The interaction between photochemical and biological processes in the degradation of marine dissolved organic matter (DOM) was investigated with seawater from a coastal southeastern U.S. salt marsh. Seawater supplemented with humic substances was exposed to alternating cycles of sunlight (equivalent to 8 h of midday sun) and dark incubations with natural bacterial populations (l-2 weeks in length). Photochemical degradation of the DOM was monitored during sunlight exposure by direct measurements of dissolved inorganic carbon (DIC) and carbon monoxide (CO) formation in 0.2-Frn filtered seawater. Bacterial degradation was monitored during dark incubations by tritiated leucine uptake and changes in bacterial numbers in bacterivore-free incubations and by direct measurements of DOM loss. The alternating cycles of sunlight and microbial activity resulted in more complete degradation of bulk DOM and marine humic substances than was found for nonirradiated controls (i.e. with microbial activity alone) by a factor of up to 3-fold. Increased decomposition was due both to direct losses of carbon gas photoproducts (DIC and CO in a 15 : 1 ratio) and to enhanced microbial degradation of photodegraded DOM, with approximately equal contributions from each pathway. Mass balance calculations indicated that low-molecular-weight carbon photoproducts, currently considered to be the compounds responsible for stimulating bacterial activity following photodegradation of DOM, were insufficient to account for the enhanced bacterial production observed. Thus, higher molecular weight, chemically uncharacterized fractions of DOM may also be modified to more biologically available forms during exposure to natural sunlight. Photochemical processes play a number of important roles in the biogeochemical cycling of dissolved organic matter (DOM) in natural waters. Water optical properties, biological processes, and trace element distributions have all been demonstrated to be affected, either directly or indirectly, by

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1997.42.6.1317

Interaction of photochemical and microbial processes in the degradation of refractory dissolved organic matter from a coastal marine environment

Absorption of sunlight by chromophoric dissolved natural organic matter (CDOM) is environmentally significant because it controls photic zone depth and causes photochemistry that affects elemental cycling and contaminant fate. Both the optics (absorbance and fluorescence) and photochemistry of CDOM display unusual properties that cannot easily be ascribed to a superposition of individual chromophores. These include (i) broad, unstructured absorbance that decreases monotonically well into the visible and near IR, (ii) fluorescence emission spectra that all fall into a single envelope regardless of the excitation wavelength, and (iii) photobleaching and photochemical quantum yields that decrease monotonically with increasing wavelength. In contrast to a simple superposition model, these phenomena and others can be reasonably well explained by a physical model in which charge-transfer interactions between electron donating and accepting chromophores within the CDOM control the optical and photophysical properties. This review summarizes current understanding of the processes underlying CDOM photophysics and photochemistry as well as their physical basis.

The importance of charge-transfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties

Interest in reactive oxygen species (ROS) in the environment was originally stimulated by certain atmospheric problems such as smog formation, stratospheric ozone depletion, and, more recently, acid rain. As a result of the need to understand the causes of these problems better, gasphase reactions of tropospheric oxidants and related primary photochemical processes have been intensely studied, and atmospheric photochemistry has become a highly developed subdiscipline.

Reactive Oxygen Species in Natural Waters

It is generally accepted that methyl iodide is a major contributor to the flux of iodine from the ocean to the atmosphere, but its sources in the ocean are largely unknown, an exception being production by certain species of coastal macrophytic algae. Preliminary results reported here indicate production of methyl iodide in filtered seawater that has been irradiated with either sunlight or an artificial light source having a spectral output closely approximating to sunlight at sea level. The production was enhanced when the water samples were deoxygenated and by the addition of iodide to this water. The results appear to be consistent with a mechanism involving reaction between photochemically produced methyl radicals and iodine atoms. It is estimated that the process is capable of making a significant contribution to global iodine fluxes.

Photochemical production of methyl iodide in seawater

Colored dissolved organic matter (CDOM) and humic substances contain a nonmetallic redox-cycling component capable of catalyzing superoxide (O2-) dismutation. First-order rate coefficients (kpseudo) measured for this O2- sink in a number of coastal and Chesapeake Bay water samples range up to 1.4 s-1, comparable in magnitude to catalyzed dismutation by Cu species. A significant (r 2 = 0.73) correlation is observed between kpseudo and the optical absorption and salinity of individual coastal water samples, suggesting an association with non-marine-derived CDOM. The activity of this sink is not changed by acidification or boiling of samples but is removed by photooxidation, indicating that it is an organic compound, but that it is neither enzymatic nor likely to consist of tightly bound metals. The stoichiometry of hydrogen peroxide formation from O2- decay indicates that this sink is capable of a redox cycle catalyzing the dismutation of O2-. This CDOM sink combined with the organic copper sink previously ...

