Ammonia Exchange and Photorespiration in Chlamydomonas

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 NH(3) from the medium provided that the concentration of dissolved CO(2) was high (equilibrated with 4% CO(2) in air). This NH(3) uptake, about 30% of the control, is discussed in terms of glutamate dehydrogenase activity. Without CO(2), or with a low CO(2) level, a NH(3) excretion was observed, the rate of which depended on the actual concentration of the dissolved CO(2). Experiments with (15)NH(3) demonstrated that no NH(3) uptake was masked by this excretion and inversely that no excretion occurred during the uptake in the conditions where it took place. Furthermore, the NH(3) excretion observed in the absence of CO(2) increased when O(2) concentration rose to 15% and was inhibited when 10 millimolar isonicotinic acid hydrazide was supplied to the algal suspension. Thus, NH(3) 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 O(2) and the isonicotinic acid hydrazide effects. These results favor the hypothesis that NH(3) 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 NH(3) accumulation into the cell.