Most microbial organisms and animals feed on substrates that differ in elemental composition from their bodies. When substrates are nutrient-poor, we argue that organisms regulate their metabolism to release extra carbon. To quantify the degree of this metabolic regulation, here we propose an optimality framework based on the hypothesis that the organism growth rate is maximized along gradients of nutrient availability. Predictions stemming from this optimality theory are largely confirmed by a data collection (available in the Bolin Database) spanning aquatic and terrestrial systems, microbial organisms and animals, and considering nitrogen and phosphorus as limiting nutrients. This new theory, by explicitly linking respiration and nutrient availability, can improve biogeochemical models aiming at quantifying how carbon emissions vary with environmental conditions and land management. 

Manzoni S., Čapek P., Mooshammer M., Lindahl B., Richter A., and H. Šantrůčková, 2017. Optimal metabolic regulation along resource stoichiometry gradients, Ecology Letters: 20(9), 1182–1191. doi: 10.1111/ele.12815