Inhibition of gut digestive proteases by cyanobacterial diets decreases infection in a Daphnia host–parasite system

Abstract

Secondary metabolites produced by primary producers have a wide range of functions as well as indirect effects outside the scope of their direct target. Research suggests that protease inhibitors produced by cyanobacteria influence grazing by herbivores and may also protect against parasites of cyanobacteria. In this study, we asked whether those same protease inhibitors produced by cyanobacteria could also influence the interactions of herbivores with their parasites. We used the Daphnia‐Metschnikowia zooplankton host‐fungal parasite system to address this question because it is well documented that cyanobacteria protease inhibitors suppress trypsin and chymotrypsin in the gut of Daphnia, and because it is known that Metschnikowia infects via the gut. We tested the hypothesis that Daphnia gut proteases are necessary for Metschnikowia spores to be released from their asci. We then also tested whether diets that decrease trypsin and chymotrypsin activity in the guts of Daphnia lead to lower levels of infection. Our results show that chymotrypsin promotes the release of the fungal spores from their asci. Moreover, a diet that strongly inhibited chymotrypsin activity in Daphnia decreased infection levels, particularly in the most susceptible Daphnia clones. Our results support the growing literature that cyanobacterial diets can be beneficial to zooplankton hosts when challenged by parasites and uncover a mechanism that contributes to the protective effect of cyanobacterial diets. Specifically, we demonstrate that host chymotrypsin enzymes promote the dehiscence of Metschnikowia spores; when cyanobacteria inhibit the activity of chymotrypsin in hosts, this most likely traps the spore inside the ascus, preventing the parasite from puncturing the gut and beginning the infection process. This study illustrates how secondary metabolites of phytoplankton can protect herbivores against their own enemies.

Food quantity and quality modulates inducible defenses in a common predator–prey system

“…Eutrophication is often associated with the occurrence of cyanobacterial blooms. Cyanobacteria are nutritionally inadequate for most aquatic consumers due to poor ingestibility, the production of harmful secondary metabolites, and the lack of essential lipids (Carmichael 1994;DeMott et al 2001;von Elert et al 2003;Wilson and Hay 2007;Sanchez et al 2024). Conditions of sufficient carbon but limited lipid supply (i.e., food quality) may occur regularly in nature, especially in eutrophic waters (Müller-Navarra et al 2004;Taipale et al 2019).…”