Variation in the optimum pH of hydrolytic enzyme activities in tropical rain forest soils

Abstract

Extracellular enzymes synthesized by soil microbes play a central role in the biogeochemical cycling of nutrients in the environment. The pH optima of eight hydrolytic enzymes involved in the cycles of carbon, nitrogen, phosphorus, and sulfur, were assessed in a series of tropical forest soils of contrasting pH from the Republic of Panama. Assays were conducted using 4-methylumbelliferone-linked fluorogenic substrates in modified universal buffer. Optimum pH values varied markedly among enzymes and soils. Enzymes were grouped into three classes based on their pH optima: (i) enzymes with acidic pH optima that were consistent among soils (cellobiohydrolase, {beta}-xylanase, arylsulfatase); (ii) enzymes with acidic pH optima that varied systematically with soil pH, with the most acidic optima in the most acidic soils ({alpha}-glucosidase, {beta}-glucosidase, N-acetyl-{beta}-glucosaminidase); and (iii) enzymes with pH optima in either the acid and alkaline range depending on soil pH (phosphomonoesterase, phosphodiesterase). The optimum pH of phosphomonoesterase was consistent among soils, being pH 4-5 for acid phosphomonoesterase and pH 10-11 for alkaline phosphomonoesterase. In contrast, the optimum pH for phosphodiesterase activity varied systematically with soil pH, with the most acidic optima (pH 3.0) in the most acidic soils and the most alkaline optima (pH 10) in near-neutral soils. Arylsulfatase activity had a very acidic pH optimum in all soils ([≤] pH 3.0) irrespective of soil pH. The differences in optimum pH may be linked to the origin of the enzymes and/or the degree of stabilization on solid surfaces. The results have important implications for the interpretation of hydrolytic enzyme assays using fluorogenic substrates.