Antibiotics affect the growth responses of <Emphasis Type="Italic">Daphnia magna</Emphasis> to poor food quality

We tested the hypothesis that exposure to antibiotics alters the growth and reproductive responses of Daphnia magna to changing stoichiometric food quality. To do so, we measured growth and reproduction of differentially P-nourished Daphnia in the presence and absence of sublethal concentrations of antibiotics. We found that exposure to an antibiotic cocktail significantly reduced an index of the microbial load of Daphnia and altered its growth responses to changing dietary P-content. Growth rates of Daphnia consuming the most P-rich and P-poor food increased with antibiotic exposure but were negatively or not affected in animals eating mildly to moderately P-limiting food. Similar effects were found in a subsequent experiment where daphnid neonates were exposed to natural bacterial communities prior to receiving antibiotics and being fed different food C:P ratios. In contrast, antibiotic effects on Daphnia reproduction were either not detected (number and size of broods) or were relatively minor (day of first reproduction). We also found no evidence that gut flora provides defense against pathogenic bacterial infection; instead, infection rates in Daphnia by a bacterial microparasite, Pasteuria ramosa, decreased in animals that had experienced prior antibiotic exposure. Our results demonstrate that antibiotic exposure reduced the microbial load and altered growth rates of an important zooplankton herbivore. Given the mediating role of animal’s food C:P ratio, our results show that interactions between Daphnia and its microbial symbionts vary in strength and nature partly with the host’s nutritional state.