Methyl viologen (MV) causes severe oxidative stress by generating superoxide in the photosystem. The marine Chlamydomonas strain W80 is highly tolerant to MV (inhibitory concentration 50% [IC(50)]= 110 mu M), and another marine Chlamydomonas strain HS5 shows also relatively a high tolerance (IC(50) = 12 mu M). These two marine strains and a freshwater Chlamydomonas reinhardtii, which is highly sensitive to MV (IC(50) = 0.03 mu M), were compared with respect to their reactive oxygen species (ROS) eliminating enzymes (superoxide dismutase,catalase,glutathione peroxidase, and ascorbate peroxidase), intracellular free amino acids, and antioxidant activities of the cell extracts. The marked difference between the marine Chlamydomonas strains and C. reinhardtii is the much higher (more than 5 fold) ascorbate peroxidase (APX) activity in the marine strains. The marine strains also kept the high APX activities (more than 100% of non-stressed condition) under the MV stressed condition, while the APX activity in C. reinhardtii was significantly decreased (36% of non-stressed condition) under the stressed condition, indicating that APX activity potentially contributes to the oxidative stress tolerance in Chlamydomonas. In addition, the levels of intracellular free proline, which is supposed to ameliorate oxidative stress, were several tens of times higher in the marine Chlamydomonas strains than in C. reinhardtii. (C) 2011, The Society for Biotechnology, Japan. All rights reserved.