Impact of DNOM photo-degradation on its bioavailability in freshwater lakes

Summary

The effect of photo-degradation on amount and functional characteristics of dissolved natural organic matter (DNOM) in regards to its bioavailability is not fully understood. It is postulated that larger compounds of DNOM, including humic acids (HA,) are broken down into fulvic acids (FA), which has a lower molecular weight and thus is more soluble and bioavailable (Maier et al, 2009). In order to better understand the factors controlling the photo-degradation process DNOM was exposed to artificial solar irradiation (Q-Sun Xenon Test Chamber) for different time periods. The exposed samples were then incubated for 25 days. Changes in DNOM concentrations and characteristics during the photo-oxidation and pre- and post- incubation were measured by use of UV-VIS and fluorescence spectroscopy, TOC analyzer, pH and cation analysis. By breaking down conjugated double-bonds photo-degradation transforms colored refractory DNOM to more biodegradable forms. UV-VIS spectroscopy revealed a loss of absorbance (pre- and post-incubation) in the visible range wavelength (400 to 450 nm), demonstrating this photo-bleaching effect. With short-term photo-exposure (four hours or less), UV absorption increased during post-incubation, indicating an enrichment of aromatic compounds during the biodegradation incubation. The samples exposed to sunlight for 12 hours or more had similar UV absorption pre- and post-incubation. This may suggest that aromatic compounds are relatively stable against photo- and biodegradation, as demonstrated by Kalbitz et al (2003). Fluorescence spectroscopy showed that both HA and FA peak intensities increased in an excitation-emission matrix (EEM) with increasing time of photo-exposure as well as during incubation, with the largest increase for FA. The increase in the humic matter intensity is weak and the peak drifts towards FA with increased photo-exposure time. The shifts in fluorescence imply that the photochemical oxidation process induces structural changes to the DNOM (Mostofa et al., 2005). Collective assessments of changes in absorbency and fluorescence indicate that photo-degradation processes leads to smaller and thereby more bioavailable molecules. In addition, subsequent microbial activity indicates that the structural changes induced by DNOM photo-degradation affecting bioavailability do not occur in a short time span of four hours. Other collected data are under current analysis and will be presented at the Nordic-Baltic IHSS 2013 meeting. References: Kalbitz, K., J. Schmerwitz, D. Schwesig, E. Matzner, 2003. Biodegradation of soil-derived dissolved organic matter as related to its properties. Geoderma 113, 273–291. Maier, R. M., I. L. Pepper, C. P. Gerba. (2009). Environmental microbiology. Amsterdam, Academic Press. Mostofa K. M. G., T. Yoshioka, E. Konohira, E. Tanoue, K. Hayakawa, M. Takahashi, 2005, Three-dimensional fluorescence as a tool for investigating the dynamics of dissolved organic matter in the Lake Biwa watershed. Limnology 6, 101-115.