To cope with changing external conditions, plants undergo dynamic acclimation processes that remodel their photosynthetic machinery, optimizing energy use while minimizing damage to photosystems (PS). Key photoprotective mechanisms include non-photochemical quenching (NPQ), which dissipates excess excitation energy, and alternative electron transport (AET) pathways, which prevent over-reduction of the photosynthetic electron transport chain. This study provides a comprehensive analysis of how various photoprotective mechanisms contribute to long-term acclimation to high and fluctuating light in Physcomitrium patens, a moss that exhibits well-conserved photoprotective responses bridging algae and vascular plants. Our results demonstrate that modulation of photoprotection around PSII and PSI is critical for maintaining photosynthetic efficiency and enable acclimation to variable light conditions. P. patens mutants deficient in NPQ or AET exposed to high or fluctuating light displayed growth defects, reduced photosynthetic efficiency and unbalanced PSI and PSII activity compared to WT plants. These findings indicate that photosynthetic response under varying light conditions depends on the complementary action of multiple protective strategies, rather than a single dominant photoprotective mechanism.