Algal diets and natural xenobiotics impact energy allocation in cladocerans. II. Moina macrocopa and Moina micrura

In freshwater lakes, changing mineral and biochemical food qualities as well as exposure to natural xenobiotics, such as humic substances (HSs) are common challenges to life history traits of zooplankters. In this contribution, we tested the bottom-up effect of different food qualities alone and in concert with exposure to increasing HS concentrations in a clone of each Moina micrura (inhabitant of Brazilian coastal lagoons) and Moina macrocopa (invasive species). Carbon, nitrogen, and phosphorus contents, fatty acid (FA) and the amino acid (AA) patterns of three coccal green algae of the summer aspect in the lagoons were analyzed. All algae were deficient in P, differed in their content of α-linolenic acid (ALA), histidine, and arginine and were lacking long-chained polyunsaturated fatty acids (PUFAs >C18). M. micrura did not grow on Desmodesmus armatus. Maximal mean lifespan and total lifespan reproduction were achieved on Pseudokirchneriella subcapitata. M. macrocopa fed all algae offered. Multiple regressions showed that ALA triggered longevity and the P-content reproduction. Exposure to HSs revealed that M. micrura reduced either its lifespan on Monoraphidium minutum or its reproduction on P. subcapitata. Conversely in M. macrocopa, lifespan on M. minutum was extended by a factor of 1.7 and offspring numbers increased by 1.4 upon HS exposure. On D. armatus an originally very low reproductive output increased maximal by a factor 4.6. Relating HS-mediated lifespan and offspring modulations of the 1.08mM exposure to food components, it appears that ALA controls lifespan modulation even under HS stress, whereas offspring modulation seems to be controlled by C and total AA content. This indicates that, upon HS exposure, the investment of energy in body maintenance or reproduction would depend at least partly on the biochemical composition of the food available. This also shows that even short-chained dietary PUFAs may play a major role in keeping lipid peroxidation low. A comparison of the two Moina species tested revealed that M. macrocopa has the capacity to outcompete M. micrura, since it had the greater clutch size, never reduced the reproductive output upon HS-exposure, and fed all diets offered.