Ammonia Exchange and Photorespiration in Chlamydomonas

Abstract

Two hours after the addition of L-methionine-DL-sulfoximine to the cell suspension, glutamine synthetase activity was inhibited by more than 90% in air-grown Chlamydomonas reinhardii. Cells continued to take up NH3 from the medium provided that the concentration of dissolved CO2 was high (equilibrated with 4% CO2 in air). This NH3 uptake, about 30% of the control, is discussed in terms of glutamate dehydrogenase activity. Without CO2, or with a low CO2 level, a NH3 excretion was observed, the rate of which depended on the actual concentration of the dissolved CO2. Experiments with 15NH3 demonstrated that no NH3 uptake was masked by this excretion and inversely that no excretion occurred during the uptake in the conditions where it took place. Furthermore, the NH3 excretion observed in the absence of CO2 increased when O2 concentration rose to 15% and was inhibited when 10 millimolar isonicotinic acid hydrazide was supplied to the algal suspension. Thus, NH3 excretion in the presence of L-methionine-DL-sulfoximine seems to be related to a photorespiratory process inasmuch as it presents the same properties with regard to the O2 and the isonicotinic acid hydrazide effects. These results favor the hypothesis that NH3 produced in the medium originates from the glycine to serine reaction. On the other hand, partial inhibition (50%) of photosynthesis by L-methionine-DL-sulfoximine was attributed to uncoupling between electron transfer and photophosphorylation due to NH3 accumulation into the cell.