CH. Xl] BACTERIA IN THE SEA 269 



of carbon food — the simply constituted organic acids appear to be 

 a very suitable source of carbon. Because of its simpler tastes 

 and its reaction towards oxygen Azotohacter is probably of more 

 significance in the sea as a nitrogen-fixing organism than is 

 Clostridium. But it is important to note that between them these 

 two microbes can live and grow" in a great variety of conditions. 



Marine plants, plankton and the surfaces of the higher animals 

 thus afford a habitat for our two bacteria. Here they find their 

 carbon food, for they are resident in a mass of mucus which affords 

 the necessary nutriment. Their mineral salts they obtain from 

 solution in sea water ; their oxygen and nitrogen from the same 

 source. The association or symbiosis between the marine plants 

 and the bacteria is a helpful one from the point of view^ of either. 

 The waste carbonaceous matter of the plants or animals provides 

 the carbon food of the bacterium, and the nitrite or nitrate 

 elaborated by the bacterium is a source of food for the plant. 



Denitrifying bacteria. If, from the point of view of the 

 experimental chemist, the fixation of free nitrogen is an operation of 

 much difficulty so also is the reverse reaction, the decomposition of 

 the oxygen compounds of this element. Yet again w^e find that 

 just this decomposition can be effected by the activity of bacteria. 

 Denitrifying bacteria have been known to occur on the land for 

 some considerable time, but not until quite recently were they 

 known in the sea. Brandt, in 1898, predicted their occurrence in 

 the latter element from theoretical considerations, and in 1902 

 Baur^ described two species from the Baltic, and almost simultane- 

 ously Gran- described several species from the North Sea off the 

 coasts of Holland. Baur's species were Bacterium lohatum and B. 

 actinopelte ; and Gran's species were B.repens, B. trivialis and B. 

 henseni. 



Baur made use of a solution containing potassium hydrogen 

 phosphate, magnesium sulphate and carbonate, ammonium sulphate, 

 and salt in the proportion in which this occurs in sea water. He 

 inoculated this solution with mud from the bottom of a marine 



1 Baur, "Ueberzwei denitrificenden Bakterien aus der Ostsee," Wiss. Meeresunt. 

 Kiel Komm. Bd. vi. Abth. Kiel, 1902. 



- Gran, "Studien liber Meeresbakterien. I. Keduktion von Nitraten und Nitri- 

 ten," Bergens Museums Aarbog, No. 10, 1901. 



