Ecosystems as Enzymes: Investigating Fundamental Controls on Stream Ecosystem Metabolism

Abstract:

Stream ecosystem respiration (ER) and gross primary production (GPP) link lotic systems to global biogeochemical cycles and are central to ecosystem functioning. The rates of these metabolic processes ultimately drive the magnitude of mass and energy fluxes through ecosystems. Understanding fundamental controls on ER and GPP allows for the prediction of ecosystem metabolism under future conditions and climates. Both ER and GPP comprise a series of complex enzyme-mediated chemical reactions, each with fundamental rate-limiting steps. For example, glycolysis may limit ER and carbon fixation may limit GPP. In this work, we combine concepts from the Metabolic Theory of Ecology (MTE) and chemical reaction kinetics to develop quantitative predictions about fundamental controls on ER and GPP. From theory, we predict that daily ER is controlled by temperature, availability of organic carbon, availability of oxygen, and the size distribution of aerobic organism biomass. Daily GPP, likewise, is predicted to be controlled by temperature, availability of photosynthetically active radiation, availability of inorganic carbon, and the size distribution of autotroph biomass. We apply these predictions to metabolism time series from spring-fed rivers. Springs exhibit relatively stable input water temperature, chemistry, discharge, as well as persistent community composition, which enable the isolation of temporal cross-correlations between GPP and ER, preclude the importance of allochthonous carbon in ER, and create thermal experiments in response to air temperature changes. When all expected metabolic controls are considered, we find that the thermal sensitivities of ER and GPP are consistent with MTE, a result previous studies across stream ecosystems have failed to consistently achieve. This suggests that it is only by accounting for all rate-limiting controls, in addition to temperature, that the appropriate activation energies are revealed. These observations support the inference that whole ecosystem metabolism is universally controlled by the underlying rate-limited enzyme-catalyzed reactions within individual organisms.