Photochemical Production of Inorganic Nitrogen

Since most of the organic nitrogen is of limited bioavailability to autotrophs, photochemical transformation of recalcitrant DON into bioavailable forms implies an important (and unaccounted for) potential source of assimilable nitrogen. The study of Bushaw et al. (1996) was followed by several investigations containing 

Since only absorbed light can initiate photo transformations, it may be expected that samples of different optical properties show different photoproduction rates. For comparative purposes, the rates are thus usually absorbance-normafized. However, the inconsistency when comparing results from different studies is not resolved by such normalization and the variabifity of normafized rates exceeds an order of magnitude (Table 10.1). An attempt to find a correlation between available bulk characteristics (DON, DOC, pH, absorbance) and irradiation effects proved unsuccessful (Grzybowski, 2003). Additional confusion is introduced by reports on lack of ammonium release and even its removal during irradiation, observed in apparently similar samples (Table 10.1). 

The method descriptions in most of the pubfished experiments are not detailed enough to allow us to elucidate whether differences in experimental setup are responsible for the observed variability. For example, data on radiation intensity at the surface of the irradiated water layer are not sufficient in the case of optically thick samples where a significant fraction of the light is absorbed within the upper layer of the sample. Accordingly, differences among samples that differ in the fraction of incident light that is absorbed may not be proportional to the photochemical reactivity of the organic matter. 

Another factor responsible for the observed variability in positive results may be adsorption of ammonium and other nitrogen compounds onto humic substances (the main photoreactive fraction of DOM). Hence, detection of photoammonification may to some extent reflect photochemicaUy induced desorption of ammonium from large organic molecules, rather than the mineralization of organic nitrogen. This phenomenon, however, would be limited to fresh and low salinity waters, since at higher salinities ammonium ions are replaced by cationic macroconstituents of seawater. 

Maracaibo Lake water, pH = 7.72 Concentrate of Satiha River fulvic acids 

---

Koopmans, D. J., and Bronk, D. A. (2002) Photochemical production of inorganic nitrogen from dissolved organic nitrogen in waters of two estuaries and adjacent surficial groundwaters. Aquat. Microb. Ecol. 26, 295-304. 

---