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

Abstract

Most marine DOC is thought to be biologically-recalcitrant, especially that in the deep ocean pool (N 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 (H 2 O 2 ) and superoxide (O 2 − ), 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 ([O 2 − ] ss ) and first-order decay constants (k pseudo ) 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-173 nM h −1 for O 2 − and from 1-8.3 nM h −1 for H 2 O 2 , indicating that a large portion of photoproduced O 2 − does not lead to H 2 O 2 formation. In fact, the photoproduction ratio of O 2 − to H 2 O 2 averaged 4:1 in paired experiments, instead of the 2:1 stoichiometry expected for O 2 − dismutation to form H 2 O 2 . Continued irradiation for up to 48 h showed photoproduced H 2 O 2 to be much lower in deep samples compared to surface samples with accumulation slowing or stopping in deep samples despite both measurable [O 2 −] ss and photon absorbance by colored dissolved organic matter (CDOM). These results are consistent with a loss of source material (i.e. CDOM) for O 2 − photoproduction and a shift to a predominantly oxidative pathway for O 2 − decay. Low photoproduction rates, loss of continued accumulation with extended radiation, and an apparent loss of O 2 − source material argues that the deep refractory DOC pool is less photochemically reactive than previously suggested.

Apparent Quantum Efficiency Spectra for Superoxide Photoproduction and Its Formation of Hydrogen Peroxide in Natural Waters

“…Samples were filtered either directly from Niskin bottles or pumped through 0.2 µm filters (Whatman; Polycap-75AS) into 10 or 20 L acid-cleaned polyethylene carboys (Nalgene) and stored at 4 • C until use, for a period of up to 1 month. Our experience indicates that storage for seawater samples does not impact sample optics or the photochemical production rates for H 2 O 2 (Powers et al, 2015). While all equipment in contact with seawater samples was acid cleaned as described above and nitrile gloves were used in handling samples, the R/V Savannah and the laboratory was not equipped with special trace metal processing facilities and thus, limited trace metal contamination could be present.…”

“…An assumption of all AQY calculations based on a single time point measurement is that the product is generated as a linear function of photon dose. With these experiments, as done in Powers and Miller (2014), it is unclear whether this requirement is met, especially when a lack of reciprocity for H 2 O 2 photoproduction over irradiation time has been reported elsewhere (Kieber et al, 2014;Powers et al, 2015). Results of SOD addition experiments were also variable in this study, so a 2:1 stoichiometry between O − 2 and H 2 O 2 was assumed and O − 2 photoproduction rates were predicted using this stoichiometry together with the AQY for H 2 O 2 .…”

“…In natural waters, Rusak et al (2011) observed a ratio of 0.37 H 2 O 2 per O − 2 consumed but used unfiltered samples that are not reflective of solely photochemical production and abiotic decay. Powers et al (2015) measured photochemical production of H 2 O 2 and O − 2 together via chemiluminescent detection, resulting in a stoichiometry between O − 2 and H 2 O 2 closer to 4:1 in seawater collected in the Gulf of Alaska.…”

Depth‐Resolved Photochemical Lability of Dissolved Organic Matter in the Western Tropical Pacific Ocean

“…Aside from biodegradation, DOM is also subject to photochemical processing in the photic zone, leading to bleaching of chromophoric DOM (CDOM), direct mineralization (DOC loss), and production of biolabile or bio‐refractory organic matter (Moran & Zepp, 1997; Obernosterer et al, 1999). Despite being physically limited to the sunlit layer, photooxidation may affect DOM cycling within the entire water column, given that the average radiocarbon age of DOC in the deep ocean is far longer than the turnover times of the ocean (centuries to one millennium), allowing the old DOC to circulate between the surface and deep oceans multiple times during its lifetime (Mopper et al, 1991; Powers et al, 2015). The photoreactivity of DOM may, however, be depth dependent due to dissimilar chemical compositions of DOM at different depth regimes.…”

“…Medeiros et al (2015) revealed that the molecular signatures of DOM in the deep water of the Gulf of Alaska become more similar to that in surface water after irradiation. The photoreactivity of the deep RDOM in that region is nevertheless comparable to or lower than that of DOM in surface water in terms of hydrogen peroxide and superoxide photoproduction (Powers et al, 2015). Fluorescent DOM (FDOM) in deep water from the Bermuda Atlantic Time‐series Study (BATS) site, however, photo‐decomposes considerably faster than does FDOM in surface water (Timko et al, 2015).…”