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The fate of soil organic carbon (SOC) in boreal forests is dependent on the integrative ecosystem response to climate change. For example, boreal forest productivity is often nitrogen (N) limited, and climate warming can enhance N cycling and primary productivity. However, the net effect of this feedback on the SOC reservoir and its longevity with climate change remain unclear due to difficulty in detecting small differences between large and variable carbon (C) fluxes needed to determine net changes in soil reservoirs. The diagenetic state of SOC – resulting from the physicochemical and biological transformations that alter the original biomolecular composition of detrital inputs to soil over time – is useful for tracing the net response of SOC at the timescales relevant to climate change not usually discernible from fluxes and stocks alone. Here, we test two hypotheses using a mesic boreal forest climate transect: (1) the SOC diagenetic state is maintained across this climosequence, and (2) the maintenance of the SOC diagenetic state is a consequence of coupled soil C and N cycling, signifying the role of enhanced N cycling supporting SOC inputs that maintain SOC stocks within the warmer-climate forests. Shifts in nonvascular to vascular plant inputs with climate observed in these and other boreal forests highlighted the need to carefully separate biogeochemical indicators of SOC source from those signifying diagenetic alteration. We thus evaluated and applied lignin biomarkers to assess the diagenetic alteration of SOC in these boreal forest organic soils and directly compared the lignin diagenetic state with that of soil organic nitrogen (SON) assessed through amino acid composition. The lignin diagenetic state remained constant across the climate transect, indicating a balance between the input and removal of lignin in these mesic boreal forests. When combined with previous knowledge of these forest ecosystems, including the diagenetic state of SON and direct measures of C fluxes and stocks, the results indicate a coupled increase in C and N cycling with climate warming that supports forest productivity and maintains SOC stocks. This balance could markedly shift as other factors begin to limit forest productivity (e.g., trace nutrients, water) with further climate change or affect forest nutrient allocation (e.g., forest age or compositional change). Further application of the approach presented here could be used to detect the limits of this and other ecosystem–climate feedbacks, by providing a tractable and parameterizable index of the lignin state across large spatial scales, necessary for ecosystem-scale parameterizations.

This study traces dissolved organic matter (DOM) in different water masses of the Arctic Ocean and its effect on the distributions of trace elements (TEs; Fe, Cu, Mn, Ni, Zn, Cd) using fluorescent properties of DOM and the terrigenous biomarker lignin. The Nansen, Amundsen, and Makarov Basins were characterized by the influence of Atlantic water and the fluvial discharge of the Siberian rivers with high concentrations of terrigenous DOM (tDOM). The Canada Basin and the Chukchi Sea were characterized by Pacific water, modified through contact with productive shelf sediments with elevated levels of marine DOM. Within the surface layer of the Beaufort Gyre, meteoric water (river water and precipitation) was characterized by low concentrations of lignin and terrigenous DOM fluorescence proxies as DOM is removed during freezing. High-resolution in situ fluorescence profiles revealed that DOM distribution closely followed isopycnals, indicating the strong influence of sea-ice formation and melt, which was also reflected in strong correlations between DOM fluorescence and brine contributions. The relationship of DOM and hydrography to TEs showed that terrigenous and marine DOM were likely carriers of dissolved Fe, Ni, Cu from the Eurasian shelves into the central Arctic Ocean. Chukchi shelf sediments were important sources of dCd, dZn, and dNi, as well as marine ligands that bind and carry these TEs offshore within the upper halocline (UHC) in the Canada Basin. Our data suggest that tDOM components represent stronger ligands relative to marine DOM components, potentially facilitating the long-range transport of TE to the North Atlantic. Key Points Dissolved Organic Matter (DOM) distribution in the Arctic Ocean is largely controlled by sea ice formation and melt processes DOM distribution in the Arctic Ocean reveals its potential as a tracer for halocline formation and freshwater source assignments Terrigenous and marine DOM are carriers of trace elements from shelves to the open Arctic Ocean, but terrigenous DOM represent stronger ligands

