The influence of food quality (P :C ratio) on RNA:DNA ratio and somatic growth rate of Daphnia

Abstract

Growth experiments with juvenile Daphnia galeata were performed to investigate how fast their RNA:DNA ratio responds to changes in food quality, the relationship between RNA:DNA ratio and somatic growth rate, and the effect of food quality (P :C ratio) on the RNA:DNA ratio. RNA and DNA concentrations in individual daphnids were measured with a single‐dye (RiboGreen) fluorometric method. Algae were cultured in chemostats and different P :C ratios were obtained by altering the dilution rate and the P content of the medium. The RNA:DNA ratio of the daphnids responded within 5 h to differences in food quality. The RNA:DNA ratio was highly correlated with the somatic growth rate (r2 = 0.94), and the RNA:DNA ratio increased with increasing food P :C ratio below a threshold P :C ratio (by atoms) of approximately 0.005 (C : P ≈ 200). The response in RNA:DNA ratio to changes in food quality is rapid and consistent with previously reported patterns for somatic growth rate. This enables shortterm experiments that reduces the problems with keeping both food quantity and quality constant during incubation. Analysis of zooplankton RNA:DNA ratios in short‐term experiments has a potential for addressing questions about when and where food quantity, quality, or both limit zooplankton growth in nature.