Feeding in the frequency domain: coarser‐grained environments increase consumer sensitivity to resource variability,covariance and phase

Theory predicts that resource variability hinders consumer performance. How this effect depends on the temporal structure of resource fluctuations encountered by individuals remains poorly understood. Combining modelling and growth experiments with Daphnia magna, we decompose the complexity of resource fluctuations and test the effect of resource variance, supply peak timing (i.e. phase) and co‐limiting resource covariance along a gradient from high to low frequencies reflecting fine‐ to coarse‐grained environments. Our results show that resource storage can buffer growth at high frequencies, but yields a sensitivity of growth to resource peak timing at lower ones. When two resources covary, negative covariance causes stronger growth depression at low frequencies. However, negative covariance might be beneficial at intermediate frequencies, an effect that can be explained by digestive acclimation. Our study provides a mechanistic basis for understanding how alterations of the environmental grain size affect consumers experiencing variable nutritional quality in nature.     

Tissue-Specific Fatty Acids Response to Different Diets in Common Carp (Cyprinus carpio L.)

Fish depend on dietary fatty acids (FA) to support their physiological condition and health. Exploring the FA distribution in common carp (Cyprinus carpio), one of the world''s most consumed freshwater fish, is important to understand how and where FA of different sources are allocated. We investigated diet effects on the composition of polar and neutral lipid fatty acids (PLFA and NLFA, respectively) in eight different tissues (dorsal and ventral muscle, heart, kidney, intestine, eyes, liver and adipose tissue) of common carp. Two-year old carp were exposed to three diet sources (i.e., zooplankton, zooplankton plus supplementary feeds containing vegetable, VO, or fish oil, FO) with different FA composition. The PLFA and NLFA response was clearly tissue-specific after 210 days of feeding on different diets. PLFA were generally rich in omega-3 polyunsaturated FA and only marginally influenced by dietary FA, whereas the NLFA composition strongly reflected dietary FA profiles. However, the NLFA composition in carp tissues varied considerably at low NLFA mass ratios, suggesting that carp is able to regulate the NLFA composition and thus FA quality in its tissues when NLFA contents are low. Finally, this study shows that FO were 3X more retained than VO as NLFA particularly in muscle tissues, indicating that higher nutritional quality feeds are selectively allocated into tissues and thus available for human consumption.     

Covariance modulates the effect of joint temperature and food variance on ectotherm life‐history traits

Understanding animal performance in heterogeneous or variable environments is a central question in ecology. We combine modelling and experiments to test how temperature and food availability variance jointly affect life‐history traits of ectotherms. The model predicts that as mean temperatures move away from the ectotherm's thermal optimum, the effect size of joint thermal and food variance should become increasingly sensitive to their covariance. Below the thermal optimum, performance should be positively correlated with food–temperature covariance and the opposite is predicted above it. At lower temperatures, covariance should determine whether food and temperature variance increases or decreases performance compared to constant conditions. Somewhat stronger than predicted, the covariance effect below the thermal optimum was confirmed experimentally on an aquatic ectotherm (Daphnia magna) exposed to diurnal food and temperature variance with different amounts of covariance. Our findings have important implications for understanding ectotherm responses to climate‐driven alterations of thermal mean and variance.     

U‐shaped response Unifies views on temperature dependency of stoichiometric requirements

Temperature and nutrient availability, which are major drivers of consumer performance, are dramatically affected by global change. To date, there is no consensus on whether warming increases or decreases consumer needs for dietary carbon (C) relatively to phosphorus (P), thus hindering predictions of secondary production responses to global change. Here, we investigate how the dietary C:P ratio optimising consumer growth (TER_C:P: Threshold Elemental Ratio) changes along temperature gradients by combining a temperature‐dependent TER_C:P model with growth experiments on Daphnia magna. Both lines of evidence show that the TER_C:P response to temperature is U‐shaped. This shape indicates that consumer nutrient requirements can both increase or decrease with increasing temperature, thus reconciling previous contradictive observations into a common framework. This unified framework improves our capacity to forecast the combined effects of nutrient cycle and climatic alterations on invertebrate production.     

