Inhibition of phosphatase activity by dissolved humic substances and hydrolytic reactivation by natural ultraviolet light

1. Phosphatases released extracellularly by aquatic micro-organisms often complex with humic compounds that are released from decomposing tissues of plants and imported in dissolved and colloidal forms to lakes and rivers. 2. Dissolved humic substances from several natural sources formed complexes with phosphatases of bacterial and algal origin and reduced hydrolytic activity by noncompetitive inhibition. Restoration of the hydrolytic enzyme activities from the humic substances-enzyme complexes increased progressively over time when exposed experimentally to natural and artificial ultraviolet (UV) irradiance. 3. Greater phosphatase restoration occurred from humic acid-phosphatase complexes when humic acids were extracted from dissolved organic matter (DOM) of mixed natural plant sources, than when humic acids were isolated from a decomposing single plant species. 4. The data support a previously suggested hypothesis that phosphatases and other enzymes in aquatic ecosystems can complex with humic substances that dominate the DOM pool. These humic substances-enzyme complexes, in which the enzyme is temporarily inactivated, can be transported with water movements and displaced to other sites within the ecosystem. Upon exposure to UV irradiance in the photic zone, functional enzymes can be released. The potential for inactivation and storage of enzyme activity, relocation within the ecosystem, and subsequent reactivation holds important implications for regulation of nutrient cycling in fresh waters.