Chlamydomonas reinhardtii, a unicellular eukaryotic organism, has the potential to produce biomass in aquatic environments. Ionizing radiation, known for its genotoxic effects, can damage DNA and induce mutations in living organisms. This study aimed to understand the impact of γ-irradiation at varying doses (50 Gy to 400 Gy) on C. reinhardtii, focusing on survivability and photosynthetic activity. The results demonstrated that gamma irradiation caused alterations in cell number and photosynthetic activities in a dose-dependent manner under circadian conditions. Notably, a dose of 200 Gy resulted in enhanced photosynthetic activity. The observed reductions in cell number and photosynthetic efficiency at higher doses are likely due to random mutagenesis and cellular damage in the chloroplast. Understanding the effects of ionizing radiation and dosage levels provides insights into the epigenetic stress responses and priming effects of photosynthesis and carbon-concentrating mechanisms (CCM) in C. reinhardtii.