344 Annals New York Academy of Sciences 



to corroborate these findings (Rabinowitch, 1945; Zeller, 1951). Before leav- 

 ing the subject of growth at low temperatures it must be stressed that in all 

 cases the growth is slow, usually requiring weeks and sometimes months before 

 definitive results are obtained. 



At the upper end of the temperature scale it has long been known that some 

 bacteria and blue-green algae exist in hot springs with temperatures in the 

 range of 80° to 88° C. For summaries of existing information the works of 

 Copeland (1938), Precht et al. (1955), and Allen (1960) should be consulted. 



Baker et al. (1955), have cultured a strain of Bacillus stearothermophilus at 

 80° C. No attempt was made to determine whether growth would still occur 

 at higher temperatures. According to ZoBell (1958) thermophilic sulfate re- 

 ducing bacteria isolated from subterranean deposits have been cultured in the 

 laboratory at temperatures to 65° to 85° C. These forms were originally ob- 

 tained from depths of 6000 to 12,000 feet, at which temperatures in situ ranged 

 from 60° to 105° C. and hydrostatic pressures from 200 to 400 atmos. ZoBell 

 (1958) also states: "The maximum temperature at which the thermophilic cul- 

 tures are active is increased by compression. At 1000 atmospheres one culture 

 reproduced and produced HoS at 104° C. No attempt has been made to as- 

 certain whether bacteria will grow at temperatures higher than 104° C. when 

 compressed, but indications are highly suggestive of the possibilities in view of 

 the protective effect of high pressure on the thermal tolerance of bacteria." 

 The case referred to represents the highest temperature so far recorded for the 

 growth and reproduction of any organism. 



Eh and pH 



The best general treatment of the environmental limits of Eh and pH 

 for growth and reproduction is that given by Baas Becking et al. (1960). 

 These workers have summarized paired Eh-pH data for the growth of diverse 

 microorganisms in natural environments and laboratory cultures. Although 

 the Eh values may in some cases not represent truly reversible potentials they 

 at least give a reproducible and reasonably accurate picture. Their results 

 are shown graphically in figure 1. When the data for all microorganisms are 

 combined and compared to Eh-pH measurements in natural surface waters of 

 the earth, a complete overlap is observed. This suggests that there is probably 

 no major aqueous environment that cannot be colonized by some microor- 

 ganism. The range for growth and reproduction of microorganisms was found 

 to lie between 850 mv. and —450 mv. on the Eh scale (when expressed as Eh at 

 the prevailing pH) ; and between values of 1 .0 and 10.2 on the pH scale. These, 

 however, do not represent the true extremes because the authors considered 

 only data for which paired measurements of Eh and pH were available. 



Some environmental extremes of pH that can be tolerated by reproducing 

 populations may now be cited. Thiobacilli are well known for their abihty to 

 grow in acid solutions. In fact, they tend to show optimal growth in the pH 

 range of 1 to 3, many growing poorly above pH 7. Carbon dioxide is the sole 

 carbon source, and energy is obtained from the oxidation of reduced forms of 

 sulfur to sulfate under aerobic conditions. Growth and reproduction can 

 occur at pH values in the neighborhood of 0, and cultures receiving no initial 



