Interactions of marine biogeochemical cycles and the photodegradation of dissolved organic carbon and dissolved organic nitrogen

Recent human-induced changes in atmospheric composition are having major effects on UV radiation reaching the Earth's surface. Experimental and modeling results discussed in this paper indicate that these changes in UV radiation and accompanying changes in photochemical reactions in the sea are an important element of global change that impacts marine biogeochemical cycles. Exposure to solar radiation enhances the oxidation of marine dissolved organic carbon (DOG) resulting in fading in the UV and blue spectral region, loss of fluorescence, and formation of trace carbon gases including carbon dioxide, carbon monoxide, and carbonyl sulfide. Photochemical production of such trace gases occurs most rapidly in coastal regions of the sea. In addition to the direct production of trace gases, photodegradation of biologically-refractory organic matter produces organic substrates that can be readily mineralized or taken up by microbiota. Photodegradation of organic matter also converts biologically-resistant dissolved organic nitrogen (DON) to ammonium and other forms of labile nitrogen. This process can increase fertility and productivity of certain N-limited ecosystems. Action spectra indicate that organic matter photodegradation is primarily induced by the UV part of the solar spectral irradiance in seawater. Methods that are being used to study these reactions in the laboratory and field are discussed as are approaches to modeling the effects of these photoreactions on marine biogeochemical cycles.