A hybrid continuous-discrete single-cell model for Chlorella vulgaris integrating photophysiology, internal nutrient quotas and multiple fission.

Population-scale models of microalgal growth represent the culture as average biomass and do not explicitly describe discrete cell-cycle events such as commitment, size variation and daughter-cell number. This limitation is relevant for Chlorella vulgaris, which divides by multiple fission and produces a variable number of autospores according to its physiological state. This study proposed a hybrid continuous-discrete single-cell model integrating photophysiology, internal nitrogen and phosphorus quotas, chlorophyll dynamics, the functional state of photosystem II, reactive oxygen species, viability and discrete cell-commitment rules. The model was calibrated by differential evolution against eight quantitative endpoints compiled from the literature and compared, under the same protocol, with a quota-only model and a parametric empirical model. After calibration, the model reproduced the continuous endpoints with a standardized RMSE of 0.123, the discrete endpoints of the control and, under terbutryn, the reduction of the target autospore number before cell death. In the leave-one-source-out analysis, it showed the lowest error for the morphological endpoints and a classification accuracy of 1.00 for the target autospore number and dark-phase division, against 0.33 for the quota-only model. The convergence, sensitivity and parameter-recovery analyses indicated numerical stability and good identifiability of the parameters associated with commitment and multiple fission. It also reproduced the decline of Fv/Fm and the rise of ROS under PSII inhibition. The model constitutes a reproducible, hypothesis-generating framework whose predictive application requires prospective experimental validation.