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Background: The Arctic Ocean receives massive freshwater input and a correspondingly large amount of humic-rich organic matter of terrestrial origin. Global warming, permafrost melt, and a changing hydrological cycle will contribute to an intensification of terrestrial organic matter release to the Arctic Ocean. Although considered recalcitrant to degradation due to complex aromatic structures, humic substances can serve as substrate for microbial growth in terrestrial environments. However, the capacity of marine microbiomes to process aromatic-rich humic substances, and how this processing may contribute to carbon and nutrient cycling in a changing Arctic Ocean, is relatively unexplored. Here, we used a combination of metagenomics and metatranscriptomics to assess the prevalence and diversity of metabolic pathways and bacterial taxa involved in aromatic compound degradation in the salinity-stratified summer waters of the Canada Basin in the western Arctic Ocean. Results: Community-scale meta-omics profiling revealed that 22 complete pathways for processing aromatic compounds were present and expressed in the Canada Basin, including those for aromatic ring fission and upstream funneling pathways to access diverse aromatic compounds of terrestrial origin. A phylogenetically diverse set of functional marker genes and transcripts were associated with fluorescent dissolved organic matter, a component of which is of terrestrial origin. Pathways were common throughout global ocean microbiomes but were more abundant in the Canada Basin. Genome-resolved analyses identified 12 clades of Alphaproteobacteria, including Rhodospirillales, as central contributors to aromatic compound processing. These genomes were mostly restricted in their biogeographical distribution to the Arctic Ocean and were enriched in aromatic compound processing genes compared to their closest relatives from other oceans. Conclusion: Overall, the detection of a phylogenetically diverse set of genes and transcripts implicated in aromatic compound processing supports the view that Arctic Ocean microbiomes have the capacity to metabolize humic substances of terrestrial origin. In addition, the demonstration that bacterial genomes replete with aromatic compound degradation genes exhibit a limited distribution outside of the Arctic Ocean suggests that processing humic substances is an adaptive trait of the Arctic Ocean microbiome. Future increases in terrestrial organic matter input to the Arctic Ocean may increase the prominence of aromatic compound processing bacteria and their contribution to Arctic carbon and nutrient cycles. 2somghBCVjjHAhscSmFASg Video Abstract [ABSTRACT FROM AUTHOR]

Lignin is one of the more refractory components of dissolved organic matter (DOM) and its concentration and composition are used to trace terrigenous DOM in the ocean. In this study, lignin oxidation products (LOP), and chromophoric and fluorescent DOM were measured in Polar Mixed Layer (PML), Atlantic water (AW), and Canada Basin Deep Water (CBDW) layers in the Canada Basin to elucidate the composition and source of terrestrial DOM. Significant differences in total lignin phenol concentration (Σ11) and cinnamyl to vanillyl ratio (C/V) measured with CuO oxidation in PML, AW, and CBDW samples were found. The fluorescence intensities of humic-like C1 were correlated with Σ11 in PML, congruent with the humic-like terrestrial origin. However, no significant correlation was found when all layers are considered (p > 0.05), suggesting a change in the molecular composition of C1. The protein-like C3 and C5 intensities were strongly correlated with Σ11 (rs = 0.55–0.64) corroborating that lignins can contribute to the protein-like fluorescence in the water column. The principal component analysis (PCA) confirmed the origin and composition of DOM vary between the different water masses, especially in terms of C/V, DOC, and fluorescent components. Significant relationships between humic-like fluorescence intensities and C/V reveal the significant contribution of woody sources to humic material in the Canada Basin. Elevated S/V and humic-like C6 levels were associated with a cold eddy DOM sample but further studies are required to validate the use of these DOM characteristics to trace deep eddy. • The lignin phenol composition varies between water masses in the Canada Basin. • The humic-like levels are correlated with lignin concentrations in surface water only. • The linkages with protein-like suggest a biological contribution to the lignin levels. [ABSTRACT FROM AUTHOR]

