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 - Lukas, Marcus A1 - Wacker, Alexander T1 - Daphnia's dilemma: adjustment of carbon budgets in the face of food and cholesterol limitation JF - The journal of experimental biology N2 - We studied the carbon (C) metabolism in Daphnia when the amount of C (food quantity) and/or the content of biochemical nutrients (food quality) was limiting. Growth performances and C budgets of Daphnia magna (assimilation, faeces egestion, excretion and respiration measured by [C-14]-tracing) were analysed when animals were raised on different food quantities and concentrations of cholesterol, an essential biochemical food compound. Cholesterol is of special interest because it not only acts as limiting nutrient but also contributes to the overall C pool of the animals. As the tissue cholesterol concentration in Daphnia is quite low, we hypothesized the selective exclusion of cholesterol from C budgeting and tested this using radiolabelled cholesterol. Somatic growth rates of D. magna were highest at high quantity and quality and were reduced to a moderate value if either the food quantity or the cholesterol concentration was low. Growth was lowest at low food quantity and quality. The measurements of C budgets revealed high regulative response to low food quality at high food quantity only. Here, low dietary cholesterol caused bulk C assimilation efficiency (AE) to decrease and assimilated (excess) C to be increasingly respired. Additionally, Daphnia enhanced efficient adjustment of C budgets when facing cholesterol limitation by (1) increasing the AE of the cholesterol itself and (2) not changing cholesterol respiration, which was still not detectable. In contrast, at low food quantity, Daphnia is unable to adjust for low food quality, emphasizing that food limitation could overrule food quality effects. KW - Biochemical limitation KW - Carbon budgets KW - Zooplankton KW - Carbon pathway KW - Food quality KW - Food quantity Y1 - 2014 U6 - https://doi.org/10.1242/jeb.094151 SN - 0022-0949 SN - 1477-9145 VL - 217 IS - 7 SP - 1079 EP - 1086 PB - Company of Biologists Limited CY - Cambridge ER - TY - JOUR A1 - Lukas, Marcus A1 - Wacker, Alexander T1 - Acclimation to dietary shifts impacts the carbon budgets of Daphnia magna JF - Journal of plankton research N2 - Daphnia responds to low availability of carbon (food quantity) or limiting concentrations of nutrients relative to carbon (C) in excess (food quality) by respectively saving or discharging C via different pathways. We investigated which kind of food limitation leads to a faster regulation in Daphnia C budgets, and whether the pre-assimilative C pathways, ingestion and faeces egestion and the post-assimilative C pathways, excretion and respiration, are regulated concurrently. Daphnia magna were exposed to dietary shifts in different food quantities or qualities; food quality was varied in terms of the essential component, cholesterol. After acclimation to the new diet ranging from 0 to 96 h, C budgets were measured by a radiotracer technique. Dietary shifts in quantity and quality caused Daphnia to quickly adjust their C budgets within 6 h, but different C pathways were affected. A shift to low food quantity reduced Daphnia respiration indicating C retention. In contrast, sudden low quality food caused increased faeces egestion to discharge excess C. Furthermore, we observed a delayed increase in excretion but no change in respiration within the time frame studied. Such time-shifted responses appear to be an appropriate means to keep the costs of physiological adjustments relatively low, which in turn would benefit Daphnia performance. KW - carbon pathway KW - cholesterol KW - zooplankton KW - food quality KW - food quantity Y1 - 2014 U6 - https://doi.org/10.1093/plankt/fbu018 SN - 0142-7873 SN - 1464-3774 VL - 36 IS - 3 SP - 848 EP - 858 PB - Oxford Univ. Press CY - Oxford 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 -