantly correlated in cellulase, triacylglycerol lipase and beta-glucosidase when switched from high to low-quality food. We conclude that one of the factors causing the often observed low biomass and energy transfer efficiency from cyanobacteria to Daphnia is probably the switch to a cost-effective overall increase of gene expression and activity of digestive enzymes of this herbivore.