Food quality shapes gradual phenotypic plasticity in ectotherms facing temperature variability.

Organisms exhibit reversible physiological adjustments as a response to rapidly changing environments. Yet such plasticity of the phenotype is gradual and may lag behind environmental fluctuations, thereby affecting long-term average performance of the organisms. By supplying energy and essential compounds for optimal tissue building, food determines the range of possible phenotypic changes and potentially the rate at which they occur. Here, we assess how differences in the dietary supply of essential lipids modulate the phenotypic plasticity of an ectotherm facing thermal fluctuations. We use three phytoplankton strains to create a gradient of polyunsaturated fatty acid and sterol supply for Daphnia magna under constant and fluctuating temperatures. We used three different fluctuation periodicities to unravel the temporal dynamics of gradual plasticity and its long-term consequences for D. magna performance measured as juvenile somatic growth rate. In agreement with gradual plasticity theory, we show that in D. magna, fluctuation periodicity determines the differential between observed growth rates and those expected from constant conditions. Most importantly, we show that diet modulates both the size and the direction of the growth rate differential. Overall, we demonstrate that the nutritional context is essential for predicting ectotherm consumers' performance in fluctuating thermal environments.

Quantifying the energetic cost of food quality constraints on resting metabolism to integrate nutritional and metabolic ecology.

Consumer metabolism controls the energy uptake from the environment and its allocation to biomass production. In natural ecosystems, available energy in food often fails to predict biomass production which is also (co)limited by the relative availability of various dietary compounds. To date, the link between energy metabolism and the effects of food chemical composition on biomass production remains elusive. Here, we measured the resting metabolic rate (RMR) of Daphnia magna along ontogeny when undergoing various (non-energetic) nutritional constraints. All types of dietary (co)limitations (Fatty acids, Sterols, Phosphorus) induced an increase in mass-specific RMR up to 128% between highest and lowest quality diets. We highlight a strong negative correlation between RMR and growth rate indicating RMR as a promising predictor of consumer growth rate. We argue that quantifying the energetic cost imposed by food quality on individual RMR may constitute a common currency enabling the integration of nutritional and metabolic ecology.

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 (TERC:P : Threshold Elemental Ratio) changes along temperature gradients by combining a temperature-dependent TERC:P model with growth experiments on Daphnia magna. Both lines of evidence show that the TERC: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.

Phytoplankton community responses to temperature fluctuations under different nutrient concentrations and stoichiometry.

Nutrient availability and temperature are important drivers of phytoplankton growth and stoichiometry. However, the interactive effects of nutrients and temperature on phytoplankton have been analyzed mostly by addressing changes in average temperature, whereas recent evidence suggests an important role of temperature fluctuations. In a laboratory experiment, we grew a natural phytoplankton community under fluctuating and constant temperature regimes across 25 combinations of nitrogen (N) and phosphorus (P) supply. Temperature fluctuations decreased phytoplankton growth rate (rmax ), as predicted by nonlinear averaging along the temperature-growth relationship. rmax increased with increasing P supply, and a significant temperature × P × N interaction reflected that the shape of the thermal reaction norm depended on nutrients. By contrast, phytoplankton carrying capacity increased with N supply and in fluctuating rather than constant temperature. Higher phytoplankton N:P ratios under constant temperature showed that temperature regimes affected cellular nutrient incorporation. Minor differences in species diversity and composition existed. Our results suggest that temperature variability interacts with nutrient supply to affect phytoplankton physiology and stoichiometry at the community level.

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.

Dietary Fatty-Acid Compositions Are more Strongly Reflected in Fatty than Lean Dorsal Fillets of Common Carp (Cyprinus carpio L.).

In this fish-feeding study, we tested similarity patterns between fatty acids (FA) in diets and common carp (Cyprinus carpio) of fish ponds used for semi-intensive aquaculture, containing naturally occurring pond zooplankton and different feeds (marine or terrestrial feeds) until carp reached market size. We evaluated if and how total lipid contents in dorsal fillets can reflect dietary FA compositions in farm-raised common carp and hypothesized that increasing total lipid contents in dorsal fillets significantly increase the similarity between dietary and dorsal fillets' FA compositions. Results of this study showed that carps had higher total lipids when supplied with marine feeds and dietary FA compositions were indeed more strongly reflected in fatty (i.e. high total lipid contents) than in leaner dorsal fillets (low total lipid contents). Increasing total lipid contents in dorsal fillets significantly increased the similarity between the dietary and dorsal fillets' FA compositions. In contrast, leaner dorsal fillets had FA patterns that were more distinct from dietary FA. Total lipid contents higher than ~60 mg/g dry weight in dorsal fillets had only limited effects on increasing the similarity between FA compositions of diets and dorsal fillets, and were independent of feed sources. It is thus suggested that higher total lipid contents in dorsal fillets can be used as a proxy to predict dietary FA profiles in common carps, or perhaps even in farm-raised fish in general.

