46 



PROBLEMS OF LAKE BIOLOGY 



bacteria growing on the glass. A compari- 

 son of these witli cultures isolated directly 

 from the water showed that different 

 species w^ere obtained by the two methods. 

 All of the pure cultures isolated directly 

 from the water were capable of growing 

 upon submerged slides in the laboratory. 



The submerged slide method offers an- 

 other approach to the study of lake bac- 

 teria. The distribution of morphologically 

 identifiable but non-cultivable species may 

 be followed through different habitats. 

 From a correlation of the distribution with 

 phj^sical and chemical characters of the 

 habitats, one may surmise something re- 

 garding the physiological requirements of 

 the organism, and proceed to devise a suit- 

 able culture medium. The iron bacterium, 

 Siderocapsa major, is readily recognizable. 

 Searching for this organism on slides sub- 

 merged in a variety of lakes shows that it 

 occurs only in hard-water drainage lakes 

 neutral or alkaline in reaction. Since fer- 

 rous iron is insoluble in alkaline water, this 

 organism must deposit iron by utilizing 

 organic iron compounds. Presumably the 

 organism could be grown in a medium with 

 organic iron, slightly alkaline in reaction, 

 though this has not been tried yet (Hard- 

 man and Henrici 1939). 



Bacteria occur in all of the ecologic 

 groups of aquatic life recognized by the 

 limnologist. Some of our earliest descrip- 

 tions of bacteria concern material obtained 

 from the "wasserspiegel," the iridescent 

 film of neuston organisms which collects 

 upon the surface of still waters. Bacterial 

 pellicles develop, however, only upon small 

 shallow bodies of w^ater high in organic 

 matter, and are not a factor in lakes. The 

 bacteria suspended or swimming in the 

 water, and which grow in plate cultures or 

 appear in microscopic examinations of con- 

 centrated lake water are to be included in 

 the plankton. Both microscopic and cul- 

 tural studies of bottom deposits show that 

 the benthic bacteria are vastly greater in 

 numbers than anywhere in the water above. 

 Probably, however, bacteria are more char- 

 acteristically a part of the periphyton than 

 of any other ecologic group. This term, 

 introduced by Behning (1928) and not yet 



in general use by limnologists, is the equiva- 

 lent of what the Germans call "aufwuchs- 

 organismen," and refers to those aquatic 

 organisms which grow attached to sub- 

 merged surfaces. There is much evidence 

 that the bacteria in the water are not, for 

 the most part, free-floating, but are at- 

 tached to algae or other plankton organ- 

 isms. There is evidence that the great 

 abundance of bacteria in bottom deposits 

 is due largely to the fact that they are car- 

 ried to the bottom by the sedimentation of 

 larger organisms. Rocks and rooted plants, 

 piles and piers, the bottoms of boats, all 

 submerged surfaces soon become coated 

 with a film of periphytic bacteria. It is 

 quite possible that the few bacteria free in 

 the water are but motile reproductive 

 bodies, swarmers, derived from attached 

 perii3hytie bacteria, just as one may trap in 

 the plankton motile spores of sessile algae. 



ZoBell and Anderson (1936) have shown 

 that the growth of bacteria in stored water 

 samples is probably due mainly to the in- 

 fluence of the surface of the vessel in which 

 the water is stored. That is, an inert sur- 

 face like glass is sufficient to cause an in- 

 crease in tlie bacteria. They showed that 

 this increase is roughly proportional to the 

 amount of surface introduced into the en- 

 vironment. This effect is probabl}^ due to 

 the concentration of organic matter ad- 

 sorbed onto the surface. AVhile the total 

 amount of dissolved matter in the water 

 may be sufficient to support a larger popu- 

 lation of bacteria, the concentration is too 

 low to permit of their fullest development; 

 when the concentration is increased by ad- 

 sorption the bacteria grow^ further. That 

 such a concentration of organic matter 

 from the w^ater on submerged surfaces 

 actually occurs has been demonstrated by 

 Stark, Stadler, and McCoy (1938). It is 

 clear, therefore, that the occurrence of sur- 

 faces either as a support for the bacteria, 

 or in concentrating their food elements, is 

 a factor of fundamental importance in the 

 growth of aquatic bacteria. 



In the preceding paragraphs I have pre- 

 sented a rather condensed resume of the 

 status of lake bacteriology from the stand- 

 point of hydrobiology. The papers cited do 