Parallel factor analysis (PARAFAC) has facilitated an explosion in research connecting the fluorescence properties of dissolved organic matter (DOM) to its functions and biogeochemical cycling in natural and engineered systems. However, the validation of robust PARAFAC models using split-half analysis requires an oft unrealistically large number (hundreds to thousands) of excitation-emission matrices (EEMs), and models with too few components may not adequately describe differences between DOM. This study used self-organizing maps (SOM) and comparing changes in residuals with the effects of adding components to estimate the number of PARAFAC components in DOM from two data sets: MS (110 EEMs from nine leaf leachates and headwaters) and LR (64 EEMs from the Lena River). Clustering by SOM demonstrated that peaks clearly persisted in model residuals after validation by split-half analysis. Plotting the changes to residuals was an effective method for visualizing the removal of fluorophore-like fluorescence caused by increasing the number of PARAFAC components. Extracting additional PARAFAC components via residuals analysis increased the proportion of correctly identified size-fractionated leaf leachates from 56.0±0.8 to 75.2±0.9%, and from 51.7±1.4 to 92.9±0.0% for whole leachates. Model overfitting was assessed by considering the correlations between components, and their distributions amongst samples. Advanced residuals analysis improved the ability of PARAFAC to resolve the variation in DOM fluorescence, and presents an enhanced validation approach for assessing the number of components that can be used to supplement the potentially misleading results of split-half analysis. [ABSTRACT FROM AUTHOR]

Fluorescent dissolved organic matter (FDOM) samples collected from water masses of Beaufort Sea (n = 12) and the North Atlantic Subtropical Gyre (NASG; n = 6) were assessed based on susceptibilities of its parallel factor analysis (PARAFAC) modeled fluorescent components to photobleaching over 72 hours of simulated solar irradiation. 315 excitation–emission matrix spectra (EEMs) were PARAFAC modeled yielding 4 humic-like (C1–3 and C5) and 1 protein-like (C4) fluorescent components. Protein-like C4 and humic-like C5 did not adhere to first order kinetics and did not yield decay rate constant ( k ) or half-life ( t 1/2 ) values as part of this study. Humic-like C1 was found to be most resilient to photodegradation with lowest k values. Principal component analysis (PCA) illustrated a shift towards C1% with photo-exposure in addition to distinguishing compositions of C1–5% in Beaufort Sea halocline and Atlantic layer as well as NASG deep water masses. When Beaufort Sea water masses were considered, k of C2 was found to be lower in upper (UH) and mid (MH) halocline layers when compared to lower halocline (LH) and underlying Atlantic layer (AL; p < 0.10). This suggested that C2 in Pacific-derived seawater was less photoreactive than C2 in Atlantic-derived seawater in Beaufort Sea, likely as a result of distinct formation pathways of these water masses. Similarly, C2 in upper Labrador Seawater (ULSW) and ‘classical’ Labrador seawater (LSW) was less photolabile than in deeper NASG layers (Iceland-Scotland and Denmark Strait overflow waters; ISOW and DSOW). Interestingly, despite similar formation pathways of AL in Beaufort Sea and deeper waters sampled from NASG (ISOW + DSOW), lower k for C1 was found in NASG. [ABSTRACT FROM AUTHOR]

