By ARTHUR T. HENRICI
DEPARTMENT OF BACTERIOLOGY AND IMMUNOLOGY, UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MINN.
Je mehr der Einblick in das Leben eines Sees vertieft werden soll, desto mehr müssen auch die Bakterien in den Bereich der Untersuchung gezogen werden; ja man darf sogar behaupten, dass für das Verständnis der oft stark ineinandergreifenden Lebenszyklen in einem See, besonders im eutrophen See, überhaupt für den gesamten Stoffhaushalt in eienem stehenden Gewässer, die Kenntnis der Tätigkeit sowohl der im freien Wasser als der im Schlamme lebenden Bakterien unerlässlich ist.
-- Huber-Pestalozzi ( 1938).
THIS quotation may well serve to indicate the nature of the problem to be solved in studying the bacteria of lakes. A similar quotation might be derived from a number of general works on limnology, for most limnologists have realized that bacteria play an important part in the economy of lakes. But definite information upon this phase of lake study is almost completely lacking; for bacteriology has lagged considerably behind the other biological sciences in participating in the study of lake ecology. Whereas botanists and zoologists have long since made their collections and classified their species, and have studied the distribution of them, so that they are now able to synthesize the results of their investigations from the standpoint of lake-types, etc., bacteriologists have yet to learn, for the most part, what species of bacteria live characteristically in water, and have only the most general ideas as to what they do there or how they are distributed. It is highly desirable that limnologists become more interested in bacteria, and bacteriologists more interested in limnology.
Our lack of knowledge of water bacteria is due largely to a lack of interest on the part of bacteriologists, who have been so concerned with the relations of bacteria to disease, agriculture, and industry that they have paid but little attention to bacteria of no "Practical" importance. Most of the work on bacteria in water has been carried out either from the standpoint of potability (the occurrence of disease-producing bacteria in the water) or from the standpoint of pollution (the self-purification of streams). Only a scattered few have done any work on the bacteria characteristic of the fresh water itself. The culture media used for the study of bacteria of medical importance are not suitable for the cultivation of the autochthonous water bacteria, and from the work of public health bacteriologists there developed an idea that there are no bacteria characteristic of water. The introduction of sodium caseinate ( Nährstoff Heyden) medium by Düggeli ( 1924) and by Fred Wilson, and Davenport ( 1924), provided indications that there are true water bacteria, i.e., species not derived from the surrounding soil or other pollution. During the past 15 years there has developed a slowly-growing literature on lake bacteria as such. This literature has been reviewed by Baier ( 1935). Only certain papers will be referred to here.
We incline toward the view that the main function of bacteria in lakes must be the decomposition of dead organic matter, leading to a mineralization of the elements composing this organic matter, so that these elements are again available to photosynthetic plants for the synthesis of new organic matter. Thus the lake bacteria link the ends of the food-chain, convert it into a cycle.
But this is not the only role of bacteria in lakes, and may not be the most important one. Our knowledge of bacteria in all habitats has been obtained through the use of artificial culture media, the composition of which (mainly organic) has been determined by habit established in studying bacteria of medical importance. Thus we know best the heterotrophlic bacteria, those that obtain their energy by the oxidation