Chemical characteristics of fulvic acids from Arctic surface waters: Microbial contributions and photochemical transformations

Abstract

[1] Dissolved organic matter (DOM) originating from the extensive Arctic tundra is an important source of organic material to the Arctic Ocean. Chemical characteristics of whole water dissolved organic matter (DOM) and the fulvic acid fraction of DOM were studied from nine surface waters in the Arctic region of Alaska to gain insight into the extent of microbial and photochemical transformation of this DOM. All the fulvic acids had a strong terrestrial/higher plant signature, with uniformly depleted d 13C values of À28%, and low fluorescence indices around 1.3. Several of the measured chemical characteristics of the Arctic fulvic acids were related to water residence time, a measure of environmental exposure to sunlight and microbial activity. For example, fulvic acids from Arctic streams had higher aromatic contents, higher specific absorbance values, lower nitrogen content, lower amino acid-like fluorescence and were more depleted in d 15 N relative to fulvic acids isolated from lake and coastal surface waters. The differences in the nitrogen signature between the lake and coastal fulvic acids compared to the stream fulvic acids indicated that microbial contributions to the fulvic acid pool increased with increasing water residence time. The photo-lability of the fulvic acids was positively correlated with water residence time, suggesting that the fulvic acids isolated from source waters with larger water residence times (i.e., lakes and coastal waters) have experienced greater photochemical degradation than the stream fulvic acids. In addition, many of the initial differences in fulvic acid chemical characteristics across the gradient of water residence times were consistent with changes observed in fulvic acid photolysis experiments. Taken together, results from this study suggest that photochemical processes predominantly control the chemical character of fulvic acids in Arctic surface waters. Our findings show that hydrologic transport in addition to biogeochemical alteration of the organic matter must be considered in order to predict the ultimate fate of Arctic DOM.

Fluorescence spectroscopy opens new windows into dissolved organic matter dynamics in freshwater ecosystems: A review

“…Quinone moieties have also been suggested to contribute to humic DOM fluorescence and research has shown that . 50% of DOM fluorescence is potentially due to such structures (Cory and McKnight 2005). Although the exact chemical compounds responsible for DOM fluorescence are still partly undefined, there is much knowledge to be gained about their ecological relevance through tracing their dynamics in the field and laboratory experiments (Baker and Spencer 2004;Cory et al 2007).…”

“…However, the FI was modified by Cory and McKnight (2005) to the ratio of em wavelengths at 470 nm and 520 nm, obtained at ex 370.…”

“…One of the simplest and most widely used is the two-dimensional fluorescence index (FI), which provides information about the source (e.g., microbial or terrestrial higher plant material) or degree of degradation of DOM because it reflects the relative contribution of aromatic vs. nonaromatic DOM (Table 1; McKnight et al 2001;Cory and McKnight 2005). Fluorescence DOM measurements are also commonly collected as three-dimensional excitationemission matrices (EEMs; Coble et al 1990;McKnight et al 2001), which are produced from multiple emission spectra collected at successively increasing excitation wavelengths (Fig.…”

“…Although protein-like fluorescence has great potential for use in a range of ecological applications, much still needs to be learned, particularly in freshwater ecosystems with high DOM concentrations, about the molecular size (Maie et al 2007) and structure of DOM that contributes to protein-like fluorescence ). Excitation-emission matrices can also be analyzed using the multivariate modeling technique parallel factor analysis (PARAFAC; Bro 1997; Stedmon et al 2003;Cory and McKnight 2005), a three-way method that decomposes the fluorescence signature of DOM into individual components and provide estimates of the relative contribution of each component to total DOM fluorescence. Thus, PARAFAC components provide information about the biochemical composition, origin, and biogeochemical role of aquatic DOM.…”

Assessing the Effect of Sterilization on the Radiocarbon Signature of Freshwater Dissolved Organic Carbon

“…It has also been demonstrated that freshwater systems do not simply act as conduits between land and ocean, but are active zones of dissolved organic matter cycling through biological processing (Battin et al 2008(Battin et al , 2009, photodegradation (Cory et al 2007), freezing and dehydration (Hudson et al 2009), and physical processes such as colloid formation and particle settling (Battin et al 2008;Aiken et al 2011). For example, studies have shown high CO 2 degassing from large rivers (Cole and Caraco 2001).…”