Shedding light on blue-green photosynthesis: A wavelength-dependent mathematical model of photosynthesis in Synechocystis sp. PCC 6803
Fig 2
Simulation of the electron and gas fluxes in Synechocystis.
A-D: Simulated steady-state electron flux through linear (blue), cyclic (green), alternate (flavodiiron+terminal oxidases, orange) and respiratory (red) electron pathways for light intensities between 10 μmol(photons) m−2 s−1 and 300 μmol(photons) m−2 s−1. The model has been parameterized to yield approximately 15 electrons PSI−1 s−1 linear electron flow (blue) for a fraction of 65% under saturating CO2 conditions, as measured in wild type (WT, A) [46]. The model predicts flux distributions under sub-saturating air CO2 level (B) and for the flavodiiron (Flv1/3, C) and NAD(P)H Dehydrogenase-like complex 1 (NDH-1) knockout mutants (D). Each value represents a steady-state flux under continuous light exposure. Simulations were run using 670 nm light (Gaussian LED, σ = 10 nm). E: Barplot showing the mean flux distribution for light intensities over 10 μmol(photons) m−2 s−1 ± sd). F: Simulation of oxygen production and consumption (respiration + three terminal oxidases) rates for increasing light intensities, as compared to measured rates provided by Schuurmans et al. (2014) [47]. Data points are the mean from measurements of cells with 625 nm illumination with 50 mM NaHCO3. Error bars show ± sd. G: The simulated carbon fixation rates are displayed with the measurement used for parameterization [48]. Simulations were run using a 625 nm light (gaussian LED, σ = 10 nm). We calculated light attenuation in the culture using Eq (S67) in S1 Appendix with default pigment concentrations and a constant 2 mg L−1 sample chlorophyll concentration. H: O2 production under variation of the external CO2 concentration in vivo and in vitro. The data was used for parametrization by fitting the parameter fCin, representing the ratio of intracellular to extracellular CO2 partial pressure which is increased by the carbon concentrating mechanism, which was varied in the simulation between 100 and 1000. Benschop et al. [49] measured oxygen evolution with 800 μmol(photons) m−2 s−1 light of Synechocystis sp. PCC 6803 grown under 20 ppm CO2 and varied the dissolved CO2 in the medium (Ci). The data was extracted from graphs using https://www.graphreader.com/. The simulation used a cool white LED. Our simulated O2 evolution rates are ca. half of the measured rates. Within the CO2 concentration range above ambient air (400 ppm) the rate dynamics are well reproduced with fCin = 1000. The model overestimates oxygen evolution at very low Ci concentrations. A black box with the letter “P” marks the data used for parameterization. RMSE quantifies the residuals of the respective simulation. In A and E, the residuals measure the difference to an experimentally measured LET flux of 15 electrons PSI−1 s−1 and 65% of the total PSI electron flux in the WT. The difference to the residuals of a model with initial parameters, not improved with the Monte Carlo results, is in parentheses.