THE LAW OF THE MINIMUM IN THE SEA 209 



the cultures, and after 112 days all the five tubes dealt with 

 still frothed, and the metaphenylene-diamine reaction was 

 positive, showing that there was still nitrite in the solution. 

 Finally, at a temperature of o° C. no growth and no gas formation 

 was observed, although the cultures were kept for 107 days. 

 These experiments were made with B. actinopelte in mixed culture, 

 but pure cultures gave similar results. B. lobatum was also 

 experimented with, and much the same behaviour of the micro- 

 organism was observed. It can apparently denitrify at o° C, 

 but the action is extremely slow. 



Here, then, is a possible, perhaps a very probable, explanation 

 of the relative scarcity of nitrogen compounds in the warmer 

 seas and the decrease in the amount of these which has been 

 observed in the colder seas in the warmer months of the year. 

 Denitrifying bacteria such as those mentioned have been shown 

 by Baur and others to be widely distributed, in the Baltic at 

 least, both in the mud at the sea-bottom as well as in the free 

 water. Probably the distribution will be extended very generally 

 to all seas as soon as adequate investigations have been made. 

 Such observations are being carried out on the cruises of the 

 International Fishery Investigations, but the results of these 

 are not yet published. If, as seems to be the case, these bacteria 

 are common in the sea, then a considerable amount of denitri- 

 fication must go on there. It is true that these organisms 

 require organic matter — carbohydrates or organic acids — in 

 order that they may exercise their function of decomposing 

 nitrogenous salts, and also organic nitrogenous matter as well. 

 They can only act on the latter when it has been nitrified by 

 the nitrifying bacteria which we know are present in the sea. 

 When by their own activity the nitrates, nitrites, and ammonia 

 resulting from the activity of the latter micro-organisms have 

 been reduced, then the denitrifying bacteria are condemned 

 (vcrdammte) to inactivity, but there is always a fresh supply 

 of the latter substances brought down to the sea by rivers, 

 and originated also by the decomposition of the dead bodies 

 of the organisms which die in the sea, so that the process is 

 a continuous one. 



The optimum temperature of the denitrifying bacteria 

 hitherto described is from 24° C. to about 28 C. It is of 

 course possible that forms may be described which are able 

 to denitrify at a temperature little above o° C, but it is unlikely 



