Data from: Toxins or medicines? Phytoplankton diets mediate host and parasite fitness in a freshwater system

Diets must satisfy the everyday metabolic requirements of organisms and
can also serve as medicines to combat disease. Currently, the medicinal
role of diets is much better understood in terrestrial than in aquatic
ecosystems. This is surprising because phytoplankton species synthesize
secondary metabolites with known antimicrobial properties. Here, we
investigated the medicinal properties of phytoplankton (including
toxin-producing cyanobacteria) against parasites of the dominant
freshwater herbivore, Daphnia. We fed Daphnia dentifera on green algae and
toxic cyanobacteria diets known to vary in their nutritional quality and
toxin production, and an additional diet of Microcystis with added pure
microcystin-LR. We then exposed Daphnia to fungal and bacterial parasites.
Anabaena, Microcystis and Chlorella diets prevented infection of Daphnia
by the fungal parasite Metschnikowia, while Nodularia toxins increased
offspring production by infected hosts. In contrast to their medicinal
effects against Metschnikowia, toxic phytoplankton generally decreased the
fitness of Daphnia infected with the bacterial parasite, Pasteuria. We
also measured the amount of toxin produced by phytoplankton over time.
Concentrations of anatoxin-a produced by Anabaena increased in the
presence of Metschnikowia, suggesting parasite-induced toxin production.
Our research illustrates that phytoplankton can serve as toxins or
medicines for their consumers, depending upon the identity of their
parasites.