Asymmetrical flow field-flow fractionation (AF4) with sequential on-line UV/visible and fluorescence detectors was used to investigate the composition of dissolved organic matter (DOM) in permeate and retentate fractions isolated by tangential flow ultrafiltration (TFF) at various concentration factors (i.e. ratio of initial volume to the retentate volume; CF). The permeation coefficient model, which defines the log-log relationship between DOM in the permeate fractions and CFs, described the permeation behaviour of DOM with regression coefficientsr2 > 0.99. The dominance of higher-molecular weight retentate chromophoric DOM (CDOM) observed in TFF was consistent with the results of AF4. The weight-averaged molecular weights (Mw) of the integral permeate and retentate at CF = 20 were determined to be 1160 and 2320 by AF4, respectively, while their molecular weight distributions (MWD) were centered at 1120 and 1600 Da.Mw, MWD, and aromaticity (i.e. ratio of absorbance at 250 and 365 nm; E2/E3) in permeate fractions were altered significantly during the early stages of TFF (CF < 9). These changes, however, were not evident in excitation-emission matrix fluorescence properties as determined using the parallel factor analysis model. The application of AF4 to TFF fractions suggests that the choice of CF may have an important impact on the size distribution and aromaticity of permeate fractions, whereas fluorescence properties appear insensitive to concentration factor. These results suggest that the choice of CF is crucial only in the study of the permeate fraction where similar CF (i.e. > 9) should be used to obtain meaningful comparison among samples. [ABSTRACT FROM PUBLISHER]

Abstract: The spatial and vertical distribution of colored dissolved organic matter (CDOM) in the North Pacific Ocean, Bering, Chukchi and Beaufort Seas was investigated with respect to water circulation. Four fluorescent components were identified in 546 samples using parallel factor analysis (PARAFAC): terrestrially-derived humic-like (C1), UV humic-like (C2), microbially derived humic-like (C3) and proteinaceous/tryptophan-like (C4) components. Deep water upwelling upon contact with the Aleutian Islands influenced optical properties of CDOM in Bering Sea surface water, where higher absorption coefficient at 355nm (a355), fluorescence intensities of all CDOM components and lower spectral slope (S275–295) were found compared to surface waters of the Gulf of Alaska (p<0.01). Further north, a marked increase in bioprocessed material (C4) was found in surface waters in Chukchi Sea. This finding was attributed to elevated biological activity in surface Chukchi water. Humic-like C1–C3 showed considerable dilution with increasing latitude from coastal to offshore regions in surface Beaufort Sea, suggesting a terrestrial predominance. This contrasts well with protein-like C4 where no significant gradient was found in Beaufort Sea surface water, supporting in situ generation. Finally, the vertical decrease of protein-like C4 relative to humic-like components C1–3 suggests that humic-like CDOM is produced at depth as a byproduct of remineralization. [Copyright &y& Elsevier]

Abstract: Dissolved organic matter (DOM) is a ubiquitous constituent of natural waters and is comprised of a variety of chemically heterogeneous molecular structures and functional groups. DOM is often considered to be a major ligand for metals in most natural waters and its reactivity is thought to be strongly dependent on its chemical composition and structure. In this study, a combination of UV/visible, emission excitation matrix fluorescence (EEM) and 1H NMR spectroscopies were used to characterize DOM from the Athabasca River (Alberta, Canada). The chemical characterization of river DOM showed that the most upstream samples located in agricultural areas were blue-shifted and less aromatic and contained more hydrogens connected with oxygen functional groups than those in the wetland dominated area in the Athabasca oil sand deposit region. The presence of paramagnetic ions (Fe and Al) was not found to significantly affect the structural composition of DOM as revealed by 1H NMR. Such change in the quality of DOM may have a profound impact on metal binding in the Athabasca River watershed. [Copyright &y& Elsevier]

