Bushaw, K. L., Zepp, R. G., Tarr, M. A., Schulz-Jander, D., Bourbonniere, R. A., Hodson, R. E., Miller, W. L., Bronk, D. A., and Moran, M. A. 1996. Photochemical Release of Biologically Available Nitrogen from Dissolved Organic Matter. Nature 381:404-407.
         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.