Microbial Thermogenesis in the Decomposition of Plant Materials

Abstract

In expts. in Dewar flasks, moistened oat straw attained temps. usually between 45[degree] and 48[degree] in times varying from 36 to 60 hrs. Pre-extraction of the straw with water, dilute acid, or dilute alkali much reduced the maximum temp. reached and increased the time elapsing before the max. was reached. In an adiabatic apparatus, temp. maxima of 70[degree]-73[degree] were obtained in 40 hrs. or less. The rate of heat evolution was not uniform and 2 well-defined maxima were apparent, one about 40[degree] and one about 60[degree], coinciding with the points of maximum activity of mesophilic and thermophilic populations. The extent of decomposition was less than 4%. The rate of heat evolution and the rate of evolution of CO2 were closely related. Expts. involving several temp. ascents, the purpose of which was the exhaustion of readily fermentable constituents, were not conclusive, inasmuch as the active flora is modified by the high temps. The addition of an aqueous extract of straw caused a temp. of 66[degree] to be reached in 24 hrs. with a max. rate of 14.9[degree] in 3 hrs. between 35.5[degree] and 50.4[degree]. In subsequent ascents the rate of heat evolution progressively decreased as the readily available constituents were removed. Pre-extraction with reagents of increasing severity markedly diminished the rate of heat evolution but had no serious effect on the maximum temp. obtained. Two maxima were invariably found in the temp. rate curves. The absence of additional available N had no effect on the thermogenic processes in one temp. ascent. With a mixed population the 1st maximum in rate of heat evolution (about 3[degree] per hr. at 40[degree]) was always greater than the 2d maximum (less than 2[degree] per hr. at about 60[degree]). This was due to the prior removal of some of the most readily available constituents by the mesophilic flora, since in expts. commenced at 50[degree] the thermophilic peak was much higher. Numbers of mesophilic and thermophilic bacteria were found to increase enormously at the time of most rapid heat evolution in their respective ranges. Low final counts of mesophilic organisms were obtained, suggesting that the active mesophilic population is composed mainly of non-spore formers. Pure-culture decompositions were carried out with Aspergillus fumigatus and an unidentified bacillus. Both proved to be active only in the mesophilic range, but to be capable of raising the temp. of the straw from 25[degree] to nearly 55[degree].