MEASURING PHOTOSYNTHETIC ACTIVITY 229 



b. Luminous Bacteria. 



Another method of detecting the oxygen produced in photosynthesis 

 was devised by Beijerinck. The method depends upon the phenomenon 

 of luminescence which is produced in certain bacteria in the presence of 

 oxygen ; in the absence of oxygen the bacteria emit no Hght. The method 

 is best adapted to the use of plate cultures of luminous bacteria (Micrococ- 

 cus phosphoreus Cohn), in seawater containing marine algae. The cul- 

 ture containing an alga can also be sealed in a tube ; when all the oxygen 

 in the tube has been consumed by respiration, the bacteria cease to lumi- 

 nesce. After illumination of but a few minutes if the tube is then taken 

 into the dark, the luminescence is plainly visible. Molisch = claims that 

 such a tube which has been allowed to become dark, becomes clearly 

 luminescent if it is illuminated for only a few seconds with a match. 

 The amount of oxygen produced by photosynthesis by so feeble and brief 

 an illumination must be exceedingly small and is less than can be detected 

 by any chemical means or probably even by any physical method. Harvey 

 and Morrison ° have determined the concentration of oxygen which causes 

 "just perceptible" luminescence of luminous bacteria. This they find to 

 be at a pressure of oxygen of about 0.005 mm. of mercury or 1 part by 

 weight of oxygen in 3,700,000,000 cc. of sea water. 



c. Motile Bacteria Method. 



By means of this method, which was introduced by Englemann, many 

 of the earlier observations on photosynthesis were made. The method de- 

 pends upon the fact that certain aerobic bacteria are motile in the pres- 

 ence of oxygen and quite inactive in the absence thereof. It is essential 

 that the appropriate bacteria be employed. Those which have been most 

 generally used are the forms usually grouped under the general term 

 Bacterium termo (Proteus vulgaris, Hauser). Certain Spirillum forms 

 may also be employed, but these are exceedingly sensitive, so that great 

 care must be used in order to avoid experimental error. Care must also 

 be exercised to ensure absolutely pure cultures which are free from 

 anaerobic forms. Slope or stab cultures may be made on bullion-agar, 

 which are allowed to develop at 25°. The cultures should not be more 

 than one to two weeks old and the bacteria taken from the edges of the 

 growth in order to obtain the motile forms. 



The plant material, the photosynthetic activity of which is to be ob- 

 served, is of course confined to small objects ; and some experience in 

 the use of the microscope is essential. Algal filaments, sections of leaves 

 or single cells are placed on a slide in a fluid containing the bacteria. 

 It is desirable to add sufficient bacteria so that a drop shows slight 

 turbidity. The solutions to be used should contain a small percentage 



"Molisch, "Lichtentwickelung in den Pflanzen," Leipzig, 1905, p. IS. "Leuchtende 

 Pflanzen," Jena, 1904. 



'Harvey and Morrison, Jour. Gen. Physiol., 6, 13 (1924). 



