Can the Metabolic Theory of Ecology predict respiration in marine bacteria?

Here we present an enzyme kinetic model (EKM) of respiratory oxygen consumption based on the substrate control of the ETS (Fig. 2 and 3). It argues that R is controlled by the maximum velocity, Vmax, of the enzyme reactions that controls the process (i.e., the ETS), the temperature, and the substrate availability, S (Fig. 3). Kinetics of this thermal-substrate regulation are described by the Arrhenius and Michaelis-Menten equations. The EKM equation takes the form: where Ea is the molar Arrhenius activation energy, Rg is the molar gas constant, Km is the Michaelis-Menten constant, and T is temperature. Here we apply the EKM and the MTE to predict a respiration times-profile throughout the exponential, steady state, and nutrient-limited phases of the marine bacterium Vibrio natriegens in an acetate-based culture (Fig. 1). Both models were tested by comparing their output with the measured RO2 time-profile (Fig. 4 top). They were evaluated quantitatively by least-square regression analysis (Fig.4 bottom). When the predictive capability of both models was compared, serious flaws in the MTE rendered it inaccurate during nutrient limitation. In contrast, the EKM worked well throughout the entire period of the experiment. Results suggest that respiratory control is achieved through changes in the in vivo, νETS , activity of the ETS by substrate modulation of the in vitro ETS, AETS, (Vmax). We conclude that EKM holds promise for predicting respiration at the different physiological states and time-scales important to microbiological studies. )( ) )( ( max 1 01 SK eSV R m TTR E g a M. Borja Aguiar-González1, May Gómez2, Elisa Berdalet3, Sylvie Roy4 & Ted Packard2 1 Dpto de Física, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain. 2 Dpto de Biología, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain. 3 Institut de Ciències del Mar (CSIC), Barcelona, Spain. 4 3579A Delson drive, P.O. Box 348, Navan, Ontario K4B 1J5, Canada. Can the Metabolic Theory of Ecology predict respiration in marine bacteria? Respiratory oxygen consumption is caused by enzymatic activity of the respiratory electron transfer system (ETS). However, in spite of this understanding, respiration models continue to be based on allometric equations relating respiration to body size, body surface, or biomass. The Metabolic Theory of Ecology (MTE) is a recent example. It is based on Kleiber’s Law relating respiration (R) and biomass (M) in the form, R = K1 M3/4, where K1 is a constant. This law holds because biomass packages the ETS. Consequently, we have been proposing to bypass biomass and model respiration directly from its causal relationship with the ETS activity (R = f (ETS)). Table 1. Kinetic parameters that were used to calculate the in vivo ETS activity (νETS ) and the intracellular NADH and NADPH concentrations. During Nutrient Limitation MTE does not predict bacteria respiration! Fig.1 Experiment VnAc110593 showing time-course observations of AETS, RO2, protein and acetate. Fig. 2 The inspiration for considering Michaelis-Menten as the throttle mechanism for respiration. Here respiration falls in parallel with the nutrient supply. Fig.3 Modelled intracellular NADH and NADPH for experiment VnAc110593. P is acetate, M is cell protein and δ, ω, λ and η are constants (Table 1). Enzyme Kinetic Model (EKM) Fig. 4 (a) Modelled time-profile of RO2 based on in vitro ETS from EKM; (b) EKM Predicted-Respiration versus Respiration; (c) Modelled time-profile of RO2 based on allometric relationship (cell protein) from MTE.; (d) MTE PredictedRespiration versus Respiration. A contribution from ICM-CSIC, BLOS, ULPGC, and the projects: EXOME (CTM-2008-01616), and OITHONA (CTM2007-60052). First author has been supported by a FPU grant from the Spanish Ministry of Science and Innovation. (a) (c) (b) (d)