A microcalorimetric approach for investigating stoichiometric constraints on the standard metabolic rate of a small invertebrate.

Understanding the determinants of metabolism is a core ecological topic since it permits to link individual energetic requirements to the ecology of communities and ecosystems. Yet, besides temperature, metabolic responses to environmental factors remain poorly understood. For example, it is commonly assumed that dietary stoichiometric constraints increase metabolism of small invertebrates despite scarce experimental support. Here, we used microcalorimetric measurements to determine the standard metabolic rate (SMR) of Daphnia magna fed stoichiometrically balanced (C/P: 166) or imbalanced (C/P: 1439). Daphnids fed imbalanced maintained their stoichiometric homeostasis within narrow boundaries. However, they consistently increased their SMR while decreasing their growth rate. Our measurements demonstrate that homeostatic regulation implies higher metabolic costs, thereby reducing available energy for growth. We demonstrate that microcalorimetry is a powerful and precise tool for measuring small-sized organisms' metabolic rate, thus opening promising perspectives for understanding how environmental factors, such as nutritional constraints, affect organismal metabolism.

Linking primary producer diversity and food quality effects on herbivores: A biochemical perspective.

Biodiversity can strongly influence trophic interactions. The nutritional quality of prey communities and how it is related to the prey diversity is suspected to be a major driver of biodiversity effects. As consumer growth can be co-limited by the supply of several biochemical components, biochemically diverse prey communities should promote consumer growth. Yet, there is no clear consensus on how prey specific diversity is linked to community biochemical diversity since previous studies have considered only single nutritional quality traits. Here, we demonstrate that phytoplankton biochemical traits (fatty acids and sterols) can to a large extent explain Daphnia magna growth and its apparent dependence on phytoplankton species diversity. We find strong correlative evidence between phytoplankton species diversity, biochemical diversity, and growth. The relationship between species diversity and growth was partially explained by the fact that in many communities Daphnia was co-limited by long chained polyunsaturated fatty acids and sterols, which was driven by different prey taxa. We suggest that biochemical diversity is a good proxy for the presence of high food quality taxa, and a careful consideration of the distribution of the different biochemical traits among species is necessary before concluding about causal links between species diversity and consumer performance.

Diet quality determines lipase gene expression and lipase/esterase activity in Daphnia pulex.

We studied the short- (12 h) and long-term (144 h) response of Daphnia pulex lipases to quality shifts in diets consisting of different mixtures of the green alga Scenedesmus with the cyanobacterium Synechococcus, two species with contrasting lipid compositions. The lipase/esterase activity in both the gut and the body tissues had fast responses to the diet shift and increased with higher dietary contributions of Synechococcus When screening the Daphnia genome for TAG lipases, we discovered a large gene-family expansion of these enzymes. We used a subset of eight genes for mRNA expression analyses and distinguished between influences of time and diet on the observed gene expression patterns. We identified five diet-responsive lipases of which three showed a sophisticated short- and long-term pattern of expression in response to small changes in food-quality. Furthermore, the gene expression of one of the lipases was strongly correlated to lipase/esterase activity in the gut suggesting its potentially major role in digestion. These findings demonstrate that the lipid-related enzymatic machinery of D. pulex is finely tuned to diet and might constitute an important mechanism of physiological adaptation in nutritionally complex environments.

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.

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.

From aquatic to terrestrial food webs: decrease of the docosahexaenoic acid/linoleic acid ratio.

Fatty acid composition of the adipose tissue of six carnivorous mammalian species (European otter Lutra lutra, American mink Mustela vison, European Mink Mustela lutreola, European polecat Mustela putorius, stone marten Martes foina and European wild cat Felis silvestris) was studied. These species forage to differing degrees in aquatic and terrestrial food webs. Fatty acid analysis revealed significant differences in polyunsaturated fatty acid composition between species. More specifically, our results underline a gradual significant decrease in the docosahexaenoic acid (DHA)/linoleic acid (LNA) ratio of carnivore species as their dependence on aquatic food webs decreases. In conclusion, the use of the DHA/LNA ratio in long-term studies is proposed as a potential proxy of changes in foraging behaviour of semi-aquatic mammals.