Dissolved lignin phenols, chromophoric dissolved organic matter (DOM), and in situ fluorescence were determined in waters of the Laptev Sea and major Arctic basins, and they were compared with dissolved iron (dFe) distributions to elucidate the sources, molecular characteristics and distributions of iron-binding ligands in the Arctic Ocean . In the Transpolar Drift region (TPD), concentrations of dFe were positively correlated with concentrations of lignin phenols and multiple optical proxies of DOM composition and source. Strong relationships between dFe and visible and ultraviolet wavelength fluorescent DOM indicated that vascular plant and algal-derived DOM contributed to the dFe-ligand pool. These observations are consistent with previous studies suggesting the association of dFe with humic terrigenous and marine organic ligands. The primary sources of iron-binding ligands appear to be the riverine discharge of terrigenous DOM, marine organic matter produced on the shelves, and degradation products of plankton-derived organic matter in the shelf sediments . A stronger relationship between dFe and visible wavelength CDOM fluorescence than with lignin phenols suggested the presence of multiple terrigenous ligands, such as aromatic tannins. The aromatic nature of these terrigenous ligands was indicated by a strong relationship between dFe and the absorption coefficient at 254 nm. A strong negative correlation between the p-hydroxyl to vanillyl lignin phenols ratio and dissolved iron concentrations indicated recently-discharged terrigenous DOM (tDOM) was an important source of iron-binding ligands. Given the strong relationships of marine and terrigenous DOM with dissolved iron, iron-binding functional groups appear to occur in diverse molecules of multiple sources. Examples of such iron-binding functional groups included catechols and carboxylates found in lignins and tannins of terrigenous origins and carboxyl-rich alicyclic molecules (CRAM) of terrigenous and marine origins. The observed dFe distributions in the Arctic Ocean could not be explained by the presence of a single ligand type, but rather by a potpourri of ligand molecules of varying concentrations and binding strengths. This molecular diversity of ligands and associated binding strengths ultimately controls the distribution and transport of dFe in the Arctic Ocean and beyond.

Data from GEOTRACES cruises GN01 (HLY1502) and GN04 (PS94) have been archived at the Biological & Chemical Oceanography Data Management Office (BCO-DMO; https://www.bco-dmo.org/deployment/638807) and PANGAEA (https://www.pangaea.de/?q=PS94&f.campaign%5B%5D=PS94) websites, respectively. The inorganic carbon data are available at the NOAA Ocean Carbon Data System (OCADS ; doi :10.3334/CDIAC/OTG.CLIVAR_ARC01_33HQ20150809). In addition to these national data archives, the data used in this paper is available as a supplementary downloadable excel file; International audience; A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river‐influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high‐resolution pan‐Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25‐50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle‐reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv (106 m3 s‐1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.

The persistence of the vast pool of recalcitrant dissolved organic carbon (RDOC) in the deep ocean is fundamental to both global carbon cycling and global climate. Yet, quantitative understanding of the mechanisms which produce and utilise RDOC is still in its infancy. Moving from a conceptual framework to quantitative understanding in marine carbon cycling can take the international research community several decades, precious time which is not available in this era of anthropogenic climate change. This perspective paper sets out an implementation strategy to move efficiently from conceptual hypotheses of marine carbon storage to quantification of the capacity for marine carbon storage through the microbial carbon pump (MCP), its sensitivity to global change and potential for modification. The aim is to excite and facilitate the international scientific community to hasten this process via an internationally co-ordinated multidisciplinary research initiative.

