ZoBell — 192 — Marine Microbiology 



in 41 cakes of ice examined in 1943. The average aerobic plate count was 

 4 per ml. at 37° C. and 7 per ml. at 20° C. Fifteen samples were sterile or 

 showed fewer than two bacteria per ml. Jensen asserted that Escherichia 

 coli died off in ice more rapidly at 0° to — 5° C, the temperature of natural 

 ice on lakes, than at — 20° C. or lower. E. coli remained viable for long 

 periods when frozen in distilled water at — 16°, — 40°, and — 79° C. 

 Species of Achromohacter, Aerobacter, Bacillus, Cellulomonas, Chromohac- 

 terium, Flavobacterium, Micrococcus, Proteus, Pseudomonas, Serratia, and 

 Spirillum were found in newly harvested lake and river ice. Bacteria were 

 found in polar ice and snow by Gazert (1902), McLean (1918), and 

 Darling and Siple (1941). 



It was directed to the attention of the author by Dr. E. B. Fred of the 

 University of Wisconsin that more bacteria have been found in water im- 

 mediately under the ice on Lake Mendota than at greater depths. This 

 observation was confirmed by demonstrating the presence of from two to 

 three times as many bacteria in the water adjacent to a 40-cm. layer of ice 

 covering Lake Mendota as in water samples collected at depths of from 

 two to five meters beneath the ice. The ice itself generally contained 

 fewer than a hundred bacteria per ml. compared with several thousand 

 bacteria per ml. found in the underlying water. There are several pos- 

 sible explanations, none of which has been established as the cause of the 

 concentration of bacteria next to the ice: {a) Nutrients may be somewhat 

 more concentrated in the water adjacent to ice because as water solidifies, 

 most of the dissolved solids remain in the liquid fraction, {b) Ice may pro- 

 vide a beneficial solid surface for increased bacterial activity or for adsorp- 

 tion of nutrients (see page 84). (c) Trihydrol, a form of water which pre- 

 dominates in ice, may stimulate the growth and activity of bacteria, as 

 suggested by the work of Barnes and Jahn (1934). Harvey (1933) noted 

 that recently melted ice water seemed to stimulate the growth of diatoms. 



While ice cannot be relied upon to prevent the spoilage of fish and other 

 flesh foods, it has a preservative value. According to Tarr and Sunder- 

 land (1940), ice prepared from water treated with o.i per cent of either 

 benzoic acid or sodium nitrite is relatively effective for preserving fish 

 fillets and other flesh foods. Jensen (1943) reports that chlorine-water 

 ice, azochloramide ice, katadyn silver ice, sodium propionate ice, and 

 other kinds of germicidal ice show promise for various purposes. 



Ice presents special problems where dilution and sedimentation are 

 depended upon to minimize the danger from sewage discharged into rivers 

 or lakes. IJnless proper precautions are taken^ the warmer, less dense 

 water from sewage effluents may flow over the surface of colder, more 

 dense water, and under certain conditions even over the surface of ice for 

 considerable distance. Such a condition is very noticeable in Great Salt 

 Lake where fresh water flowing over the surface of the denser saline water 

 in mid-winter freezes to form a sheet of ice, over which more fresh water 

 flows farther and farther from land, until higher temperatures or turbu- 

 lence breaks up the ice sheet. 



Bacteria and allied microorganisms preserved in glacial ice may be 

 transported hundreds of miles to sea in icebergs. Icebergs play an impor- 

 tant role in the transportation of sedimentary materials at high latitudes. 



