LIGHT-INDUCED ABSORBANCY CHANGES IN 

 RHODOMICROBIUM VANNIELII 



JOHN M. OLSON and SIGEHIRO MORITA 



Biology Department, Brookhaven National Laboratory, Upton. New York 

 and Department of Microbiology , Dartmouth Medical School, 

 Hanover, New Hampshire 



In both purple and green photosynthetic bacteria, light absorbed by 

 chlorophyll(s) causes the oxidation of cytochrome (s) under appropriate 

 conditions (1-5), In several strains of purple bacteria the cytochrome 

 changes are masked by a pronounced red shift of the carotenoid ab- 

 sorption peaks (6,7). These general observations have been found to 

 apply also to the obligate, 2ina.evohicx>hotoheterotroph,RJiodo)mcrobium 

 vannielii, which is similar to the purple bacteria in its complement of 

 photosynthetic pigments. 



METHODS 



The bacteria were grown as described previously (8) and harvested 

 by centrifugation. The cells were resuspended in 0.05 M phosphate 

 buffer, pH 7.4, for observation in a Chance- type double-beam spectro- 

 photometer (9) in which the measuringbeams were deflected by a half- 

 silvered mirror vertically down through the sample cuvette to mini- 

 mize changes in light transmission due to the settling of the filamentous 

 mass of bacteria in the cuvette. Monochromatic actinic light (0.80/i) 

 was projected through the half-silvered mirror down onto the sample 

 cuvette from a slide projector with appropriate filters. Actinic light 

 intensity was monitored by a calibrated photodiode. A blue- green filter 

 between sample cuvette and photo multiplier transmitted the measuring 

 light while attenuating the actinic light to a negligible level. 



In most experiments the sample cuvette was completely filled with 

 bacterial suspension. However, in experiments in which the transition 

 from anaerobiosis to aerobiosis was observed, a thin layer (2-3 mm) 

 of a dense cell suspension covered the bottom of the cuvette while the 

 remainder (7-8 mm) was flushed with either wet N2 or O2. 



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