Chlorella vulgaris heterotrophy-to-phototrophy conversion dynamics are mostly independent of light intensity

Trophic conversion - a sequential cultivation strategy combining heterotrophic and phototrophic growth - offers a promising route for large-scale microalgae production by coupling the high biomass yields of heterotrophy with the biochemical advantages of phototrophy. Despite its potential, the cellular mechanisms governing this transition remain poorly understood. Here is presented the first mechanistic dissection of trophic conversion in Chlorella vulgaris, using isoactinic light conditions (30-600 µmol photons/m²/s) and inocula with varied physiological states. A consistent five-step cellular response was identified: initiation of gene expression, recovery of PhotoSystem II activity, reactivation of cell division, restoration of pigment biosynthesis, and establishment of steady-state growth, all culminating in full restoration of the phototrophic phenotype. These phases follow distinct timelines (3 h, 9-17 h, 24 h, 32 h, and 72 h, respectively) and exhibit specific light dependencies, as characterized in this study. These findings provide a mechanistic framework to guide the optimization of trophic conversion at the process level.