DISSOLVED organic material in marine and freshwater ecosystems constitutes one of the Earth’s largest actively cycled reservoirs for organic matter(1). The bacterially mediated turnover of chemically identifiable, low-molecular-mass components of this pool has been studied in detail for nearly three decades, but these compounds constitute less than 20% of the total reservoir(2). In contrast, little is known about the fate of the larger, biologically more refractory molecules-including humic substances-which make up the bulk of dissolved organic matter. Here we report results from bacterial bioassays and photochemical studies indicating that exposure to sunlight causes dissolved organic matter to release nitrogen-rich compounds that are biologically available, thus enhancing the bacterial degradation of humic substances. We demonstrate that ammonium is among the nitrogenous compounds released and is produced most efficiently by ultraviolet wavelengths. Photochemical release of ammonium from dissolved organic matter has important implications for nitrogen availability in many aquatic ecosystems, including nitrogen-limited high-latitude environments and coastal oceans, where inputs of terrestrial humic substances are high.