Dissolved lignin phenols, chromophoric dissolved organic matter (CDOM) absorption, and fluorescence were analyzed along cross-slope mooring locations in the Barents, Laptev, and East Siberian Seas to gain a better understanding of terrigenous dissolved organic carbon (tDOC) dynamics in Arctic shelf seas and the Arctic Ocean. A gradient of river water and tDOC was observed along the continental shelf eastward into the East Siberian Sea. Correlations of carbon-normalized yields of lignin-derived phenols supplied by Siberian rivers with river water fractions and known water residence times yielded in situ decay constants of 0.18–0.58 yr−1. Calculations showed ∼50% of annual tDOC discharged by Siberian rivers was mineralized in estuaries and on Eurasian shelves per year indicating extensive removal of tDOC. Bioassay experiments and in situ decay constants indicated a reactivity continuum for tDOC. CDOM parameters and acid/aldehyde ratios of vanillyl (V) and syringyl (S) lignin phenols showed biomineralization was the dominant mechanism for the removal of tDOC. Characteristic ratios of p-hydroxy (P), S, and V phenols (P/V, S/V) also identified shelf regions in the Kara Sea and regions along the Western Laptev Sea shelf where formation of Low Salinity Halocline Waters (LSHW) and Lower Halocline Water (LHW) occurred. The efficient removal of tDOC demonstrates the importance of Eurasian margins as sinks of tDOC derived from the large Siberian Rivers and confirms tDOC mineralization has a major impact on nutrients budgets, air-sea CO2 exchange, and acidification in the Siberian Shelf Seas.

Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5–100 m) and mesopelagic (200–750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42 ± 4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected varying ecosystem productivity and water masses. Surface distributions of chlorophyll-a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0°S–61.5°S) to ice-covered regions (61.5°S–62.5°S). This trend was evident from concentrations and DOC-normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice-free stations. Relatively low mole percentages of bacterially derived d-amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal the occurrence and spatial heterogeneity of bioavailable substrates in Antarctic waters during winter.

We construct mass-balance based estimates of carbon (C) export fractions from the water column across the Chukchi Sea shelf. Export is calculated as the difference between phytoplankton drawdown of dissolved inorganic C (DIC) and the accumulation of autochthonous particulate and dissolved organic C in the water column. Organic carbon (Corg) exports of >50% of DIC drawdown are ubiquitous across the shelf, even during, or shortly after, phytoplankton blooms, suggesting widespread and strong pelagic-benthic coupling. Export fractions on the shelf were generally greater in the less-productive Alaska Coastal Water than in the more productive Bering Shelf-Anadyr Water. Additionally, export fractions were greater in 2011 than in 2010, highlighting the significant spatial and inter-annual variability of the fate of Corg in this ecologically and biogeochemically important, and rapidly changing, ecosystem.

Concentrations of dissolved organic carbon (DOC) and major biochemicals (amino acids and carbohydrates) were measured during five cruises (2009–2010) to the Louisiana margin in the northern Gulf of Mexico. Concentrations of amino acids and carbohydrates were elevated at mid-salinities and were indicative of plankton production of dissolved organic matter (DOM) in surface waters. Hot spots of two compositionally distinct types of labile DOM were identified based on the relative abundances of amino acids and carbohydrates. Amino acid-rich hot spots occurred sporadically in regions of high phytoplankton biomass and were mostly observed between dusk and dawn, reflecting a grazing source. In contrast, carbohydrate-rich hot spots were more widespread and were often found in nutrient-poor waters, indicating the production of carbon-rich DOM associated with nutrient limitation. Major biochemical indicators and bioassay experiments indicated labile DOM comprised a relatively small fraction of the DOC. Most DOM was degraded and had a semi-labile nature. Substantial accumulations of marine (plankton-derived) DOC were observed in surface waters, particularly at mid-salinities during the summer. Microbial alteration of marine DOC and nutrient limitation of microbial utilization of carbon-rich DOM appeared largely responsible for the accumulation of DOC. The reservoir of accumulated marine DOC in the shelf surface mixed layer ranged from 0.11 Tg C to 0.23 Tg C, with the lowest and highest values occurring during winter and summer. Substantial cross-shelf export of semi-labile marine DOM occurred during the summer and provided a major carbon and energy subsidy to microbial food webs in offshore waters.

