Dissolved organic matter in marine and freshwater ecosystems
constitutes one of the largest active organic matter reservoirs on earth. 
The bacterially-mediated turnover of chemically identifiable, low molecular
weight components of this pool has been studied in detail for nearly three
decades, but these compounds constitute less than 20% of the total
reservoir.  In contrast, little is known about the fate of higher molecular
weight, biologically more refractory molecules that make up the bulk of
dissolved organic matter, including aquatic humic substances.  Here we
report results from bacterial bioassays indicating that exposure to sunlight
causes the release of nitrogen-rich compounds from humic substances which
are biologically available and enhance the subsequent bacterial degradation
of the humic substances.  Photochemical studies demonstrated that ammonium
is among the nitrogenous compounds released, produced most efficiently by
wavelengths in the UV region.  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.