The chlorophyllide a oxygenase (CAO) plays a crucial role in the biosynthesis of chlorophyll b (Chl b). In the moss Physcomitrium patens (P. patens), two distinct gene copies, PpCAO1 and PpCAO2, are present. In this study, we investigate the differential expression of these CAOs following light exposure after a period of darkness (24 h) and demonstrate that the accumulation of Chl b is only abolished when both genes are knocked out. In the ppcao1cao2 mutant, most of the antenna proteins associated with both photosystems (PS) I and II are absent. Despite of the existence of LHCSR proteins and zeaxanthin, the mutant exhibits minimal non-photochemical quenching (NPQ) capacity. Nevertheless, the ppcao1cao2 mutant retains a certain level of pseudo-cyclic electron transport to provide photoprotection for PSI. These findings shed light on the dual dependency of Chl b synthesis on two CAOs and highlight the distinct effects of Chl b deprival on PSI and PSII core complexes in P. patens, a model species for bryophytes.