Photobleaching of open-ocean dissolved organic matter (DOM) is typically treated as a removal mechanism; however, photobleaching also encompasses a poorly characterized suite of transformative ...processes. To examine the qualitative changes to DOM optical properties during photobleaching, 674m N. Pacific DOM, concentrated and desalted by reverse osmosis with electrodialysis (RO/ED), was subjected to 68days of continuous irradiation in a UV solar simulator. Approximately 84% of chromophoric and fluorescent DOM (CDOM and FDOM respectively) and 38% of dissolved organic carbon (DOC) were lost during the irradiation. Based on these results the concentration of photochemically refractory DOC in the surface pacific is estimated to be 27μmol of carbon per liter. In addition, the spectra of the remaining CDOM and FDOM were shifted towards shorter wavelengths, a result which has important implications for the interpretation of fluorescence excitation emission matrix (EEM) spectra because the relative positions of fluorescence maxima are often attributed to differences in FDOM source. Qualitative indices derived from CDOM and FDOM spectra for the irradiated deep DOM sample resembled those for surface waters of the North Pacific Ocean indicating that photobleaching has a significant influence upon the optical properties of DOM in the open ocean.
•38% of dissolved organic carbon was remineralized during the 68-day light exposure.•84% of a300, 75% of EEM Peak A and 84% of Peak C fluorescence were removed.•SUVA254 decreased by 50% and S275–295 increased from 0.0191 to 0.0388.•Fluorescence in the EEM Peak A, M, and C regions shifted to shorter wavelengths.•Light exposed deep ocean DOM had similar spectra to surface DOM.
A new approach for parameterizing dissolved organic matter (DOM) ultraviolet-visible absorption spectra is presented. Two distinct spectral slope regions (275-295 nm and 350-400 nm) within ...log-transformed absorption spectra were used to compare DOM from contrasting water types, ranging from wetlands (Great Dismal Swamp and Suwannee River) to photobleached oceanic water (Atlantic Ocean). On the basis of DOM size-fractionation studies (ultrafiltration and gel filtration chromatography), the slope of the 275-295-nm region and the ratio of these slopes ($S_{\rm{R}} $; 275-295-nm slope : 350-400-nm slope) were related to DOM molecular weight (MW) and to photochemically induced shifts in MW. Dark aerobic microbial alteration of chromophoric DOM (CDOM) resulted in spectral slope changes opposite of those caused by photochemistry. Along an axial transect in the Delaware Estuary, large variations in $S_{\rm{R}} $ were measured, probably due to mixing, photodegradation, and microbial alteration of CDOM as terrestrially derived DOM transited through the estuary. Further, $S_{\rm{R}} $ varied by over a factor of 13 between DOM-rich wetland waters and Sargasso Sea surface waters. Currently, there is no consensus on a wavelength range for log-transformed absorption spectra. We propose that the 275-295-nm slope be routinely reported in future DOM studies, as it can be measured with high precision, it facilitates comparison among dissimilar water types including CDOM-rich wetland and CDOM-poor marine waters, and it appears to be a good proxy for DOM MW.
Congo River water was filtered and then irradiated for 57 d in a solar simulator, resulting in extensive photodegradation of dissolved organic matter (DOM). Whole-water (i.e., unfractionated) DOM was ...analyzed pre-and post-irradiation using ultrahigh resolution Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), revealing the following three pools of DOM classified based upon their photoreactivity: (1) photo-resistant, (2) photo-labile, and (3) photo-produced. Photo-resistant DOM was heterogeneous, with most molecular classes represented, although only a small number of aromatics and no condensed aromatics were identified. The photoproduced pool was dominated by aliphatic compounds, although it included a small number of aromatics, including condensed aromatics. Aromatic compounds were the most photoreactive, with >90% being lost upon irradiation. Photochemistry also resulted in a significant drop in the number of molecules identified and a decrease in their structural diversity. The FT-ICR MS signatures of two classes of refractory organic matter, black carbon and carboxylic-rich alicyclic molecules (CRAM), were present in the sample prior to irradiation, indicating that the Congo River could be a significant exporter of recalcitrant DOM to the ocean. All black carbon-like molecules identified in the initial sample were lost during irradiation. Molecular signatures consistent with CRAM were also highly photo-labile, demonstrating that environmental solar irradiation levels are capable of removing these refractory compounds from aquatic systems. Irradiation also shifted the molecular signature of terrestrial DOM toward that of marine DOM, thereby complicating the task of tracking terrestrial DOM in the ocean.
