Direct and Indirect Photoreactions of Chromophoric Dissolved Organic Matter: Roles of Reactive Oxygen Species and Iron
Abstract
Photochemical transformations of chromophoric dissolved organic matter (CDOM) are one of the principal processes controlling its fate in coastal waters. Photoreactions ultimately lead to the destruction of the chromophores and hence to the loss of absorption and fluorescence (bleaching), thus acting as a sink for CDOM and photodecomposition may proceed both via direct photochemical reactions or via indirect processes, involving photochemically generated intermediates such as reactive oxygen species (ROS). Superoxide (O2) reactions with CDOM did not appear to degrade the CDOM. Instead, CDOM catalysed the dismutation of O2 to O2 and HOOH. This reactivity has the effect of limiting the steady-state concentration of O2 in most coastal waters. Reactions of CDOM with radiolytically produced hydroxyl radical (OH.) formed CO2 and several low molecular weight carboxylic acids, as well as bleached both the absorption and fluorescence at slow rates. These reactions did not increase the bioavailability of this material to a microbial consortium. Addition of iron to several coastal seawater samples and to solutions of Suwannee River Fulvic Acid neither increased the rate of photobleaching nor the apparent quantum yield (AQY) of CO. Similarly, the addition of the siderophore desferrioxamine B did not change the photobleaching rates or the CO AQYs. In combination with prior results, these findings suggest that indirect photoreactions do not increase the photobleaching rates of CDOM in coastal systems. A model of CDOM photobleaching based on the assumption of negligible indirect photobleaching processes and multiple non-interacting chromophores was created utilizing photobleaching data produced with monochromatic light to calculate the spectra and exponential decay rates of independent components. These components were then used to calculate bleaching spectra for broadband light and compared with actual bleaching spectra.
Document Details
- Document Type
- Technical Report
- Publication Date
- Feb 01, 2002
- Accession Number
- ADA405886
Entities
People
- Jared V. Goldstone
Organizations
- Massachusetts Institute of Technology