On illumination with blue light the O2-uptake of Chlorella pyrenoidosa (211-8b) in which photosynthetic O2-liberation has been suppressed by 10-5M DCMU initially decreases, but in the course of 5-10 min increases over that in preceding darkness (Fig. 1). Whereas an enhancement of O2-uptake is already induced by traces of blue radiation and saturated at about 1.5x10-10einsteins cm-2sec-1, the initial inhibition of O2-uptake can be measured only after application of more than 1.5×10-10einsteins cm-2sec-1 (Fig. 2). The long induction time that passes before a steady enhancement in O2-uptake is reached, the low energy requirement of the enhancement, and its spectral dependence with greatest efficiency of wavelengths around λ 455 nm and λ 375 nm and no effect of wavelengths beyond λ 520 nm (Fig. 3) resemble the corresponding data found earlier for an enhancement of respiration by light in a chlorophyll-free, carotenoidcontaining Chlorella mutant. It is therefore likely that the increased O2-uptake in DCMU-poisoned cells of wild type Chlorella depends on an increase in respiration. The pigment involved is not known, but from the action spectrum it could be a flavin or a cis-carotenoid. In contrast to the increase the initial decrease in O2-uptake does not show up in strong blue light only, but is also present in red light in which it stays constant throughout the period of measurement of 20 min (Fig. 4). Its intensity dependence is similar in blue and in red light; the lower efficiency of blue, which appears in Fig. 5, is at least partially due to the time interval of 5 min chosen for its determination: in these first 5 min after the beginning of blue illumination the slow increase in respiration already begins. The spectral dependence of the decrease in O2-consumption in the red part of the visible spectrum yields greatest activity around λ 680 nm, a slow drop towards λ 525 nm and a steep one towards λ 743 nm (Fig. 6). From that and the absence of any after-effect of red light on the O2-consumption in following darkness (Fig. 8), which might be expected if phytochrome action were involved, we think chlorophyll to be the pigment responsible for light-dependent inhibition of O2-uptake. A mutant of Scenedesmus, Bishop's Nr. 11, which is unable to evolve photosynthetic oxygen, behaves just like DCMU-poisoned Chlorella (Fig. 7). We therefore consider the decreased O2-consumption in the light to result from a partial inhibition of respiration and not from remaining photosynthesis unaffected by 10-5M DCMU. As photosystem I still operates in Bishop's mutant 11 as well as in DCMU-poisoned Chlorella, illumination might lead to an accumulation of ATP by cyclic photophosphorylation and thus to a lowering of the cellular ADP level. This could result in a slowing down of glycolysis and consequently of respiratory O2-uptake. © 1969 Springer-Verlag.
