Photochemical Transformations and Bacterial Utilization of High-Molecular-Weight Dissolved Organic Carbon in a Southern Louisiana Tidal Stream (Bayou Trepagnier)

UV irradiation of dissolved organic carbon (DOC) in the laboratory can produce small, labile organic compounds utilizable by microbes, but few studies have attempted to document this process in situ. 13C nuclear magnetic resonance (NMR) was used to examine the bulk chemical composition of natural and laboratory-irradiated high-molecular-weight DOC (HMW-DOC) from shaded (150 μ mol m-2 s-1 average light in surface water) and open (1500 μ mol m-2 s-1) field sites over one and a half years. 13C NMR revealed only small differences in carbon functional groups between laboratory irradiated and non-irradiated HMW-DOC. However, bacterial protein productivity per cell (BPP) was enhanced in naturally irradiated samples of HMW-DOC in a field mesocosm experiment (p < 0.05), suggesting that bacterial growth was enhanced by photochemical production of labile DOC substrates. Absorbance characteristics such as spectral slope, absorbance at 350 nm, and the absorbance ratio 250 nm/365 nm revealed that HMW-DOC was photoreactive, yet no differences in these values were found between samples irradiated with and without UV-B. In experiments conducted with simulated solar radiation in the laboratory and with natural light in the field mesocosm experiment, UV-A (320-400 nm) and photosynthetically active radiation (PAR; 400-700 nm) were more effective than UV-B (280-320 nm) in HMW-DOC photolysis.