40 



PROBLEMS OF LAKE BIOLOGY 



or fermentation of organic matter. By the 

 use of purely inorganic media a number of 

 bacterial species have been discovered 

 which are not organisms of decay. These 

 autotrophic bacteria are very probably also 

 of great importance in the metabolism of 

 lakes. They fall into two divisions : the 

 chemosynthetic species which obtain energy 

 from the oxidation of inorganic elements or 

 compounds (hydrogen, sulphur, iron, am- 

 monia, carbon monoxide), and the photo- 

 synthetic species. The latter, the red and 

 green sulphur bacteria, possess pigments 

 which, like chlorophyll, trap the energy of 

 sunlight, and use it to transform carbon 

 dioxide and hydrogen sulphide into organic 

 matter. The chemosynthetic bacteria are 

 extensively discussed by Waksman {Prin- 

 ciples of Soil Microhiology, 1932) ; the 

 photosynthetic bacteria, which are mainly 

 aquatic in habitat, have been studied by 

 van Niel (1931, 1935, and 1936) and van 

 Niel and Muller (1931). 



Heterotrophic bacteria must be listed as 

 consuming organisms, autotrophic bacteria 

 as producing organisms, in the economy of 

 lakes. Both heterotrophic and autotrophic 

 bacteria may of course serve as food for 

 part of the zooplankton. The relative im- 

 portance of bacteria as producing organ- 

 isms in lakes has been discussed by Birge 

 and Juday (1922), who concluded that 

 they are relatively unimportant as com- 

 pared with the higher phytoplankton. The 

 degree to which bacteria may serve as food 

 for the zooplankton has been discussed by 

 Baier (1935) who cites further literature. 



A consideration of the importance of bac- 

 teria as producers and consumers does not, 

 however, adequately present the role of bac- 

 teria in the economy of lakes. Hetero- 

 trophic and autotrophic bacteria together 

 are transformers of materials. Just as the 

 essential character of protoplasm is its 

 chemical instability, its power continuously 

 to transform matter and energy, so from a 

 biological standpoint the essential charac- 

 ter of a lake is its ability continuously to 

 transform matter and energy through the 

 activities of its total fauna and flora. And 

 in these transformations bacteria play a 

 very large part, since they grow and trans- 



form matter more rapidly than higher or- 

 ganisms, and because they can perform 

 chemical reactions impossible to other or- 



ganisms. 



A few examples may serve to show the 

 importance of bacteria in lake metabolism. 

 Proteins derived from higher organisms are 

 decomposed by heterotrophic bacteria, the 

 nitrogen eventually liberated as ammonia ; 

 this ammonia is oxidized by autotrophic 

 bacteria to nitrates. Under anaerobic con- 

 ditions in the bottom muds, organic matter 

 is fermented by heterotrophic bacteria that 

 liberate methane and hydrogen ; these gases 

 are oxidized in the aerated waters above by 

 autotrophic bacteria. Sulphates are re- 

 duced in the lake bottom by heterotrophic 

 bacteria which are thus able to oxidize or- 

 ganic matter under anaerobic conditions; 

 the resulting sulphides, diffusing to the 

 aerated water above, are oxidized to sul- 

 phates by autotrophic bacteria. It is this 

 complex interplay in the activities of dif- 

 ferent sorts of bacteria which is the most 

 interesting and important part of a study 

 of the role of bacteria in the economy of 

 lakes. 



These generalizations regarding the ac- 

 tivities of lake bacteria are derived from 

 our general knowledge of bacteria, not 

 based upon extensive studies of the activi- 

 ties of bacteria in the lakes themselves. A 

 proper study of lake bacteria should begin 

 with an analytic and experimental study 

 of the effects of various environmental fac- 

 tors upon individual species of water bac- 

 teria, to be followed by a synthetic study of 

 the various lake environments, observing 

 and correlating the natural occurrence of 

 these factors and of the bacterial flora 

 which they determine. We have as yet only 

 a small amount of information regarding 

 the characteristic species of water bacteria, 

 we have not learned yet how to grow many 

 of them in artificial cultures, so that we 

 cannot even make a beginning of this study. 

 All that we may state at the present time 

 regarding the influence of environmental 

 factors upon lake bacteria must be a gen- 

 eralization, again drawn from our knowl- 

 edge of other kinds of bacteria. 



We have seen that bacteria are very di- 



