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Most of the oceanic reservoir of dissolved organic matter (DOM) is of marine origin and is resistant to microbial oxidation, but little is known about the mechanisms of its formation. In a laboratory study, natural assemblages of marine bacteria rapidly (in <48 hours) utilized labile compounds (glucose, glutamate) and produced refractory DOM that persisted for more than a year. Only 10 to 15% of the bacterially derived DOM was identified as hydrolyzable amino acids and sugars, a feature consistent with marine DOM. These results suggest that microbial processes alter the molecular structure of DOM, making it resistant to further degradation and thereby preserving fixed carbon in the ocean.

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Bacterial biomarkers (D-amino acids and muramic acid) were measured in various organic matter size fractions collected in the North Pacific and North Atlantic, and they were used to quantitatively estimate bacterial contributions to particulate and dissolved organic carbon and nitrogen reservoirs. The origins and yields of biomarkers were determined in cultured marine bacteria, and the results indicated that D-amino acids are derived from numerous macromolecules in addition to peptidoglycan and are not solely from peptidoglycan. Bacterial detritus was a major component of particulate organic matter (POM) and is an important source of submicronsize particles and colloids in the ocean. Peptidoglycan was a substantial component of POM but not of dissolved organic matter (DOM). Compositional differences between POM and DOM primarily reflected the selective incorporation of specific bacterial components into these reservoirs. Autotrophic and heterotrophic bacterial sources were not quantified separately, but the presence of D-aspartic acid (D-Asx) and D-serine (D-Ser) suggested that heterotrophic sources were substantial. The average reactivity of bacterial organic matter was comparable to that of the bulk organic carbon pool. Bacteria were important sources of labile, semilabile, and refractory dissolved organic carbon. Bacterial organic matter accounted for ,25% of particulate and dissolved organic carbon and ,50% of particulate and dissolved organic nitrogen. These results demonstrate the importance of bacteria in regulating the ocean carbon and nitrogen cycles.

Continental runoff is a major source of freshwater, nutrients and terrigenous material to the Arctic Ocean. As such, it influences water column stratification, light attenuation, surface heating, gas exchange, biological productivity and carbon sequestration. Increasing river discharge and thawing permafrost suggest that the impacts of continental runoff on these processes are changing. Here, a new optical proxy was developed and implemented with remote sensing to determine the first pan-Arctic distribution of terrigenous dissolved organic matter (tDOM) and continental runoff in the surface Arctic Ocean. Retrospective analyses revealed connections between the routing of North American runoff and the recent freshening of the Canada Basin, and indicated a correspondence between climate-driven changes in river discharge and tDOM inventories in the Kara Sea. By facilitating the real-time, synoptic monitoring of tDOM and freshwater runoff in surface polar waters, this novel approach will help understand the manifestations of climate change in this remote region.

Several recent studies indicated that amino sugars were likely to be major components of marine carbon and nitrogen cycles, but there has been insufficient data to investigate this hypothesis. In the present study, hydrolyzable amino sugars were measured in a variety of marine organisms and particulate organic matter (POM) and in ultrafiltered dissolved organic matter (UDOM) from various ocean basins and depths. Glucosamine (GlcN) and galactosamine (GalN) were abundant and common to all samples. Concentrations of these two amino sugars were similar to their neutral sugar counterparts, glucose and galactose, and in surface water they accounted for up to ϳ1.5% of C and N in POM and 2.5% of C and 7.1% of N in UDOM. Chitin, a polymer of glucosamine, is produced by many marine organisms, but based on the low GlcN : GalN ratios (1.0-2.5) in POM and UDOM, it does not appear to be a primary source of glucosamine in these samples. Muramic acid, an amino sugar found only in the bacterial cell wall polymer peptidoglycan, was relatively abundant in POM, indicating that bacterial detritus is a principal component of submicron particles in seawater. Muramic acid was measured in all UDOM samples, but its concentrations were low (Ͻ1 nmol L Ϫ1 ), indicating that intact fragments of peptidoglycan were a relatively minor component of UDOM. The abundance of GlcN and GalN and their similar concentrations in POM and UDOM provide novel information suggesting a major prokaryotic source.Amino sugars are often considered to be abundant components of marine organic matter, yet there are surprisingly few measurements of amino sugars in the marine environment. Chitin, a structural polymer of the amino sugar Nacetyl-glucosamine, is produced by a large variety of marine organisms (Muzzarelli 1977), and the production of chitin in the oceans is undoubtedly quite large. Kirchman and White (1999) estimated that about 10% of marine bacterial production could be supported by chitin. In addition to chitin and its derivatives, amino sugars are found in other polysaccharides, glycoproteins, and glycolipids that are common to many organisms (Sharon 1965). Amino sugars are important structural components of prokaryotic cell walls, where they occur in peptidoglycan, lipopolysaccharides, and pseudopeptidoglycan (Brock et al. 1994). Several studies provide measurements of amino sugars in marine particles and sediments (Ittekot et al. 1984; Müller et al. 1986;Liebezeit 1993;Dauwe and Middelburg 1998), but existing in-1 Corresponding author (benner @biol.sc.edu). AcknowledgmentsWe dedicate this paper to the memory of John I. Hedges. John provided valuable comments on an earlier draft of this manuscript and many hours of stimulating discussion on the chemical composition of DOM. We thank Rumi Fukuda for providing samples of natural populations of marine bacteria collected from seawater, W. Whitman for providing cultures of methanogens, and José Bersano for assistance with the collection and sorting of copepods. We thank scientists and crew aboard the...

shelf-and river-derived elements to the central Arctic Ocean • The TPD is rich in dissolved organic matter (DOM), which facilitates long-range transport of trace metals that form complexes with DOM • Margin trace element fluxes may increase with future Arctic warming due to DOM release from permafrost thaw and increasing river discharge

Soils in the drainage basins of Arctic rivers are a major global reservoir of aged organic carbon. The fate of this old carbon is of growing concern as the effects of climate change become more evident in the Arctic. We report natural abundance 14C data indicating that dissolved organic carbon (DOC) from several Eurasian and North American rivers is predominantly young and largely derived from recently‐fixed C in plant litter and upper soil horizons. Concentrations of dissolved lignin phenols, unique organic tracers of terrestrial plant material, and 14C content in DOC were strongly correlated throughout the Arctic Ocean, indicating terrigenous DOC is mostly young and widely distributed in polar surface waters. These young ages of terrigenous DOC in rivers and the ocean indicate little of the old carbon stored in Arctic soils is currently being mobilized in the dissolved component of continental runoff.