THE ACTION OF SOME DENITRIFYING BACTEIIIA. 



149 



smaller and usually ovoid in shape, and of a somewhat darker 

 colour than those on the surface. Growth does not occur on 

 gelatin media. 



Acid formation takes place in Glucose, hut not in Cane-sugar, 

 Lactose, or Mannite media. 



Growth is much retarded by exposure to a strong light, and 

 cultures on sloped Peptone Agar are completely killed by four 

 hours' exposure to bright sunlight. 



The bacterium is a strict aerobe. 



Free growth takes place in Gran's medium, but develops much 

 slower than in the case of the denitrifying form : no growth occurs 

 if the Potassium nitrate be omitted entirely, but takes place freely 

 if a mere trace in excess of that normally present in the sea- water be 

 added, though no denitrification results. Attempts were made to dis- 

 cover whether this bacterium had any nitrifying or denitrifying 

 action in various culture media, but uniformly negative results 

 were obtained. Nitrites were neither oxidized to nitrates, nor 

 reduced to Ammonia or free Nitrogen, and Ammonium salts were 

 unaftected. No growth was obtained in any culture medium that 

 did not contain at least a trace of nitrates, so it was not practic- 

 able to ascertain whether the bacterium had a nitrifying action 

 without the necessary facilities for quantitative work. 



On one occasion samples were obtained from various depths up to 

 90 fathoms at a point in the Gulf Stream, 25 miles south of 

 the Dry Tortugas. 



The samples were plated in the Peptone Agar medium, and counted 

 with the following average results : — 



If any deductions can be made from one series of observations, it 

 would seem probable that the non-denitrifying bacteria are a deep- 



