FLUXNET-ECN Seminar Series: Lianhong Gu and Jingfeng Xiao

FLUXNET-ECN Seminar Series: Lianhong Gu and Jingfeng Xiao
https://fluxnet.org/community/fluxnet...

The light reactions of photosynthesis and the grand hypothesis of eddy covariance energy imbalance, by Lianhong Gu

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. This electric field provides 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 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 eddy covariance. 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 eddy covariance system 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.

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Assessing drought impacts on terrestrial photosynthesis with satellite data from OCO-2 and SMAP, by Jingfeng Xiao

Solar-induced chlorophyll fluorescence (SIF) measured from space (e.g., the Orbiting Carbon Observatory-2 or OCO-2) and both soil moisture and gross primary production (GPP) estimates from the Soil Moisture Active Passive (SMAP) provide new opportunities for understanding the responses terrestrial photosynthesis to water stress over large regions. In this presentation, we will explore the impacts of drought on photosynthesis using SIF (mainly from OCO-2 and partly from GOME-2), SMAP, and flux data.