438 PHOTOMECHANICAL CONSIDERATIONS 



the light- minus- dark difference spectrum for aerobic cells is very 

 similar to that for anaerobic cells (Fig. 1) and indicates the involve- 

 ment of both cytochromes and carotenoids. 



An important effect of oxygen on the cytochrome response to light 

 is the abolition of cytochrome reaction I, found in anaerobic cells (see 

 Fig. 2). This observation along with the oxygen- induced cytochrome 

 oxidation in illuminated cells indicates that oxygen affects both light- 

 sensitive and light-insensitive cytochrome systems, even though most 

 of the light-sensitive cytochrome remains reduced in the presence of 

 oxygen alone. 



Effects of electron transfer inhibitors. 



The effect of n-heptyl-hydroxyquinoline-N-oxide (HOQNO) was to 

 cause a concentration-dependent inhibition (half maximal at 2 x 10-6 

 M) of cytochrome and carotenoid reactions induced by high light in- 

 tensity. Antimycin A (50 /ig/ml), on the other hand, showed relatively 

 little effect on the light- induced reactions except to maintain the typi- 

 cal kinetics for aerobic cells in contrast to the typical kinetics for 

 anaerobic cells exhibited by the control. It appeared that this concen- 

 tration of antimycin A inhibited respiration and thus prevented the 

 suspension from becoming anaerobic without otherwise affecting the 

 light-induced reactions. 



DISCUSSION 



These observations of Rhodomicrobium are comparable in many 

 respects with the results obtained with other photosynthetic bacteria. 

 The light-induced carotenoid shifts are strikingly similar to those ob- 

 served by Smith and Ramirez (3) and Clayton (7) in Rhodopseudomonas 

 spheroides. The absorbancy changes due to light- induced cytochrome 

 oxidation are very similar to those observed in Chromatium (4), 

 Chlorobium and Chloropseiidomonas (5) in being attributable in the 

 main to c- type cytochrome (s). The cytochrome and carotenoid reactions 

 appear to be independent of one another as in Rps. spheroides . 



The presence of multiple inflection points in the curve of steady- 

 state cytochrome oxidation versus logarithm of light intensity is simi- 

 lar to the results previously observed in Chromatinu} where three in- 

 flection points were noted. The abolition of the most sensitive cyto- 

 chrome light reaction by oxygen is also common to both species. In 

 Chromatium this is effected through the oxidation of the most sensitive 

 cytochrome in the dark; in Rhodomicrobium this happens also, but in 

 addition other cytochromes not affected by light appear to be oxidized 

 at the same time. Rhodomicrobium appears to have three types of 

 cytochromes: (1) those which react to both light and oxygen, (2) those 