Concentrations of dissolved organic matter (DOM) and ultraviolet/visible light absorbance decrease systematically as groundwater moves through the unsaturated zones overlying aquifers and along flowpaths within aquifers. These changes occur over distances of tens of meters (m) implying rapid removal kinetics of the chromophoric DOM that imparts color to groundwater. A one-compartment input-output model was used to derive a differential equation describing the removal of DOM from the dissolved phase due to the combined effects of biodegradation and sorption. The general solution to the equation was parameterized using a 2-year record of dissolved organic carbon (DOC) concentration changes in groundwater at a long-term observation well. Estimated rates of DOC loss were rapid and ranged from 0.093 to 0.21 micromoles per liter per day (μM d−1), and rate constants for DOC removal ranged from 0.0021 to 0.011 per day (d−1). Applying these removal rate constants to an advective-dispersion model illustrates substantial depletion of DOC over flow-path distances of 200 m or less and in timeframes of 2 years or less. These results explain the low to moderate DOC concentrations (20–75 μM; 0.26–1 mg L−1) and ultraviolet absorption coefficient values (a 254

Deep soils contain more organic carbon than surface soils, and deep organic carbon has a longer carbon mean residence time. Relatively little is known about the degradation of organic matter in deep soils, especially over millennial time scales. Tephra (volcanic ash) deposition occasionally buries surface soils and forms deep soils rich in organic matter. In order to investigate the alteration and dynamics of buried organic matter in deep soils, we characterized common biochemicals, combined amino acids and amino sugars, in tephra layers deposited over millennial timescales in Hokkaido, Japan. Chemical analyses indicated relative abundances of specific amino acids (e.g. leucine and phenylalanine) and amino sugars, (e.g. glucosamine and galactosamine) changed with tephra soil age, indicating microbial alterations over millennial time scales. The observed changes in chemical composition were much slower than those typically observed during litter decomposition. Adsorption characteristics of the tephras developed with weathering and contributed to substantial organic carbon accumulation in the deep, aged tephra layers, such as Ta-c (deposited 1640 yr BP) and Ta-d (deposited 8940 yr BP). These observations indicate rapid microbial degradation and alteration during early stages of litter decomposition followed by much slower degradation following burial, when physicochemical processes such as sorption and occlusion and changes in environmental conditions influence organic matter accumulation and composition. Thus, microbial processes during organic matter decomposition and physicochemical processes during tephra diagenesis shape the chemical characteristics, reactivity and age of organic matter in buried tephra soils.

Vertical and horizontal distributions of total dissolved amino acids (TDAAs), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) were measured in the East/Japan Sea (EJS). The euphotic zone of this sea is N-limited, and the N : P ratio is ∼ 13 below 200 m depth. Elevated TDAA concentrations (137 ± 34 nM) and DOC-normalized yields (0.8 ± 0.2 % of DOC) were observed in deep waters ( ≥ 1000 m) of the EJS and compared with those in the deep North Pacific Ocean. Significantly high TDAA concentrations and yields were observed in a region of deep-water formation, indicating the convection of margin-derived bioavailable dissolved organic matter (DOM) to deep waters. Declining TDAA concentrations (36 ± 12 %) and yields (33 ± 13 %) were observed between 1000 and 3000 m throughout the EJS, indicating the utilization of bioavailable DOM in deep waters. Concentrations of the D-enantiomers of amino acids (Ala, Glx, Asx, and Ser) were relatively high in deep waters of the EJS, indicating substantial bacterial contributions to DOM from surface and upper mesopelagic waters. Climate warming during the past few decades in the EJS is weakening deep convection during the winter, and one consequence of this reduction in deep convection is a decline in the supply of bioavailable DOM from surface waters.

