270 BACTERIA IN THE SEA [PART III 



aquarium, and in a few days obtained an abundant growth of 

 bacteria. This mixed culture was then inoculated in a solution 

 containing fish-broth, peptone and potassium nitrite. Again an 

 abundant bacterial growth formed and ultimately the food solution 

 became quite free from nitrite. Gas was freely evolved and this 

 was shewn to be free nitrogen. Baur found that his bacteria 

 could grow in an infusion of the sea-weed Fucus and in mussel 

 broth. 



Gran found that while some of his species of denitrifying 

 bacteria could utilise simple carbonaceous food-stuffs, others were 

 more fastidious and demanded complex materials. He found also 

 that his bacteria behaved differently towards nitrogen compounds. 

 Some could reduce nitrate to nitrite, and the latter to ammonia, 

 and this compound to free nitrogen. Others again carried on the 

 reduction only so far as ammonia. All the species described by 

 both investigators were aerobic germs, but some of Gran's bacteria 

 could live and grow in the absence of oxygen. Thus, just as in the 

 case of the nitrogen-fixing bacteria, the denitrifying ones can live 

 under very varied conditions of nourishment, and this is of great 

 importance, for we never find pure cultures of bacteria in nature, 

 but always various species associated together. Now it may often 

 happen that the waste products of one species may serve as the 

 food-stuffs for others. Obviously these symbioses are of great 

 significance. 



Temperature and denitrification. Thus the denitrifying 

 bacteria can reduce nitrates, nitrites, and ammonia and set free 

 elementary nitrogen from these compounds. In a typical culture 

 of these organisms the solution after standing for a few days begins 

 to froth and evolve gas. This gas is nitrogen. By-and-by this 

 frothing ceases and if the culture be then tested for nitrites or 

 nitrates it is found that these compounds have entirely disappeared. 

 All the combined nitrogen originally present in the culture has been 

 decomposed and the element has been returned to the atmosphere. 

 Now the length of time required for the completion of this change 

 depends on the temperature ; that is the activity of the microbes, 

 to which the decomposition of the nitrogen compounds is due, 

 varies with the temperature, just of course as we find to be the 



