TY - JOUR A1 - Lukas, Marcus A1 - Wacker, Alexander T1 - Constraints by oxygen and food quality on carbon pathway regulation: a co-limitation study with an aquatic key herbivore JF - Ecology : a publication of the Ecological Society of America N2 - In food webs, herbivores are often constrained by low food quality in terms of mineral and biochemical limitations, which in aquatic ecosystems can co-occur with limited oxygen conditions. As low food quality implies that carbon (C) is available in excess, and therefore a regulation to get rid of excess C is crucial for the performance of consumers, we examined the C pathways (ingestion, feces release, excretion, and respiration) of a planktonic key herbivore (Daphnia magna). We tested whether consumer C pathways increase due to mineral (phosphorus, P) or biochemical (cholesterol and fatty acid) limitations and how these regulations vary when in addition oxygen is low. Under such conditions, at least the capability of the upregulation of respiration may be restricted. Furthermore, we discussed the potential role of the oxygen-transporting protein hemoglobin (Hb) in the regulation of C budgets. Different food quality constraints led to certain C regulation patterns to increase the removal of excess dietary C: P-limited D. magna increased excretion and respiration, while cholesterol-limited Daphnia in addition upregulated the release of feces. In contrast, the regulative effort was low and only feces release increased when D. magna was limited by a long-chain polyunsaturated fatty acid (eicosapentaenoic acid, EPA). Co-limiting oxygen did not always impact the discharge of excess C. We found the food-quality-induced upregulation of respiration was still present at low oxygen. In contrast, higher excretion of excess C was diminished at low oxygen supply. Besides the effect that the Hb concentration increased under low oxygen, our results indicate a low food-quality-induced increase in the Hb content of the animals. Overall, C budgeting is phenotypically plastic towards different (co-) limiting scenarios. These trigger specific regulation responses that could be the result of evolutionary adaptations. KW - carbon budget KW - carbon stoichiometry KW - cholesterol KW - co-limitation KW - Daphnia KW - EPA KW - hemoglobin KW - oxygen KW - phosphorus KW - polyunsaturated fatty acid KW - zooplankton Y1 - 2014 SN - 0012-9658 SN - 1939-9170 VL - 95 IS - 11 SP - 3068 EP - 3079 PB - Wiley CY - Washington ER - TY - JOUR A1 - Martin, Benjamin A1 - Jager, Tjalling A1 - Nisbet, Roger M. A1 - Preuss, Thomas G. A1 - Grimm, Volker T1 - Limitations of extrapolating toxic effects on reproduction to the population level JF - Ecological applications : a publication of the Ecological Society of America N2 - For the ecological risk assessment of toxic chemicals, standardized tests on individuals are often used as proxies for population-level effects. Here, we address the utility of one commonly used metric, reproductive output, as a proxy for population-level effects. Because reproduction integrates the outcome of many interacting processes (e.g., feeding, growth, allocation of energy to reproduction), the observed toxic effects in a reproduction test could be due to stress on one of many processes. Although this makes reproduction a robust endpoint for detecting stress, it may mask important population-level consequences if the different physiological processes stress affects are associated with different feedback mechanisms at the population level. We therefore evaluated how an observed reduction in reproduction found in a standard reproduction test translates to effects at the population level if it is caused by hypothetical toxicants affecting different physiological processes (physiological modes of action; PMoA). For this we used two consumer-resource models: the Yodzis-Innes (YI) model, which is mathematically tractable, but requires strong assumptions of energetic equivalence among individuals as they progress through ontogeny, and an individual-based implementation of dynamic energy budget theory (DEB-IBM), which relaxes these assumptions at the expense of tractability. We identified two important feedback mechanisms controlling the link between individual- and population-level stress in the YI model. These mechanisms turned out to also be important for interpreting some of the individual-based model results; for two PMoAs, they determined the population response to stress in both models. In contrast, others stress types involved more complex feedbacks, because they asymmetrically stressed the production efficiency of reproduction and somatic growth. The feedbacks associated with different PMoAs drastically altered the link between individual- and population-level effects. For example, hypothetical stressors with different PMoAs that had equal effects on reproduction had effects ranging from a negligible decline in biomass to population extinction. Thus, reproduction tests alone are of little use for extrapolating toxicity to the population level, but we showed that the ecological relevance of standard tests could easily be improved if growth is measured along with reproduction. KW - Daphnia KW - dynamic energy budget KW - ecological risk assessment KW - ecotoxicology KW - ontogenetic symmetry KW - physiological mode of action KW - PMoA KW - population dynamics KW - reproduction test KW - Yodzis-Innes Y1 - 2014 U6 - https://doi.org/10.1890/14-0656.1 SN - 1051-0761 SN - 1939-5582 VL - 24 IS - 8 SP - 1972 EP - 1983 PB - Wiley CY - Washington ER - TY - JOUR A1 - Martin-Creuzburg, Dominik A1 - Oexle, Sarah A1 - Wacker, Alexander T1 - Thresholds for sterol-limited growth of Daphnia magna: A comparative approach using 10 different sterols JF - Journal of chemical ecology N2 - Arthropods are incapable of synthesizing sterols de novo and thus require a dietary source to cover their physiological demands. The most prominent sterol in animal tissues is cholesterol, which is an indispensable structural component of cell membranes and serves as precursor for steroid hormones. Instead of cholesterol, plants and algae contain a variety of different phytosterols. Consequently, herbivorous arthropods have to metabolize dietary phytosterols to cholesterol to meet their requirements for growth and reproduction. Here, we investigated sterol-limited growth responses of the freshwater herbivore Daphnia magna by supplementing a sterol-free diet with increasing amounts of 10 different phytosterols and comparing thresholds for sterol-limited growth. In addition, we analyzed the sterol composition of D. magna to explore sterol metabolic constraints and bioconversion capacities. We show that dietary phytosterols strongly differ in their potential to support somatic growth of D. magna. The dietary threshold concentrations obtained by supplementing the different sterols cover a wide range (3.5-34.4 mu g mg C-1) and encompass the one for cholesterol (8.9 mu g mg C-1), indicating that certain phytosterols are more efficient in supporting somatic growth than cholesterol (e.g., fucosterol, brassicasterol) while others are less efficient (e.g., dihydrocholesterol, lathosterol). The dietary sterol concentration gradients revealed that the poor quality of particular sterols can be alleviated partially by increasing dietary concentrations, and that qualitative differences among sterols are most pronounced at low to moderate dietary concentrations. We infer that the dietary sterol composition has to be considered in zooplankton nutritional ecology to accurately assess potential sterol limitations under field conditions. KW - Cholesterol KW - Daphnia KW - Food quality KW - Nutrition KW - Phytosterols KW - Sterols Y1 - 2014 U6 - https://doi.org/10.1007/s10886-014-0486-1 SN - 0098-0331 SN - 1573-1561 VL - 40 IS - 9 SP - 1039 EP - 1050 PB - Springer CY - Dordrecht ER -