302 P. W. Flanagan and F. L. Bunnell 



temperature can result in a higher rate of activity near the minimum tem- 

 peratures (Bunnell et al. 1977a). In support of the second hypothesis it is 

 noted that observed relations between respiration and temperature in 

 soils do not depart statistically from linear, and computed Q.o values in 

 the soil are frequently less than 2.0. Qio values for respiration decrease 

 from the surface litter downwards into the soil. Numbers of psychro- 

 philes increase from the phyllosphere to the 10-cm depth in soil, and the 

 majority of psychrophiles and several important cold-tolerant meso- 

 philes have linear responses to increasing temperature (Figures 9-2 and 

 9-4). Because such organisms have not been recorded for temperate 

 regions it would seem that the second hypothesis is further supported. 



Temperature Influences on Substrate Utilization 



About 80 to 90% of the tundra bacteria that are cellulolytic at 15 °C 

 are unable to use cellulose as a sole source of carbon at 0°C. But 82% of 

 these same bacteria use glucose as a sole source of carbon at 0°C. These 

 observations are similar to those made in Canadian arctic lakes by Chris- 

 tensen (1974), who found 6% of the cellulolytic Cytophaga to be psy- 

 chrophilic. The bacteria capable of decomposing hydrocarbons that were 

 isolated from plots treated with oil at the Biome research area illustrate 

 similar relations with temperature. Bacteria that could use mineral oil as 

 a sole source of carbon at 15 °C could not metabolize the same substrate 

 at 0°C (Campbell et al. 1973). They could, however, use succinic acid or 

 glutamic acid as sole sources of carbon at 0° and 15 °C. 



The limited data suggest that a temperature near 0°C eliminates spe- 

 cific metabolic pathways of bacteria while other metabolic activities, e.g. 

 respiration, in the same organisms are only depressed. At low tempera- 

 ture bacteria can sustain themselves on substrates which at higher tem- 

 perature they might ignore in favor of larger molecular compounds. 

 Comparable data are not available for temperate forms of the organisms 

 found at Barrow, but one interpretation of such a switch between sub- 

 strates is that the tundra organisms are responding to the demands of a 

 colder environment. Such a response broadly conforms to hypothesis 

 one, and suggests that tundra bacteria extend their capabilities at lower 

 temperature by exploiting compounds of lower molecular weight. 



The influences of temperature on the potentials for substrate utiliza- 

 tion by fungi are greatest for simple substrates (Figure 9-1). Utilization 

 of cellulose and phenohc compounds does not cease below 5 °C and in 

 most cases continues to sub-zero levels, although usually at lower rates. 

 Potentials for fungal utilization of pectin and starch are substantially 

 decreased when the temperature falls below 5 °C but do not cease. De- 

 composers of hemicelluloses have not been fully examined, but prelimin- 



