PELAGIC PLANT LIFE 369 



rivers, as organic nitrogenous compounds, ammoniacal salts, and 

 nitrates. The result would be a constant increase, until at last 

 the sea became poisoned, were it not that it is continually being 

 absorbed by living organisms, or else being restored in some 

 form or other to the atmosphere. We now know that there is 

 very little combined nitrogen in the sea, so that it must evidently 

 be used up as fast as it arrives. The consumers of nitrogen 

 are first and foremost the seaweeds growing along the coasts, 

 and the floating algae of the open sea, but besides them there 

 are also bacteria, which exist in all sea-water, as shown by 

 Fischer. Their competition with the algae for the nitro- Fischer. 

 genous compounds is not of any great consequence, so long 

 as they do not interfere with the circulation of nitrogen other- 

 wise than by disintegrating organic compounds so as to form 

 ammonia, or by binding ammonia and nitrates in their cells 

 as albumen. 



From the bacteria-life of the soil, however, we are acquainted Nitrifying and 

 with another kind of nitrogenous metamorphosis produced by bacterS?"^ 

 bacteria. There are nitrifying species which oxidise ammonia 

 into nitrites and nitrates, without requiring organic substance to 

 enable them to live ; there are further whole series of other 

 species which can reduce nitrites and nitrates, and give off 

 nitrogen in a free state. Their action drives out of the natural 

 circulation larger or smaller quantities of this valuable nutritive 

 substance, scarce enough already, which all plants generally 

 utilise to the uttermost. How great the loss is, as compared 

 with the metamorphosis in other respects, and under what 

 conditions it takes place, are questions that require our most 

 careful attention before considering anything else. 



Baur, and others after him, succeeded in finding several Baur. 

 kinds of these denitrifying bacteria in the sea, where they 

 appeared to be widely distributed. It was found, too, that 

 they produced free nitrogen with greater rapidity when the 

 temperature was high (20° to 30° C.) than when it was low. 

 Brandt, accordingly, put forward the hypothesis, that to the 

 activity of these bacteria is due the fact that the abundance of 

 plant life does not increase as we approach the tropics, but 

 on the contrary very often decreases. This theory has now 

 for some years been considered the only explanation of the 

 irregular distribution of the plankton, but recent researches 

 have shown that it is untenable. 



The denitrifying bacteria require organic substance for their 

 existence. If they are to give off free nitrogen, they must have 



2 B 



