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Digestion of Clay-Protein, Lignin-Protein, and Silica-Protein Complexes by Enzymes and Bacteria¹

ABSTRACT

As a contribution to an understanding of the stability of organic matter in soil, and to knowledge of microbial activity at solid-liquid interfaces generally, the relative influence of clay-protein and lignin-protein complex formation in retarding protein mineralization by enzymes and bacteria is evaluated. We have found that monolayers of lysozyme adsorbed on kaolinite and bentonite can be digested by chymotrypsin, mixed soil cultures, and pure cultures of B. subtilis, B. mycoides, and Pseudomonas sp. In contrast, lignin-protein complexes are extremely resistant to attack. Montmorillonite-protein monolayer formation retards the rate of enzymatic proteolysis; however, most of the protein can be hydrolyzed under optimum conditions in one day with a resultant decrease in X-ray diffraction (001) spacings. For equal clay surface areas, a mixture of soil infusion organisms or pure cultures of soil organisms hydrolyze protein freshly adsorbed on montmorillonite or kaolinite at rates nearly equal to that of free, i.e., nonadsorbed, protein, all being compared in aqueous suspension. A decrease in X-ray (001) spacing of protein-montmorillonite complexes is observed following digestion by the organisms. Dried, rewetted montmorillonite-protein complexes are more resistant to attack by soil organisms than freshly prepared complexes. Complexes of protein with silica gel are somewhat comparable to lignin-protein complexes in being resistant to digestion by Pseudomonas sp.

Contribution from Department of Soils and Plant Nutrition, University of California, Berkeley. Parts of dissertations submitted to the Graduate Division by Eva F. Estermann and G. H. Peterson in partial fulfillment of the requirements for the Ph.D. degree. This investigation was supported in part by a research grant (G-4236) from the National Institutes of Health, U. S. Public Health Service. Presented before Div. III, Soil Science Society of America, Nov. 20, 1957, at Atlanta, Ga.

² Junior Research Biochemist, formerly Research Assistant (now at Purdue University, Lafayette, Indiana), and Professor of soil Chemistry, respectively.

Received for publication May 5, 1958. Accepted for publication July 15, 1958.