Catalase activity increased in Peridinium gatunense (formerly P. cinctum fa. westii) cells during the decline of the seasonal spring bloom period in Lake Kinneret. This was correlated with the low ambient total CO2 concentration. The relationship was confirmed in laboratory experiments where maximum catalase activity occurred under an atmosphere composed of 30% O2 and 0.003% CO2. Conversely, high CO2 concentrations inhibited catalase activity. The rise in catalase activity was not directly due to increasing environmental pH, as in vitro and in vivo measurements showed a characteristic broad pH curve with a constant activity from pH 6–10 for catalase.
Photoinhibition of catalase occurred above 250 μmol photons · m−2· s−1. However, at high photoinactivating irradiances, photoinhibition was ameliorated under high pO2/pCO2. Such conditions prevail in the Kinneret at the end of the spring. We propose that the enhancement of photorespiration (under high pO2/pCO2) induces a temporary burst in catalase activity despite the progressively photoinhibitory conditions of early summer.