Fatty acid retention under temporally heterogeneous dietary intake in a cladoceran

Omega‐3 (ω3) and ‐6 (ω6) polyunsaturated fatty acids (PUFA) are essential for all aquatic animals, but their dietary availability can be highly heterogeneous in space and time. The way consumers retain PUFA across such heterogeneous feeding conditions remains poorly understood. In a series of feeding experiments, we investigated how retention efficiencies (i.e. amount in consumer biomass/amount ingested) of PUFA and bulk carbon responded to heterogeneous PUFA intake in Daphnia magna. Heterogeneous PUFA intake was achieved by exposing D. magna to algal diets of different PUFA content and composition for specific time periods. The retention efficiency of carbon did not change among dietary treatments. At shorter exposure to PUFA‐rich diet, retention efficiencies of most PUFA were 2–3 times higher than that of bulk carbon, clearly indicating PUFA bioaccumulation in D. magna. Increasing exposure to PUFA‐rich diet caused exponential decrease of retention efficiencies for most PUFA. However, D. magna receiving more PUFA were richer in these compounds despite lower retention efficiency. Eicosapentaenoic (20:5ω3) and arachidonic acid (20:4ω6) and their precursors were always supplied in the same proportions (3.6:1), but the 20:5ω3/20:4ω6 ratio in D. magna (an important measure of nutritional quality for consumers) increased with exposure time to these PUFA from 2.2:1 to 3.8:1, thus eventually matching the diet. Our results suggest that D. magna is an efficient gatherer, accumulator, and repository of PUFA under low/fragmented dietary availability. However, at higher availabilities, PUFA are not always bioaccumulated in D. magna. Hence, the efficiency of PUFA transfer by daphnids in food webs may depend on temporal PUFA availability and its range of variation. Finally, we show that heterogeneity in PUFA intake may also affect higher trophic levels by influencing nutritionally critical PUFA ratios of zooplankton.     

Transformation of 5-ene steroids by the fungus Aspergillus tamarii KITA: Mixed molecular fate in lactonization and hydroxylation pathways with identification of a putative 3β-hydroxy-steroid dehydrogenase/Δ–Δ isomerase pathway

The fungus Aspergillus tamarii metabolizes progesterone to testololactone in high yield through a sequential four step enzymatic pathway which, has demonstrated flexibility in handling a range of steroidal probes. These substrates have revealed that subtle changes in the molecular structure of the steroid lead to significant changes in route of metabolism. It was therefore of interest to determine the metabolism of a range of 5-ene containing steroidal substrates. Remarkably the primary route of 5-ene steroid metabolism involved a 3β-hydroxy-steroid dehydrogenase/Δ⁵–Δ⁴ isomerase (3β-HSD/isomerase) enzyme(s), generating 3-one-4-ene functionality and identified for the first time in a fungus with the ability to handle both dehydroepiansdrosterone (DHEA) as well as C-17 side-chain containing compounds such as pregnenolone and 3β-hydroxy-16α,17α-epoxypregn-5-en-20-one. Uniquely in all the steroids tested, 3β-HSD/isomerase activity only occurred following lactonization of the steroidal ring-D. Presence of C-7 allylic hydroxylation, in either epimeric form, inhibited 3β-HSD/isomerase activity and of the substrates tested, was only observed with DHEA and its 13α-methyl analogue. In contrast to previous studies of fungi with 3β-HSD/isomerase activity DHEA could also enter a minor hydroxylation pathway. Pregnenolone and 3β-hydroxy-16α,17α-epoxypregn-5-en-20-one were metabolized solely through the putative 3β-HSD/isomerase pathway, indicating that a 17β-methyl ketone functionality inhibits allylic oxidation at C-7. The presence of the 3β-HSD/isomerase in A. tamarii and the transformation results obtained in this study highlight an important potential role that fungi may have in the generation of environmental androgens.     

Early spring food resources and the trophic structure of macroinvertebrates in a small headwater stream as revealed by bulk and fatty acid stable isotope analysis

Hydrobiologia - Energy and organic matter flow in forested headwater stream trophic webs is generally more dependent on allochthonous than autochthonous organic matter. However, we propose that...