Dietary essential amino acids affect the reproduction of the keystone herbivore Daphnia pulex

Abstract

Recent studies have indicated that nitrogen availability can be an important determinant of primary production in freshwater lakes and that herbivore growth can be limited by low dietary nitrogen availability. Furthermore, a lack of specific essential nitrogenous biochemicals (such as essential amino acids) might be another important constraint on the fitness of consumers. This might be of particular importance for cladoceran zooplankton, which can switch between two alternative reproductive strategies--the production of subitaneously developing and resting eggs. Here, we hypothesize that both the somatic growth and the type of reproduction of the aquatic keystone herbivore Daphnia is limited by the availability of specific essential amino acids in the diet. In laboratory experiments, we investigated this hypothesis by feeding a high quality phytoplankton organism (Cryptomonas) and a green alga of moderate nutritional quality (Chlamydomonas) to a clone of Daphnia pulex with and without the addition of essential amino acids. The somatic growth of D. pulex differed between the algae of different nutritional quality, but not dependent on the addition of dissolved amino acids. However, in reproduction experiments, where moderate crowding conditions at saturating food quantities were applied, addition of the essential amino acids arginine and histidine (but not lysine and threonine) increased the total number and the developmental stage of subitaneous eggs. While D. pulex did not produce resting eggs on Cryptomonas, relatively high numbers of resting eggs were released on Chlamydomonas. When arginine and histidine were added to the green algal diet, the production of resting eggs was effectively suppressed. This demonstrates the high, but previously overlooked importance of single essential amino acids for the reproductive strategy of the aquatic keystone herbivore Daphnia.

Figure 1. Growth on algae with and without EAA addition.

Somatic growth rates (mean of n = 3 ± SD) of D. pulex fed Chlamydomonas or Cryptomonas, both either without additional EAA or with the addition of dissolved arginine and histidine or lysine and threonine at concentrations of 25 µmol L⁻¹; identical lowercase letters indicate no significant difference after Tukey's post-hoc HSD test following one-way ANOVA.

Figure 2. Reproduction on algae with and without EAA addition – exp. 1.

Cumulative production of subitaneous eggs (A) and ephippia (resting eggs, B) by D. pulex fed Chlamydomonas or Cryptomonas, both either without additional EAA or with the addition of dissolved arginine and histidine or lysine and threonine at concentrations of 25 µmol L⁻¹; egg numbers are means of n = 3 ± SD; identical lowercase letters indicate no significant difference after Tukey's post-hoc HSD test following one-way ANOVA for the subitaneous eggs and Mann-Whitney U-tests following Kruskal-Wallis ANOVA for the resting eggs.

Figure 3. Reproduction on algae with and without EAA addition – exp. 2.

Cumulative production of subitaneous eggs (A) and ephippia (resting eggs, B) by D. pulex fed either Cryptomonas, Chlamydomonas, Chlamydomonas with the addition of dissolved arginine, histidine or both arginine and histidine at concentrations of 25 µmol L⁻¹; egg numbers are means of n = 3 ± SD; identical lowercase letters indicate no significant difference after Tukey's post-hoc HSD test following one-way ANOVA.

Figure 4. Development of subitaneous eggs.

Developmental stage of D. pulex subitaneous eggs (% of total) depending on the food alga and the addition of dissolved EAA at concentrations of 25 µmol L⁻¹; values given are means of n = 3 jars with 40 individuals each after 6 days of juvenile and pre-adult growth.

Figure 5. Amino acid content of algae.

Arginine (A) and histidine (B) content of Chlamydomonas and Cryptomonas per mg of particulate organic carbon (n = 3); no significant differences were found using a one-way ANOVA; note the different scaling of plots A and B.