Extracellular enzymes in terrestrial, freshwater, and marine environments: perspectives on system variability and common research needs

Abstract

Extracellular enzymes produced by het- erotrophic microbial communities are major drivers of carbon and nutrient cycling in terrestrial, freshwater, and marine environments. Although carbon and nutrient cycles are coupled on global scales, studies of extracellular enzymes associated with terrestrial, freshwater, and marine microbial communities are not often compared across ecosystems. In part, this disconnect arises because the environmental parame- ters that control enzyme activities in terrestrial and freshwater systems, such as temperature, pH, and moisture content, have little explanatory power for patterns of enzyme activities in marine systems. Instead, factors such as the functional diversity of microbial communities may explain varying patterns of enzyme activities observed in the ocean to date. In any case, many studies across systems focus on similar issues that highlight the commonalities of microbial community organization. Examples include the effec- tive lifetime of enzymes released into the environ- ment; the extent to which microbial communities coordinate enzyme expression to decompose complex organic substrates; and the influence of microbial community composition on enzyme activities and kinetics. Here we review the often-disparate research foci in terrestrial, freshwater, and marine environ- ments. We consider the extent to which environmental factors may regulate extracellular enzyme activities within each ecosystem, and highlight commonalities and current methodological challenges to identify