Domain of the Marine Microbiologist 11 



were killed by being plated with molten nuti'ient agar poured 

 at 42 to 45 C (64). The maximum temperature for growth and 

 the thermal death point for most marine bacteria are only a few 

 degrees higher than their optimal growth temperature. The 

 temperature optima and tolerance of microorganisms is influenced 

 by salinity (45) and hydrostatic pressure (21, 65). 



Hydi'ostatic pressure increases with depth in the sea almost 

 exactly as a straight-line function, the pressure-depth gradient 

 being about 0.1 atm/m. At the greatest known depths (60) 

 pressures of nearly 1100 atm prevail. Approximately one-half of 

 the ocean area, or one-third of the total area of the earth, is 

 covered with 3800 or more meters of water, an aquatic environ- 

 ment characterized by pressures ranging from 380 to 1100 

 atm. Such pressures retard the growth of most sur- 

 face-dwelling bacteria, yeasts, and fungi from terrestrial soil, 

 sewage, freshwater bodies, and shallow (less than 3000 to 4000 

 m) layers of sea water. A good many, but not all, surface-dwelling 

 microbes lose their viability after several days in nutrient media 

 compressed to 600 atm (43). Few, if any, though, appear to be 

 injured by a few minutes' compression to 1000 atm. In unenriched 

 water and soil samples, surface-dwelling bacteria have been found 

 to be viable after several days' compression to 1500 atm at 5 to 25 

 C. Such observations suggest that the pressure tolerance of bac- 

 teria depends in part upon the chemical composition of the milieu. 

 Considerable difference has been observed in the pressure toler- 

 ance of different microbial species. 



Many bacteria recovered from the deep sea floor (7,000 to 

 10,000 m) grow in nuti'ient media at ambient pressures, i.e., 700 

 to 1000 atm (25, 67). A few of these deep sea bacteria, to which 

 the term barophilic has been applied, appear to grow onb' at 

 pressures matching their native habitat (65, 66). But other deep 

 sea bacteria grow as well or even better at 1 atm than when 

 compressed (25, 67). Bacteria in this latter categoiy (a) may 

 be passive mutants which ha\e come into being on the deep 

 sea floor, (b) they may be shallow water forms which have 

 settled and survived on the deep sea floor, or (c) they may be 

 eurybaric species capable of growing throughout a wide range 

 of hydrostatic pressure under certain conditions. Little is known 



