Probing the photochemical reactivity of deep ocean refractory carbon (DORC): Lessons from hydrogen peroxide and superoxide kinetics

Most marine DOC is thought to be biologically-recalcitrant, especially that in the deep ocean pool (> 1000 m). In particular, the deep waters of the North Pacific should contain the most recalcitrant DOC because they do not form locally, with deep DOC having survived long isolation from the surface during global-scale thermohaline circulation. One of the proposed removal pathways involves photochemical reactions when refractory DOC circulates through sunlit surface waters (Mopper et al., 1991). Here, we reevaluate the general photoreactivity of refractory DOC by investigating the photochemical production of two reactive oxygen species (ROS), hydrogen peroxide (H2O2) and superoxide (O2−), using controlled irradiations at sea and in the laboratory. The photoproduction of these two ROS were compared between deep and surface water samples collected in the Gulf of Alaska. For irradiated samples, initial superoxide steady-state concentrations ([O2−]ss) and first-order decay constants (kpseudo) were similar between surface and deep waters, ranging from ~ 1-4 nM and 4-12 × 10− 3 s− 1, respectively. Initial photoproduction rates were comparable between surface and deep waters, ranging from 22-173 nM h− 1 for O2− and from 1-8.3 nM h− 1 for H2O2, indicating that a large portion of photoproduced O2− does not lead to H2O2 formation. In fact, the photoproduction ratio of O2− to H2O2 averaged ~ 4:1 in paired experiments, instead of the 2:1 stoichiometry expected for O2− dismutation to form H2O2. Continued irradiation for up to 48 h showed photoproduced H2O2 to be much lower in deep samples compared to surface samples with accumulation slowing or stopping in deep samples despite both measurable [O2−]ss and photon absorbance by colored dissolved organic matter (CDOM). These results are consistent with a loss of source material (i.e. CDOM) for O2− photoproduction and a shift to a predominantly oxidative pathway for O2− decay. Low photoproduction rates, loss of continued accumulation with extended radiation, and an apparent loss of O2− source material argues that the deep refractory DOC pool is less photochemically reactive than previously suggested.