THE BIOLOGICAL DECOMPOSITION OF PLANT MATERIALS. PART III. PHYSIOLOGICAL STUDIES ON SOME CELLULOSE‐DECOMPOSING FUNGI

Abstract

SUMMARY. Since the major part of the loss of organic matter in the biological decomposition of mature plant materials is accounted for by the loss in cellulose, the present investigation was directed to the nature of the fungi assisting in this process. Certain fungi were isolated, which though actively attacking cellulose in straws, do not make more than meagre growth on celluloseagar plates. All were found to have their optimum temperature above that usual for fungal growth; three, indeed, could develop at 50° C. The availability of various nitrogen and carbon compounds to these organisms was tested. Particular attention was directed to those sugars naturally occurring in that class of substances known as hemicelluloses. The pentose sugars, though readily available, did not appear so suitable as their sterically related hexoses. Variation, in the form of “sectoring” of the colonies, was observed in the case of one organism only, namely Sepedonium sp. A wedge of a light‐spored variant appeared, which has remained constant throughout several culture generations. Certain carbohydrates, xylose and fructose, in particular, favoured the appearance of this variant. Since the decomposition of straw and other plant materials by a mixed microflora is often accompanied by a rise in temperature, the thermogenic power on sterile straw of the organisms individually was examined. A considerable rise in temperature was observed in some cases, and some rise in all. The highest temperature attained was 49.0° C, which is outside the temperature range on a synthetic medium of the organism giving it, namely Trichoderma sp. That this was not due to chemical oxidation following the rise due to fungal activity was shown by the fact that sub‐cultures of the organism were obtained from the straw which had reached this high temperature. The thermogenic powers of certain simple combinations of these organisms were tested, and in each case a depressing effect was observed. In the cases in which two organisms only were involved, the combination gave a temperature near to that of the one with lesser thermogenic power. The evolution of CO2 closely paralleled the production of heat, when the heat‐retaining ability of the vacuum flasks is taken into account. The period of maximum heat production was shown to correspond with that of rapid loss of hemicelluloses. Cellulose decomposition, which is the next phase, does not appear to result in the production of much heat. Although microbial thermogenesis is an incidental, and not an essential effect of metabolism, it may markedly affect a decomposition by bringing the substrate to the optimum temperature, or even by modifying the flora involved. The association of organisms may be competitive or co‐operative in thermogenic power. In competitive association there is competition for constituents of similar availability, and the heat produced by organisms of different thermogenic power in competitive association will be an intermediate figure. In co‐operative association certain stages in the decomposition may be more effectively and rapidly carried out by one organism than another, or intermediate or by‐products readily utilised. In this way the decomposition may be more rapid and more complete, and heat may be produced from substances which might otherwise not be attacked. Certain associations of organisms may, therefore, be of greater thermogenic power than the individuals participating. The author is indebted to Sir John Russell, F.R.S., Director of the Rothamsted Experimental Station, for placing at his disposal the facilities of that Station, and to the Department of Scientific and Industrial Research for Senior Research Award, during the tenure of which this work has been carried out.Especially are his thanks due to Dr W. B. Brierley, Head of the Department of Mycology, whose suggestions and advice throughout have been invaluable.