Effects of algal food quality on free amino acid metabolism of Daphnia

Abstract

Zooplankton frequently face dietary limitation caused by imbalances between their nutrient requirements and the nutrient content of their phytoplankton food. While insufficient nutrition is acknowledged to affect growth and reproduction of zooplankton, the effect of dietary limitation on other aspects of their metabolism remains poorly studied. One understudied biochemical response associated with nutrient stress is changes to the free amino acid (FAA) composition of body tissues resulting from changes in dietary supply. We examined changes in FAA composition in two cladoceran species (Daphnia magna and Daphnia pulex) in response to stress created by ecologically relevant food conditions: low food quantities, high algal carbon: nitrogen (C:N) ratios, and high algal C: phosphorus (P) ratios. We measured 14 FAA after extracting them from lyophilized whole Daphnia grown under these contrasting algal diets. Generally, low food quantity and low algal P increased body FAA content in both daphnid species while many essential FAA decreased due to N-limitation. While significant responses to diet were seen for each daphnid species, the nature of these responses depended on the type of nutritional stress, the FAA examined, and on the species. Non-parametric discriminant analysis exhibited a high predictive ability to classify unknown zooplankton into the correct diet category. The observed changes in body FAA profiles of Daphnia illustrate strong effects that algal food quality can have on the biochemistry of planktonic consumers. Furthermore these results demonstrate body FAA composition of zooplankton may provide an important source of information about zooplankton nutrient limitation especially combined with other nutrient-dependent molecular responses.