Quantitative ecology of psychrophilic microorganisms.

To obtain information on the importance of psychrophiles in nature, 95 samples of soil, water, mud, and various foods were quantitatively assayed for their content of psychrophilic bacteria and fungi and also for mesophilic and thermophilic bacteria and fungi. Thousands to millions of psychrophilic bacteria were present per gram of soil and represented 0.5 to 86% of the bacterial population. Also, about 25% of the fungi in uncultivated soil were psychrophilic. In stream and river water, psychrophilic bacteria constituted 16 to 47% of the bacterial population; in lake water, 41 to 76%; and in lake mud, 11 to 33%. Large numbers of psychrophilic bacteria were present in dairy products, meats, and other foods, and accounted for 35 to 93% of the bacterial population of meats. In contrast, thermophilic bacteria usually comprised 1% or less of the bacterial population in all of the materials examined. The data indicate that psychrophiles are both ubiquitous and numerous in nature, and probably play important roles in the cycles of matter.