THE DISTRIBT'TIOX OF BACTERIA IX LAKES 



41 



verse in their food requirements. In tliis 

 group nature lias experimented with every 

 possible mode of nutrition. It would be 

 difficult to imagine a natui'ally-occurring 

 environment that would not provide chemi- 

 cals suitable for the growth of some sort of 

 bacterium. They are equally versatile with 

 regard to other environmental factors. 

 There are bacteria known which can grow" 

 at temperatures below the freezing point ; 

 others that can grow at 80° C. There are 

 bacteria that can grow in distilled water, 

 others that can grow in damp salt. There 

 are bacteria tliat can groAv in acidities lower 

 than pH 1, others in alkalinities as high as 

 pH 13. There are bacteria that can grow 

 in the complete absence of oxygen, others 

 exposed to the air. From the standpoint of 

 foodstuffs, temperatures, osmotic pressures, 

 hydrogen-ion concentrations, oxidation-re- 

 duction potentials, bacteria are adapted to 

 a wider range than any other group of liv- 

 ing organisms, Avith the possible exception 

 of the blue-green algae. One cannot 

 imagine any sort of lake environment that 

 would not provide a habitat for some sort 

 of bacteria. 



The problem of the relationship of bac- 

 teria to the productivity of lakes is very 

 similar to the problem of their relationship 

 to soil fertility. AVe have already de- 

 veloped an extensive soil bacteriology. The 

 aquatic habitat differs from soil in two im- 

 portant particulars. Soil is normally well 

 aerated; only when it is waterlogged is 

 oxygenation reduced, and under such con- 

 ditions the soil takes on the characters of 

 an aquatic habitat. Lakes are often an- 

 aerobic for part of the year in their deeper 

 portions, entirely so in their bottom de- 

 posits, where bacteria are most numerous. 

 Thus anaerobic bacteria must be much more 

 important in the ecology of lakes than in 

 soil, and the decomposition of organic mat- 

 ter in lake bottoms must be very slow and 

 imperfect as compared with that in the soil. 

 A number of limnologists have recognized 

 the importance of bacteria in the oxygen 

 regimen of lakes ; this topic has been ex- 

 tensively discussed by Grote (1934) and by 

 Kusnetzow (1935). It is the tremendous 

 oxygen-consuming power of the large num- 



bers of bacteria in the lake bottom which 

 makes for complete anaerobiosis a few milli- 

 meters below the mud-water interface, 

 which causes oxj-gen to disappear below the 

 thermocline in stratified euti'ophic lakes, 

 A\liich causes fish to suffocate in shallow 

 waters in the winter. 



Secondly, soil differs from the aquatic 

 habitat in that the latter is penetrated by 

 light. AVe have as yet but little informa- 

 tion on the effect of light on bacteria. It is 

 well known that the ultraviolet portion of 

 the spectrum is highly lethal, and it is 

 W'idely believed that light is to a large ex- 

 tent responsible for the self-purification of 

 polluted streams. Minder (1920) sought to 

 explain the summer minimum of bacteria 

 in Lake Zurich, and the low counts obtained 

 from surface samples, on the basis of lethal 

 action of sunlight. This view was also sup- 

 ported by Buchner (1893) and by Zih 

 (1932). This belief is. however, not ade- 

 quately supported by experimental evi- 

 dence ; there is confusion regarding the de- 

 gree to which the lethal ultraviolet light 

 rays penetrate into water. 



The work of Snow and Fred (1926) con- 

 firmed by a number of other later workers 

 has shown that pigmented bacteria, rela- 

 tively rare in soil, are abundant in lake 

 w^aters. From 30 per cent to 50 per cent 

 of the colonies appearing in plate cultures 

 are pigmented. This peculiarity of the lake 

 flora is shared by the sea and by streams, 

 and is probably related to the fact that 

 light can penetrate into the aquatic habitat. 

 Chromogenic bacteria are almost com- 

 pletely absent in the dark waters of dys- 

 trophic lakes. They are found on the 

 leaves of plants, and it is noteworthy that 

 the onlj" pigmented parasitic bacteria of 

 man occur most abundantly on the skin. 

 Pigment production by bacteria appears to 

 be clearly correlated with exposure to light, 

 but we do not know what function the pig- 

 ments perform. 



In earlier literature on water bacteriol- 

 ogy one finds the question raised as to 

 whether there is a characteristic auloch- 

 thonous bacterial flora of water. This 

 question was usually answered in the nega- 

 tive. But all of the earlier work was car- 



