INTRODUCTION 149 



bacteria per cubic centimeter of plankton tow, ranged from 202,000,000 

 to 347,000,000 ; 2 the ratio between numbers in the tow and numbers in 

 the sea water alone was from 225 :1 to 2270 :1. 3 The differences depend 

 on the type of plankton rather than on depth or other factors when 

 contaminated areas are avoided. Off Andros Island as many as 160,- 

 000,000 bacteria were found per cubic centimeter of mud. 4 They live 

 both at the upper surface and in decreasing numbers deeper in the mud. 

 Bacteria are present in fewer numbers on or in sand, but they are more 

 abundant in the water immediately above sandy as compared with 

 muddy bottoms. 



The number of bacteria on the continental shelf decreases with 

 distance from land except in the deeper layers of mud, where they 

 remain constant. Bottom materials from oceanic depths contain still 

 fewer bacteria and have been reported as being entirely absent in some 

 of the samples, at least by the methods used. 



The long dispute concerning the presence or absence of nitrifying 

 bacteria in the sea seems to have been settled conclusively by Waksman 

 using the facilities of the Atlantis of Woods Hole. Both the aerobic 

 Azotobacter and the anaerobic Clostridium occur. Waksman suggests 

 the following hypothesis concerning the nitrogen cycle in the sea: 3 



Decomposition of the organic nitrogenous compounds takes place 

 in the sea water but largely on the sea bottom, with the result that 

 the ammonia is then liberated. This ammonia is rapidly oxidized by 

 specific bacteria living in the bottom to nitrite and later to nitrate. 

 This nitrate remains in the sea bottom and is not reduced, due to 

 a lack of available energy for the nitrate-reducing bacteria and 

 not to a lack of such bacteria. The small amounts of ammonia found 

 in the sea water originate from the plant and animal residues in the 

 plankton and in the water. The nitrate formed in the bottom 

 gradually diffuses into the water where it remains as such. On reach- 

 ing the zone of photosynthetic activities, this nitrate is consumed 

 by the phytoplankton or is reduced by the nitrate-reducing bacteria 

 to nitrite, which may also be gradually consumed by the plants. 

 Very little nitrate reduction to gaseous nitrogen or complete deni- 

 trification is possible under normal sea conditions. Reduction of 

 nitrate to nitrite does not mean necessarily any loss of nitrogen 

 from the cycle of life in the sea. 



By such relations there is seen to be good reasons, in addition to 

 those to be given later, for the greater richness of life in the shallower 

 waters of the continental shelf, on oceanic banks, and in polar regions 

 where the circulation tends to bring the nitrites and nitrates up to the 

 lighted zone. 



