Land surface energy fluxes are commonly monitored with the eddy covariance (EC) technique. A phenomenon that has puzzled the EC community for decades is that when all the energy flux terms measured by EC are summed, the obtained value is generally less than the net available energy measured by radiation sensors. This phenomenon occurs at almost every EC site around the world. Where does the missing energy go? To answer this question, I investigate the light reactions of photosynthesis. Photosynthesis starts with the excitation of chlorophyll molecules by light. The excitation is then funneled to the reaction centers of photosystems where the energy of excitation is converted to charge separation and electron transport via a chain of redox reactions. The electron transport causes the buildup of protons in the lumen and the formation of electric field across the thylakoid membrane with the positive side in the lumen and the negative side in the stroma. The proton gradient and electric field provide the proton motive force (PMF) that drives the synthesis of high-energy molecule ATP across the membrane. At the end of the electron transport chain, the electrons reduce the lower energy molecule NADP+ to form the higher energy molecule NADPH. ATP and NADPH power the Calvin Cycle to produce sugar which stores some of the initially absorbed light energy in chemical bonds to support essentially all life on Earth. The light energy initially harvested by chlorophyll molecules is much higher than the chemical bond energy eventually stored in sugar because of the thermodynamics and because the processes of the formation of PMF and the production of ATP and NADPH also require energy. However, the difference between the light energy initially harvested and the chemical bond energy eventually stored does not appear as heat immediately to be detectable by a heat sensor such as EC. This is because it takes time for the proton motive force to dissipate and the ATP and NADPH to be consumed. At high light, the PMF may be stronger, and the rates of production of ATP and NADPH may be higher than what are needed by the Calvin Cycle. Conversely, at low light, the PMF and the production rates of ATP and NADPH that can be supported by the available light may not be able to satisfy the demand of the Calvin Cycle. Therefore, the oversupply of PMF, ATP and NADPH at high light may compensate the shortage at low light. This compensation cannot be detected as sensible heat in the energy budget equations. I hypothesize that this is the reason why the EC technique cannot close the energy budget. Technological and theoretical developments needed to test this hypothesis are now in place. This hypothesis leads to several predictions that can be experimentally or observationally checked for its falsification or confirmation.
Dr. Lianhong Gu is a Distinguished Research Staff Scientist in the Ecosystem Processes Group, Environmental Sciences Division, Oak Ridge National Laboratory. Dr. Gu got his PhD in Environmental Sciences at the University of Virginia in 1998 and joined the Oak Ridge National Lab in 2002. He conducts research in photosynthesis and ecosystem science with observational, theoretical, and experimental approaches. He is the inventor of the licensed technology Fluorescence Auto-Measurement Equipment (FAME), the software Integrated Measurement And Control System for SIF (IMACSS), and the community online tool leafweb (leafweb.org). Dr. Gu received the World meteorological Organization Norbert-Gerbier Mumm Award in 2012.