Chemistry of Superoxide Radical in Seawater: CDOM Associated Sink of Superoxide in Coastal Waters

Potential photochemical production rates of total (NO-scavengeable) free radicals were surveyed underway (> 900 points) in the eastern Caribbean and Orinoco delta in spring and fall 1988. These data document seasonal trends and large-scale (∼ 10–1000 km) variability in the pools of sunlight-generated reactive transients, which probably mediate a major portion of marine photoredox transformations. Radical production potential was detectable in all waters and was reasonably quantifiable at rates above 0.25 nmol L−1 min−1 sun−1. Radical production rates varied from ∼ 0.1–0.5 nmol L−1 min−1 of full-sun illumination in “blue water” to > 60 nmol L−1 min−1 in some estuarine waters in the high-flow season. Qualitatively, spatiotemporal potential rate distributions strikingly resembled that of “chlorophyll” (a riverine-influence tracer of uncertain specificity) in 1979–1981 CZCS images of the region [Muller-Karger et al., 1988] at all scales. Basin-scale occurrence of greatly enhanced rates in fall compared to spring is attributed to terrestrial chromophore inputs, primarily from the Orinoco River, any contributions from Amazon water and nutrient-stimulus effects could not be resolved. A major part of the functionally photoreactive colored organic matter (COM) involved in radical formation clearly mixes without massive loss out into high-salinity waters, although humic acids may flocculate in estuaries. A similar conclusion applies over smaller scales for COM as measured optically [Blough et al., this issue]. Furthermore, optical absorption and radical production rates were positively correlated in the estuarine region in fall. These cruises demonstrated that photochemical techniques are now adequate to treat terrestrial photochemical chromophore inputs as an estuarine mixing problem on a large scale, though the ancillary data base does not currently support such an analysis in this region. Eastern Caribbean waters are not markedly more reactive at comparable salinities than waters of the Gulf of Maine and North Atlantic Bight, despite large inputs of colored waters from two large tropical rivers with substantial “black water” tributaries. Other sources of reactive COM, such as grazing, sedimentary diagenesis, and “marine humus” may increase temperate waters' photoreactivity; alternatively, northern waters may be chromophore-rich because they are light-poor and photobleaching is a major sink of photoreactive COM.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/92JC02765%4010.1002/%28ISSN%292169-9291.PECW1

Photochemical free radical production rates in the eastern Caribbean

The total free radical fluxes generated by illumination of surface water samples with a solar simulator were measured in a variety of coastal surface waters along the east coast of the United States in summer 1987. Photochemically generated free radicals were scavenged with a known excess of added nitric oxide, and the nitric oxide losses were measured by difference using a semiautomated, computer-controlled system coupled to the ship/s clean surface water intake system. This first large free radical flux data set shows that radical production occurs at significant rates that are relatively uniform over large regions. However, at the time of this cruise a strong photoreactivity front divided coastal waters north of Cape Cod (3.85±0.82 nmol L−1 min−1 sun−1, n=8 sites) from coastal waters south to northern Florida (1.67±0.32 nmol L−1 min−1 sun−1, n = 20). The mean flux on an areal basis is estimated to be ∼0.7 mmol−2 d−1, or about 50 mmol m−2 for the summer season. These values are sufficiently large to produce interesting but unknown geochemical consequences. The regional uniformity and wavelength dependence of the rates are consistent with the concept that the principal mechanism of radical production is photolysis of long-lived dissolved chromophores of unknown structure, such as fulvic and humic acids. A vertical profile shows strong near-surface depletion of reactive materials under calm conditions. The dependence of total radical production rates on irradiation time, light intensity, concentration of added NO, and wave-length range of polychromatic radiation is also reported. The results support the conclusion that the method produces an operationally consistent, well-behaved estimate of photochemical free radical production potential that is not strongly affected by biological processes.

Photochemical free radical production rates: Gulf of Maine and Woods Hole‐Miami transect

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Photochemical free radical production rates in the eastern Caribbean

Photochemical free radical production rates: Gulf of Maine and Woods Hole‐Miami transect