Natural photolysis by ultraviolet irradiance of recalcitrant dissolved organic matter to simple substrates for rapid bacterial metabolism

Whole leachate and humic and fulvic acid fractions of dissolved organic matter (DOM) released from senescent littoral aquatic plants were exposed to varying spectra of ultraviolet radiation as well as natural UV of sunlight over different periods of time. Examination of the DOM by solid-state C-13 nuclear magnetic resonance and pyrolytic gas chromatography-mass spectrometry before and after photolysis revealed only subtle changes to the bulk DOM. However, the DOM exposed to natural UV radiation showed immediate stimulation of and sustained bacterial growth. Chemical analyses by high performance liquid chromatography (HPLC) of the small organic fractions generated by photolysis of humic substances showed marked, progressively increasing release of numerous small fatty acids, particularly acetic, formic, citric, pyruvic, and levulinic, among others. Use of radiolabeled humic substances demonstrated that these small compounds photolyzed from the humic substances were readily metabolized by the bacteria.