The photochemical conversion of the nitrogen fraction of
dissolved humic substances into more biologically available compounds
was studied in two estuarine sites in the southeastern US.  Marine humic
substances were isolated using an XAD-8 resin and used in bacterial
bioassays and chemical studies.  The bioassays demonstrated that humic
substances irradiated with natural sunlight supported enhanced bacterial
growth, measured as cell accumulation and protein production, due to
increased availability of both the carbon and nitrogen components. 
Chemical analyses demonstrated the photoproduction of ammonium and
dissolved primary amines from the coastal humic substances.  The total
biologically available nitrogen (ammonium, dissolved primary amines, and
other unidentified compounds) formed during a day-long irradiation at
natural solar radiation levels accounted for about 6% of the original
nitrogen associated with the humic substances.  Photochemical
modification of marine humic substances may provide a source of labile
nitrogen to estuarine and coastal ecosystems that has not previously
been considered.