THE ACTION OF SOME DENITRIFYING BACTERIA. 151 



break down nitrates at a temperature of 20' C. The optiiuuni tempera- 

 ture for denitritication produced by this bacterium appears to be about 

 20° C, as the process was less rapid at average temperatures of 17' C. 

 and 25° C. At a temperature of 32° C, rapid growth took place, Init 

 no denitritication resulted. 



It should be noted that these temperature observations were only 

 made with suljcultures from colonies on Peptone Agar and Peptone 

 Gelatin media, and there is reason to believe that the power of denitri- 

 tication becomes diminished after cultivation on such media. Further 

 and more accurate temperature experiments are required, in which the 

 culture medium is directly inoculated with freshly collected samples of 

 water. 



INVESTIGATION OF SAMPLES OF WATER FEOM THE 

 MAPiQUESAS KEYS, AND THE EXPERIMENTAL PRE- 

 CIPITATION OF CALCIUM CARBONATE BY BACTERIAL 

 AGENCY^ 



The Marquesas Keys constitute a coral atoll which forms part of the 

 long chain of Keys separating the Gulf of Mexico from the Straits of 

 Florida. AVithin the atoll the water is very shallow, and the bottom 

 consists of a fine chalky mud many feet deep. Samples of the water 

 from within the atoll were sent to me at Plymouth by post, and 

 examined fourteen days after collection. 



On plating on Peptone Agar, 800 colonies per 1 c.c. of the sample 

 were obtained. Tliese colonies appeared to be all of one species, and 

 in appearance and all cultural characteristics were identical with the 

 denitrifying form previously described as occurring around the 

 Tor tu gas. 



A suspension of these bacteria from a culture on Peptone Agar was 

 made in sterile sea-water, and a similar suspension, containing roughly 

 the same number of bacteria, was made from a third subculture on 

 Peptone Agar of tlie bacteria obtained from the station seventy miles 

 west of Ushant. 1 c.c. of each of these suspensions was then added 

 to 1000 c.c. of the modified Gran's medium : some of these cultures 

 were kept at an average temperature of 20°C. and others at o2°C., with 

 the following results : — 



At 20° C. cultures from Manjuesas .showed trace of nitrite after 45 hours. 

 „ ,, ,, gave strong nitrite reaction „ 53 ,, 



„ „ 70 miles W. ILshant showed trace of "I .,^ 



nitrite . . . . • ■' 



., „ 70 miles we.st of Ushant she wed \ w..^ 



strou'f nitrite reaction . . . -^ 



