268 BACTERIA IN THE SEA [PART III 



Clostridium and Azotohacter occur in all land soils except moor- 

 lands, and they are also present in sand dunes, especially when 

 *' Strandpflanzen " are abundant. They occur in sea water, in 

 fresh water, and in the mud and sand from the sea bottom. They 

 are present on the surfaces of both fresh water and marine plankton, 

 and on the surfaces of sea-weeds, and such animals as starfishes 

 and molluscs. They have been described from the North Sea, the 

 Baltic, and the sea off the coasts of Africa and the Malay 

 Peninsula, and from the Indian Ocean. Probably both species are 

 quite universally distributed. 



Clostridium is an anaerobe and Azotohacter is an aerobe. Again 

 the former is a bacillus, while the latter is a coccus. They have 

 this character in common, that they can take up atmospheric 

 nitrogen and fix it, forming nitrous or nitric acids, which then 

 combine with whatever bases are present. They can both be grown 

 in food solutions containing absolutely no fixed nitrogen. 

 Nevertheless in such solutions nitrites and nitrates accumulate. 



Clostridium can live in a solution containing absolutely no free 

 oxygen : indeed the presence of this gas is inimical to the activity 

 of the organism. Winogradsky cultivated it in a solution over 

 which passed a continual stream of free nitrogen. It requires, of 

 course, a source of carbon and is somewhat fastidious in this respect, 

 demanding such carbohydrates as cane-sugar, laevulose, dextrose, &c. 

 It cannot assimilate the higher alcohols, starch, cellulose or the 

 organic acids. It requires potassium and magnesium salts and a 

 source of sulphur. As it grows acid is produced, and this must be 

 neutralised by the addition of calcium carbonate to the nutritive 

 solution, free acid being detrimental to the growth of the bacillus. 

 The salinity of the solution does not matter greatly, thus it can 

 grow in both fresh and sea water and in water containing up to 

 8^0 of common salt. Though it is an anaerobic germ which is 

 harmed by oxygen it can yet live in solutions containing this gas 

 in the free condition, if along with it there are other bacteria 

 which by taking up the oxygen can shield the Clostridia from the 

 influence of the gas. This is a case of true symbiosis. 



Azotohacter resembles Clostridium in its reaction towards 

 nitrogen. But it is an aerobic bacterium and grows best in the 

 presence of abundant oxygen. It can avail itself of a wide choice 