Abstract: Flow field-flow fractionation (FlFFF) with on-line UV/Visible diode array detector (DAD) and excitation emission matrix (EEM) fluorescence detector has been developed for the characterization of optical properties of aquatic dissolved organic matter (DOM) collected in the Otonabee River (Ontario, Canada) and Athabasca River (Alberta, Canada). The molecular weight (MW) distribution of DOM was estimated using a series of organic macromolecules ranging from 479 to 66,000Da. Both the number-average (M n) and weight-average (M w) molecular weights of Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) determined using these macromolecular standards were comparable to those obtained using polystyrenesulfonate (PSS) standards, suggesting that organic macromolecules can be used to estimate MW of natural organic colloids. The MW of eight river DOM samples determined by this method was found to have an M n range of 0.8–1.1kDa, which agrees with available literature estimates. The FlFFF-DAD-EEM system provided insight into the MW components of river DOM including the optical properties by on-line absorbance and fluorescence measurement. A red-shift in emission and excitation wavelength maxima associated with lower spectral slope ratios (S R = S 275–295:S 350–400) was related to higher MW DOM. However, DOM of different origins at similar MW also showed significant difference in optical properties. A difference of 47 and 40nm in excitation and emission peak C maxima was found. This supports the hypothesis that river DOM is not uniform in size and optical composition. [Copyright &y& Elsevier]

Diffusive gradients in thin films (DGT) devices were used to investigate the temporal and spatial changes in vanadium (V) speciation in the Churchill estuary system (Manitoba). Thirty-six DGT sets and 95 discrete water samples were collected at 8 river and 3 estuary sites during spring freshet and summer base flow. Dissolved V concentration in the Churchill River at summer base flow was approximately 5 times higher than those during the spring high flow (27.3 ± 18.9 nM vs 4.8 ± 3.5 nM). DGT-labile V showed an opposite trend with greater values found during the spring high flow (2.6 ± 1.8 nM vs 1.4 ± 0.3 nM). Parallel factor analysis (PARAFAC) conducted on 95 excitation-emission matrix spectra validated four humic-like (C1 C4) and one protein-like (C5) fluorescent components. Significant positive relationship was found between protein-like DOM and DGT-labile V (r = 0.53, p < 0.05), indicating that protein-like DOM possibly affected the DGT-labile V concentration in Churchill River. Sediment leachates were enriched in DGT-labile V and protein-like DOM, which can be readily released when river sediment began to thaw during spring freshet. [ABSTRACT FROM AUTHOR]

Highlights: [•] Biochars contributed different amounts of DOC in relation to production methods. [•] DOM aromaticity and mean molecular weight varied amongst different biochars. [•] EEM fluorescence showed distinct DOM signatures for each biochar type. [•] Transformations in biochar DOM were observed over time. [ABSTRACT FROM AUTHOR]

• Terrestrial carbon remineralization reduces aragonite-saturation and increases pCO 2. • Export production and terrestrial CaCO 3 dissolution offset remineralization effects. • Changes in marine pCO 2 and Ω Ar depend on watershed geology and organic matter C:N. Recent research has focused on the changing ability of oceans to absorb atmospheric CO 2 and the consequences for ocean acidification, with Arctic shelf seas being among the most sensitive regions. Hudson Bay is a large shelf sea in northern Canada whose location at the margin of the cryosphere places it in the vanguard of global climate change. Here, we develop a four-compartment box-model and carbon budget using published and recently collected measurements to estimate carbon inputs, transformations, and losses within Hudson Bay. We estimate the annual effects of terrestrial carbon remineralization on aragonite saturation (Ω Ar , a proxy for ocean acidification) and on the partial pressure of CO 2 (pCO 2 , a proxy for air-sea CO 2 flux) within each compartment, as well as the effects of marine primary production, marine organic carbon remineralization, and terrestrial calcium carbonate dissolution. We find that the remineralization of terrestrial dissolved organic carbon is the main driver of CO 2 accumulation and aragonite under-saturation in coastal surface waters, but this is largely offset by marine primary production. Below the surface mixed layer, marine organic carbon remineralization is the largest contributor to CO 2 accumulation and aragonite under-saturation, and is partially offset by terrestrial CaCO 3 dissolution. Overall, the annual delivery and processing of carbon reduces Ω Ar of water flowing through HB by up to 0.17 units and raises pCO 2 by up to 165 µatm. The similarities between Hudson Bay and other Arctic shelf seas suggest these areas are also significantly influenced by terrestrial carbon inputs and transformation. [ABSTRACT FROM AUTHOR]