Specific UV absorbance (SUVA) is defined as the UV absorbance of a water sample at a given wavelength normalized for dissolved organic carbon (DOC) concentration. Our data indicate that SUVA, ...determined at 254 nm, is strongly correlated with percent aromaticity as determined by 13C NMR for 13 organic matter isolates obtained from a variety of aquatic environments. SUVA, therefore, is shown to be a useful parameter for estimating the dissolved aromatic carbon content in aquatic systems. Experiments involving the reactivity of DOC with chlorine and tetra- methylammonium hydroxide (TMAH), however, show a wide range of reactivity for samples with similar SUVA values. These results indicate that, while SUVA measurements are good predictors of general chemical characteristics of DOC, they do not provide information about reactivity of DOC derived from different types of source materials. Sample pH, nitrate, and iron were found to influence SUVA measurements.
An overview of Fourier transform ion cyclotron mass spectrometry and other advanced imaging techniques for the study of marine dissolved organic matter is presented. Recent advances in sample ...preparation are discussed.
Dissolved organic matter (DOM) rich water samples (Great Dismal Swamp, Virginia) were 0.1-μm filtered and UV-irradiated in a solar simulator for 30days. During the irradiation, pH increased, ...particulate organic matter (POM) and particulate iron formed. After 30days, 7% of the dissolved organic carbon (DOC) was converted to POC while 75% was remineralized. Approximately 87% of the iron was removed from the dissolved phase after 30days, but iron did not flocculate until a major fraction of DOM was removed by photochemical degradation and flocculation (>10days); thus, during the initial 10days, there were sufficient organic ligands present or the pH was low enough to keep iron in solution. Nuclear magnetic resonance and Fourier transform infrared spectroscopies indicated that photochemically flocculated POM was more aliphatic than the residual non-flocculated DOM. Photochemically flocculated POM was also enriched in amide functionality, while carbohydrate-like material was resistant to both photochemical degradation and flocculation. Abiotic photochemical flocculation likely removes a significant fraction of terrestrial DOM from the upper water column between headwaters and the ocean, but has previously been ignored. Preliminary evidence suggests that this process may significantly impact the transport of DOM and POM in ocean margin environments including estuaries.
Cyanate is a nitrogen and energy source for diverse marine microorganisms, playing important roles in the nitrogen cycle. Despite the extensive research on cyanate utilization, the sources of this ...nitrogen compound remain largely enigmatic. To unravel the sources of cyanate, distributions and production of cyanate during photochemical degradation of natural dissolved organic matter (DOM) were investigated across various environments, including freshwater, estuarine, coastal areas in Florida, and the continental and slope regions of the North American mid-Atlantic Ocean (NATL). Cyanate production was also examined during the photochemical degradation of exudates from a typical strain of
Synechococcus
, an important phytoplankton component. To deepen our understanding of the sources and production mechanisms of cyanate, its production was assessed during the photochemical degradation of a natural seawater DOM supplemented with five nitrogen–containing compounds with distinguishing structures and functional groups. Generally, cyanate exhibited higher concentrations in the Florida coastal, estuarine, and freshwater environments than the NATL. However, cyanate distribution did not consistently align with its production rates. Despite significantly low concentrations in the NATL, DOM from this region exhibited cyanate production rates comparable to estuarine and Florida coastal environments. Although relatively high cyanate concentrations were observed in the freshwaters, DOM in this environment exhibited very low cyanate production rates. A highly significant correlation was observed between cyanate and chlorophyll
a
(Chl
a
) concentrations in these areas. Moreover, in most estuarine and NATL stations, cyanate concentration and production rate in the Chl
a
maximum layer were significantly higher than in other layers. Cyanate was produced during the photochemical degradation of the
Synechococcus
exudates. The cyanate production was significantly enhanced when the natural seawater DOM was supplemented with GlycylGlycine, 4-(methylamino) benzoic acid, 4-ethyl(methyl)amino benzaldehyde or methyl 2-aminobenzoate. Our study implies that photochemical degradation of marine DOM, especially phytoplankton-derived DOM, is a substantial source of cyanate in the ocean. Additionally, cyanate may form during the degradation of peptides and small aromatic compounds in DOM, providing novel insights into the nitrogen cycle.
