370 DEPTHS OF THE OCEAN chap. 



nitrates or nitrites, though denitrification is as little a vital 

 necessity for them as alcoholic fermentation is for the fermenta- 

 tion fungi. Feeding them with sugar and ammoniacal salts will 

 result in their multiplying to an unlimited number of generations, 

 without exhibiting their power of denitrification. They can 

 attack nitrates whenever met with, utilise their oxygen, and 

 give off nitrogen, but denitrification is not of any particular 

 importance, provided the bacteria find sufficient free oxygen in 

 their surroundings. It is only when this fails that they attack 

 nitrates to any great extent. Given the requisite quantity of 

 oxygen they will enter the regular circulation, and no nitrogen 

 worth mentioning will be produced even where denitrifying 

 bacteria are living and multiplying. 



This is the case at any rate in the soil, where denitrification 

 is of no importance, unless nitrates are brought into contact 

 with considerable quantities of easily disintegrated organic 

 substance. In the sea the quantity of organic substance is 

 generally so small that a cubic centimetre of salt-water from the 

 open sea rarely contains more than 50 to 100 living bacteria 

 cells, while the nitrogenous compounds occur for the most part 

 as ammonia or inorganic compounds, and not as nitrates or 

 nitrites. It is more than likely that nitrates are not formed to 

 any great extent in sea- water. Nitrifying bacteria are met 

 with occasionally in the mud along the coasts, but they have 

 not been proved to exist in the open sea ; in any case they 

 have not the same importance there that they possess on land, 

 where numbers of them are present in every single gram of 

 cultivated earth. So it is probable that the small quantities of 

 nitrates and nitrites in the sea-water are brought either from 

 the land, or in a minor degree from the atmosphere as the 

 result of electrical discharges. Most of the combined nitrogen 

 of the sea occurs as organic compounds or as saline ammonia, 

 neither of which can be reduced by denitrification. Supposing 

 then that denitrification does play any noticeable part, it will 

 only be in more or less enclosed bays and fjords, where 

 there is a comparatively large amount of organic substance, 

 a plentiful supply of nitrates from land, and so little circulation 

 that there may be a lack of oxygen. In the open sea it is 

 negligible. 

 N.ithansohn. We must look for other conditions to explain the apparent 



irregularities in the distribution of the plankton. Nathansohn 

 was the first to notice that vertical currents are bound to 

 exercise considerable influence. If it be true that one or 



