Chapter X 



TRANSFORMATION OF ORGANIC MATTER 



The chief function of bacteria in the carbon cycle is the decomposition 

 of organic matter to CO2, water, ammonia, and certain minerals. The 

 efficiency with which they perform this function is indicated by the low 

 content of organic matter in sea water, which averages less than 5 mgm. 

 per liter. It is because bacteria are able to mineralize virtually all kinds 

 of organic matter that the sea has aptly been characterized as the world's 

 largest and most efficient septic tank. 



A second function of bacteria in the carbon cycle is that of converting 

 waste and dissolved organic matter into bacterial cell substance which can 

 be assimilated by filter-feeding and mud-eating animals (see page 173). 

 The bacteria may convert as much as 30 to 40 per cent of the organic 

 matter into bacterial cell substance while oxidizing the rest to CO2 and 

 water as a source of energy. 



Third, certain bacteria living symbiotically in the intestinal tracts of 

 animals may aid the latter in the digestion of food. There is ample evi- 

 dence that such bacteria are widely distributed, but there are no data 

 for apraising quantitatively their importance in the carbon cycle. 



A fourth function is the primary production of organic matter by 

 chemosynthetic and photosynthetic autotrophs. Below the photo- 

 synthetic zone, chemosynthetic bacteria are the only primary producers 

 which can synthesize organic matter from CO2 and water. However, in 

 spite of the academic interest centering around chemosynthetic and 

 photosynthetic bacteria, they appear, when compared with green plants, 

 to play a very minor role as primary producers in the sea. 



Quantity of organic matter decomposed : — Different workers, using 

 various methods of approach, estimate that the primary production of 

 organic matter in the sea ranges from 5 to 1000 grams of organic matter 

 per square meter per year (see literature summarized by Riley, 1941). 

 Taking 10 gm. /My year as a conservative production figure, there is pro- 

 duced in the oceans of the world a minimum of 3.7 Xio^^ kilograms of 

 organic carbon per year. Only a small fraction of this organic matter is 



Table XXXV. — Summer standing crop in Wisconsin lakes given as kilograms per hectare 

 of lake surface on a wet-weight, ash-free basis (from Juday, 1942): — 



Excluding fish, the standing crop of which is not given. 



