128 Nitrogen-Metabolism on Land and in the Sea. 
and Azotobacter Chroococcum, were found in the slime at the bottom, 
and as plankton in the waters, of the bay of Kiel. 
These two organisms can live in a medium which contains no 
nitrogen compound, but they require for their growth and for the 
absorption of nitrogen a supply of carbohydrate. As this substance 
is, of course, not to be found in sea-water it seems very probable 
that it is supplied by some plant with which the bacteria live in 
symbiosis. The slime in which the Azotobacter was found is 
covered with a fine layer of Diatoms and it seems very probable 
that the bacteria live in association with them. It was in fact 
found that the organism would not develop in pure sea-water, but 
only if the water had been “ sown ” with a little plankton. The 
Azotobacter was also found growing on the slimy surface of 
Laminaria, Fucus, etc.; it is very probably in actual symbiosis with 
them, for such growths are almost of the nature of agar-agar plate 
cultures, that substance being nothing more than the cell-wall 
material of certain marine algje. From the cell-wall material the 
bacterium could obtain the carbohydrate necessary for its growth, and 
could, perhaps, supply the alga with the nitrates formed by its 
activity. In the open sea Reinke suggests that it is possible that 
the same or similar organisms are to be found actually growing on 
the surface of the vegetable plankton such as Diatoms and 
Peridineae. 
The similarity between this symbiosis, if it should be proved to 
exist, and the well-known one between bacteria and leguminous 
plants is certainly very striking, and makes the analogy between 
circulation of nitrogen and the general metabolism on land and in 
the sea still more close. 
The question of the assimilation of free nitrogen in the sea is 
only one of the important questions in connection with the meta¬ 
bolism of the ocean, our knowledge of which is yet in its infancy. 
The astonishing difference of proportion which is to be observed 
between tropical and temperate vegetation on land and in the sea, 
respectively, is still unexplained. For while the land vegetation is 
generally much more prolific in the tropics owing to the greater sup¬ 
ply of sunlight, higher temperature and greater degree of moisture, 
yet in the sea the exact opposite is the case. In spite of the greater 
supply of radiant energy and the higher temperature the plankton 
and marine vegetation generally is much less in amount in the 
warmer waters than in temperate and colder regions. Brandt has 
put forward the interesting suggestion that owing to the higher 
