Organo-Mineral–Enzyme Interaction and Soil Enzyme Activity

Both microbially produced enzymes and mineral surfaces can be considered catalysts of chemical transformation in soils and other geological environments such as sediments and subsurface aquifers. While both are important direct agents of organic matter (OM) remineralization and transformation, mineral surfaces can also act as “heterogeneous co-catalysts,” influencing the kinetic properties of biological enzymes by providing surfaces upon which reactions can take place. Enzymes may be categorized, according to their location as “intracellular,” those present in living and proliferating cells, or “abiontic,” i.e. all others (Skujins 1976). Within the latter group, extracellular enzymes may be leaked or lysed from dead cells or actively secreted by living bacteria and fungi cells (Burns 1982). They are of particular importance to the biogeochemistry of soils and sediments, in that they hydrolyze large polymeric organic compounds into small monomers, which then can be passed through cell walls and fuel microbial respiration and growth. Thus, their activity may be the rate-limiting step in governing the degradation of OM and the remineralization of carbon and other nutrients. While classical enzyme experiments have typically been carried out in buffered solutions and under easily controllable conditions, the soil environment represents a wholly different set of conditions and extracellular enzyme activity cannot be expected to conform to the “ideal.” In addition to the variables often cited as influencing enzyme activity such as temperature, pH, and substrate availability, the activity and stability of a particular enzyme may be enormously influenced by its interaction