Two water samples from the Great Dismal Swamp National Wildlife Refuge with high dissolved organic matter (DOM) concentrations (51 and 121 mg C L
−1
) were subjected to ultraviolet (UV) light for up ...to 110 days. During the course of the irradiations, 74–88 % of the original dissolved organic carbon was lost along with 95–99 % of the absorption at 300 nm. Based on changes observed during light exposure, three pools of DOM were identified: photo-labile, photo-refractory, and photo-produced compounds. Solid-state
13
C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy were used to determine structural moieties characteristic to each of these pools. These analyses showed aromatic carbons were preferentially removed while carbohydrate-like and amide/peptide-like carbons were preserved during UV exposure. An increase in carbon normalized
13
C NMR signal in the 0–50 ppm region suggests that alkyl moieties were produced, while FTIR signal at 1,745 cm
−1
and two-dimensional
1
H–
13
C NMR results confirmed the photochemical production of acetate. Several properties typically used to trace terrigenous DOM in ocean margin and marine environments were significantly altered. Optical properties, including absorption spectral slopes and fluorescence indices, as well as carbon-normalized lignin yields shifted from terrestrial values towards those more typical of coastal or open ocean samples. The loss of terrestrial signatures during irradiation highlights the difficulty faced when quantifying the contribution of terrigenous DOM to aquatic carbon pools.
Recent studies suggest that cyanate (OCN–) is a potentially important source of reduced nitrogen (N) available to support the growth of aquatic microbes and, thus, may play a role in aquatic N ...cycling. However, aquatic OCN– distributions have not been previously described because of the lack of a suitable assay for measuring OCN– concentrations in natural waters. Previous methods were designed to quantify OCN– in aqueous samples with much higher reduced N concentrations (micromolar levels) than those likely to be found in natural waters (nanomolar levels). We have developed a method to quantify OCN– in dilute, saline environments. In the method described here, OCN– in aqueous solution reacts with 2-aminobenzoic acid to produce a highly fluorescent derivative, 2,4-quinazolinedione, which is then quantified using high performance liquid chromatography. Derivatization conditions were optimized to simultaneously minimize the reagent blank and maximize 2,4-quinazolinedione formation (>90% reaction yield) in estuarine and seawater matrices. A limit of detection (LOD) of 0.4 nM was achieved with only minor matrix effects. We applied this method to measure OCN– concentrations in estuarine and seawater samples from the Chesapeake Bay and coastal waters from the mid-Atlantic region. OCN– concentrations ranged from 0.9 to 41 nM. We determined that OCN– concentrations were stable in 0.2 μm filtered seawater samples stored at −80 °C for up to nine months.
Photochemical degradation of Congo River dissolved organic matter (DOM) was investigated to examine the fate of terrigenous DOM derived from tropical ecosystems. Tropical riverine DOM receives ...greater exposure to solar radiation, particularly in large river plumes discharging directly into the open ocean. Initial Congo River DOM exhibited dissolved organic carbon (DOC) concentration and compositional characteristics typical of organic rich blackwater systems. During a 57 day irradiation experiment, Congo River DOM was shown to be highly photoreactive with a decrease in DOC, chromophoric DOM (CDOM), lignin phenol concentrations (Σ8) and carbon‐normalized yields (Λ8), equivalent to losses of ∼45, 85–95, >95 and >95% of initial values, respectively, and a +3.1 ‰ enrichment of the δ13C‐DOC signature. The loss of Λ8 and enrichment of δ13C‐DOC during irradiation was strongly correlated (r = 0.99, p < 0.01) indicating tight coupling between these biomarkers. Furthermore, the loss of CDOM absorbance was correlated to the loss of Λ8 (e.g., a355 versus Λ8; r = 0.98, p < 0.01) and δ13C‐DOC (e.g., a355 versus δ13C; r = 0.97, p < 0.01), highlighting the potential of CDOM absorbance measurements for delineating the photochemical degradation of lignin and thus terrigenous DOM. It is apparent that these commonly used measurements for examination of terrigenous DOM in the oceans have a higher rate of photochemical decay than the bulk DOC pool. Further process‐based studies are required to determine the selective removal rates of these biomarkers for advancement of our understanding of the fate of this material in the ocean.
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