Vallentyne: Environmental Biophysics & Microbial Ubiquity 347 



media containing less than 15 per cent salt. See Clifton (1958, p. 262) for a 

 summary of Volcani's study of the Dead Sea biota. 



Solar evaporation ponds are often discolored by the growth of halophilic 

 bacteria and algae. According to Carpelan (cited by Gibor, 1956) photosyn- 

 thetic production rates in such environments are comparable to those in the 

 most productive parts of the oceans. Gibor (1956) has shown that the osmo- 

 tolerant brine flagellate, DunaUella salina, grows well in 10 X concentrated 

 artificial sea water. Some halophilic bacteria isolated from salt brines fail to 

 grow in salt solutions containing less than 16 per cent NaCl, and will survive 

 on dry crystals of salt obtained by the evaporation of brines (Browne, 1922). 

 According to Gibbons and Payne (1961) the most rapid growth rates of several 

 halophilic bacteria (Halobacterium spp. and Sarcina littoralis) occur in solutions 

 containing 20 to 25 per cent NaCl at temperatures in the range of 40° to 45° C. 

 ZoBell (1958) states that sulfate reducing bacteria grow naturally and can be 

 cultured in waters with salinities up to 300 per thousand. 



Pressure 



The effect of varying atmospheric pressure on the growth and reproduction 

 of microorganisms seems not to have been investigated in much detail. Strug- 

 hold (1961), however, passingly refers to the cultivation of soil bacteria under 

 an atmosphere with the composition and total pressure (0.1 Earth atmos.) of 

 that presumed to exist on Mars. The existence of barophilic bacteria in sub- 

 terranean deposits and deep sea sediments has been demonstrated by ZoBell 

 et al. Most organisms living in the surface regions of Earth fail to grow and 

 are killed by hydrostatic pressures of a few hundred atmospheres. In contrast 

 to these, barophilic bacteria isolated from the deep sea bottom can be cultured 

 only under hydrostatic pressures comparable to those in their natural environ- 

 ment, i.e., pressures of 1000 atmos. or more (ZoBell and Morita, 1956). The 

 viability of some barophiles is unaffected by alternate compression and decom- 

 pression between 1 and 1000 atmos. of hydrostatic pressure when applied 10 

 times within 10 minutes (ZoBell, 1958). ZoBell (personal communication) has 

 cultured deep sea bacteria under 1400 atmos. of hydrostatic pressure. 



Water 



Water is the most concentrated single molecule in protoplasm. Its depletion 

 can therefore be expected to restrict growth and reproduction. Most organ- 

 isms, microbes included, survive periods of extreme drought in dormant states, 

 often as spores. On the other hand, in the case of Pleurococcus vulgaris slightly 

 modified vegetative cells suffice to withstand prolonged drought (Fritch, 1922; 

 Fritch and Haines, 1923). According to Zeuch (1934) cell division of Pleurococ- 

 cus vulgaris can still occur at relative humidities of 68 per cent at 1° C, 55 per 

 cent at 10° C, and 48 per cent at 20° C. Aspergillus glaiicus is well known for 

 its growth on substrates where the activity of water (a„,) is as low as 0.65 to 0.70 

 (Scott, 1961). Kordyum and Bobchenko (1959) hold the opinion that many 

 microorganisms can actually use air as a habitat for growth and reproduction. 

 The growth of lichens on bare rock surfaces, bacteria and fungi in flour, and 

 many microorganisms in strongly saline media represent ecological instances of 



