Factorial Knockouts Distinguish Physical Necessity from Numerical Compensation in a PSI-LHCI Transport Model

Abstract

Mechanistic interpretation of photosynthetic energy-transfer models requires more than reproduction of experimental observables: a model intended to support mechanistic claims should also respond consistently when its proposed functional organizations are removed. Here, we evaluate whether a calibrated PSI-LHCI transport surrogate encodes physically meaningful principles by applying a 2x2 factorial knockout framework that independently removes site-energy heterogeneity and coupling-strength heterogeneity in a 155-pigment network. All perturbations were evaluated using the same Full-model calibration without parameter refitting. Although the calibrated model reproduced a high excitation-trapping yield, eliminating either energetic or coupling heterogeneity unexpectedly improved its apparent transport performance. A strict zero-coupling control confirmed that coupling itself remained necessary for network-mediated reaction-center access, whereas the supplied organization of coupling strengths was not supported by the surrogate. An audit of the model inputs further identified peripheral localization of all lowest-energy states and effective coupling scales far above those used in structure-based chlorophyll Hamiltonians. These findings do not imply that native PSI favors flat energy landscapes or uniform couplings. Instead, they show that endpoint agreement alone does not validate a mechanistic interpretation of a pigment-network model. Factorial knockout analysis provides a falsification-oriented framework for separating physical necessity from proposed organization, diagnosing numerical compensation, and identifying the constraints required for more predictive models of PSI-LHCI energy transfer.