Aguer, J.-P. | bioRxiv Science

bioRxiv2019

The Threshold Elemental Ratio of an ectotherm decreases then increases with rising temperature

Earth is currently facing unprecedented global changes, hurrying scientists to provide predictive tools to explore the futures responses of ecosystems. Among those changes, temperature increase and alterations of nutrient availabilities largely drive consumer performances, yet their interactive effect remains poorly understood. Here we investigate how the dietary C:P ratio that optimizes consumer growth (TERC:P: Threshold Elemental Ratio) changes along temperature gradients by combining a TERC:P model and growth experiments on the model organism Daphnia magna. Both lines of evidence show that the TERC:P responds to temperature in an U-shaped fashion. This shape reconciles previous contradictive observations into a common framework, thereby improving our capacity to forecast the combined effects of nutrient cycle and climatic alterations on ectotherms.

ecology↗

bioRxiv2018

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

1: Understanding the determinant of metabolism is a core ecological topic since it permits to link individuals energetic requirements to the ecology of communities and ecosystems. Yet, besides temperature, the effects of environmental factors on metabolism remain poorly understood. For example, dietary stoichiometric constraints have been hypothesized to increase maintenance metabolism of small invertebrates, yet experimental support remains scarce.\n\n2: Here, we used microcalorimetric heat flow measurements to determine the standard metabolic rate (SMR) of Daphnia magna throughout its ontogeny when fed stoichiometrically balanced (C/P ratio:166) or imbalanced (C/P ratio:1439) diets.\n\n3: When fed a stoichiometrically imbalanced diet, daphnids were able to maintain the stoichiometric homeostasis within narrow boundaries. However, they consistently increased their SMR while decreasing their somatic growth rate. Our measurements unequivocally demonstrate that homeostatic regulation implies higher metabolic costs and thereby reduces the portion of energy that can be allocated to growth.\n\n4: Our study demonstrates that microcalorimetry is a powerful and precise tool for measuring the metabolic rate of small-sized organisms and opens promising perspectives for understanding how environmental factors, such as nutritional constraints, affect organismal metabolism.

ecology↗