Shedding light on blue-green photosynthesis: A wavelength-dependent mathematical model of photosynthesis in Synechocystis sp. PCC 6803
Fig 3
Saturation pulse method using Pulse Amplitude Modulation (PAM) fluorescence measurement in vivo and in silico.
The simulated signal has been calculated using Eq (8). All experimental measurements were performed with a Multi-Color PAM (Walz, Effeltrich, Germany). A, B: Fit of simulated (black) to measured (red) PAM fluorescence dynamics during a saturation pulse light protocol. The simulation was manually fit the traces in A and improved by automated parameter variation using seven model parameters, and the parameters are used for all other model simulations. The experimental traces were measured in Synechocystis sp. PCC 6803 grown under 435 nm light (A, n = 2) or 633 nm light (B, n = 2) [59]. Simulations use the respectively measured pigment contents and ambient CO2 (400 ppm). The shown light protocol includes several different light wavelengths and intensities to trigger a response from respective photosynthetic electron transport chain components. By monitoring cell responses to these light conditions, we captured light responses via state transitions and non-photochemical quenching and relaxation (as described in the upper bar). We calculated light attenuation in the culture using Eq (S67) in S1 Appendix with the measured pigment concentrations and sample chlorophyll content in Table D in S1 Appendix. C-J: Model prediction on emitted fluorescence signal. Light protocol in A is repeated with pigment contents measured in cells grown under different monochromatic lights [59]. We calculated light attenuation with the chlorophyll content measured in each culture. K,L: Model validation comparing PAM-SP fluorescence traces in vivo (K) and in silico (L). Four experimental replicates are shown. Simulations assume 1% CO2 supplementation and use the default parameters (see A) and pigment set. The model reproduces the qualitative fluorescence dynamics during most of the experiment except for overestimating steady-state fluorescence during the strong blue light phase. The lower bar depicts the light wavelength and intensity (in parentheses, in μmol(photons) m−2 s−1) (lights used: 440 nm at 80 μmol(photons) m−2 s−1 and 1800 μmol(photons) m−2 s−1 and 625 nm at 50 μmol(photons) m−2 s−1, saturating pulse: 600 ms cool white LED at 15 000 μmol(photons) m−2 s−1). Cultures of Synechocystis sp. PCC 6803 were grown under bubbling with 1% CO2 and 25 μmol(photons) m−2 s−1 of 615 nm light for ca. 24 h. For the measurement, 1.5 mL culture was transferred to a quartz cuvette and dark-acclimated for 15 min prior to each measurement. A black box with the letter “P” marks the data used for parameterization. RMSE quantifies the residuals of the respective simulation. The difference to the residuals of a model with initial parameters, not improved with the Monte Carlo results, is in parentheses.