Interplay of oxygen and light in the photo-oxidation of dissolved organic carbon

Light energy and oxygen drive photo-oxidation of dissolved organic carbon (DOC). The longer the wavelength the greater its depth of penetration into water, changing the spectral environment with depth. We asked how oxygen concentration and light spectral composition might affect photo-oxidation processes in DOC. Outdoor experiments compared responses of fluorescence and absorbance indices to photo-oxidation of filtered (0.45 µm) Rio Negro water (Amazon Basin) under near-anoxia, normoxia and hyperoxia exposed to natural sunlight or reduced sunlight (≥340, reduced-UVR). Near-anoxia decreased all absorbance and fluorescence indices. Absorbance changed across the spectrum (≥250 nm) even under reduced-UVR provided that oxygen was present. This phenomenon maintains broader photo-oxidation and the release of CO₂ at depth. Slope_350-400 was responsive to changes in the irradiance field but not to oxygen concentration, while Slope_275-295 responded to both. Thus, larger molecules are broken down near the water's surface and medium to smaller molecules continue to be processed at depth. The production of fulvic acid-like fluorescence required both UVB and oxygen, restricting its production to surface waters. The relatively small increase in R_254/365 compared with the loss of SUVA₂₅₄ under near-anoxia indicated a slower breakdown of larger DOC molecules as oxygen becomes limiting. Breakdown of larger molecules which absorb in the 350-400 nm range, appears to involve two steps - one by radiant energy and another involving oxygen. The study results reflect the dynamic gradients in photo-oxidation with depth.