Photochemical formation of biologically available nitrogen from dissolved humic substances in coastal marine systems

The photochemical conversion of the nitrogen fraction of dissolved humic substances into more biologically available compounds was studied in 2 estuarine sites in the southeastern U.S. 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 hurnic substances. The total biologically available nitrogen [ammonium, &ssolved primary amines, and other unidentified compounds) formed during a day-long irradiation at natural solar radiat~on 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 estuanne and coastal ecosystems that has not previously been considered.