The fate of dissolved organic matter (DOM) in aquatic environments is of interest from a number of perspectives, including aquatic food web dynamics and carbon biogeochemistry. Bacterial utilization of DOM in freshwater and marine environments was investigated in several related studies of the effects of natural photochemical processes on the biological turnover of DOM. In the first study, fulvic acids (a hydrophobic component of aquatic DOM) were isolated from a boreal wetland in northern Canada. Bacterial bioassays indicated the formation of biologically-available substrates from the fulvic acids during irradiation with natural sunlight. A matrix of inorganic nutrient additions provided evidence for the photoproduction of labile carbon-rich compounds, as well as evidence for labile nitrogenous photoproducts. Chemical analyses demonstrated the photoproduction of ammonium during irradiation. DOM from a number of other freshwater and estuarine systems subjected to irradiation were likewise found to yield ammonium via photochemical processes. A model of photochemical nitrogen formation based on an estuarine sample from the southeastern U.S. indicated that the photochemical production of labile nitrogen may be particularly important in marine systems, in this case increasing the amount of terrestrial nitrogen available to bacteria by up to 20%. A second study of photochemical formation of nitrogen photoproducts was subsequently undertaken for two coastal sites in the southeastern U.S.. Bacterial bioassays and chemical analyses demonstrated the formation of labile nitrogen photoproducts, and both ammonium and dissolved primary amines were detected. Up to 6% of the refractory organic nitrogen component of DOM from these systems was converted into more biologically available forms by sunlight. In a third study, an investigation was carried out into which components of DOM are the source of biologically-available photoproducts. DOM from a boreal wetland in Canada was separated by resin techniques into hydrophobic acids (HPOA), hydrophilic acids (HPIA), and hydrophilic neutrals (HPIN). These three fractions constitute approximately 75% of the DOM pool. Bacterial bioassays indicated that all fractions yielded available photoproducts following irradiation. Although previous studies focused on the most hydrophobic (and highly colored) fraction of DOM (HPOA or humic substances), these experiments indicated that other components can be equally important as sources of biologically-available photoproducts.