Carbon is a keystone element in global biogeochemical cycles. It plays a fundamental role in biotic and abiotic processes in the ocean, which intertwine to mediate the chemistry and redox status of carbon in the ocean and the atmosphere. The interactions between abiotic and biogenic carbon (e.g. CO2, CaCO3, organic matter) in the ocean are complex, and there is a half-century-old enigma about the existence of a huge reservoir of recalcitrant dissolved organic carbon (RDOC) that equates to the magnitude of the pool of atmospheric CO2. The concepts of the biological carbon pump (BCP) and the microbial loop (ML) shaped our understanding of the marine carbon cycle. The more recent concept of the microbial carbon pump (MCP), which is closely connected to those of the BCP and the ML, explicitly considers the significance of the ocean's RDOC reservoir and provides a mechanistic framework for the exploration of its formation and persistence. Understanding of the MCP has benefited from advanced ‘omics’ and novel research in biological oceanography and microbial biogeochemistry. The need to predict the ocean's response to climate change makes an integrative understanding of the BCP, ML and MCP a high priority. In this review, we summarize and discuss progress since the proposal of the MCP in 2010 and formulate research questions for the future.

Mosses contribute an average of 20 % of upland boreal forest net primary productivity and are frequently observed to degrade slowly compared to vascular plants. If this is caused primarily by the chemically complexity of their tissues, moss decomposition could exhibit high temperature sensitivity (measured as Q10) due to high activation energy, which would imply soil organic carbon (SOC) stocks derived from moss remains are especially vulnerable to decomposition with warming. Alternatively, the physical structure of the moss cell wall biochemical matrix could inhibit decomposition, resulting in low decay rates and low temperature sensitivity. We tested these hypotheses by incubating mosses collected from two boreal forests in Newfoundland, Canada, for 959 days at 5 and 18 °C, while monitoring changes in the moss tissue composition using total hydrolysable amino acid (THAA) analysis and 13C NMR spectroscopy. Less than 40 % of C was respired in all incubations, revealing a large pool of apparently recalcitrant C. The decay rate of the labile fraction increased in the warmer treatment, but the total amount of C loss increased only slightly, resulting in low Q10 values (1.23–1.33) compared to L horizon soils collected from the same forests. NMR spectra were dominated by O-alkyl C throughout the experiment, indicating the persistence of potentially labile C. Accumulation of hydroxyproline (derived primarily from plant cell wall proteins) and aromatic C indicates selective preservation of biochemicals associated with the moss cell wall. This was supported by scanning electron microscope (SEM) images of the moss tissues, which revealed few changes in the physical structure of the cell wall after incubation. This suggests the moss cell wall matrix protected labile C from microbial decomposition, accounting for the low temperature sensitivity of moss decomposition despite low decay rates. Climate drivers of moss biomass and productivity, therefore, represent a potentially important regulator of boreal forest SOC responses to climate change that needs to be assessed to improve our understanding of carbon-climate feedbacks.

A large quantity of reduced carbon is sequestered in the ocean as refractory dissolved molecules that persist through several circuits of global overturning circulation. Key aspects of the cycling of refractory dissolved organic carbon (DOC) remain unknown, making it challenging to predict how this large carbon reservoir will respond to climate change. Herein we investigate mechanisms that remove refractory DOC using bioassay experiments with DOC isolated from surface, mesopelagic and deep waters of the Atlantic Ocean. The isolated DOC was refractory to degradation by native microbial communities, even at elevated concentrations. However, when the refractory DOC was introduced to a series of novel environmental conditions, including addition of a labile substrate, a microbial community from coastal waters and exposure to solar radiation, a substantial fraction (7–13%) was removed within 1.5 years. Our results suggest that while refractory molecules can persist in the ocean for millennia, removal is rapid when they encounter their fate. The observed and projected climate-induced slowdown of global overturning circulation could reduce the exposure of refractory molecules to disparate removal processes. Assuming a constant rate of production, the reservoir size of refractory DOC could increase as overturning circulation slows, providing a negative feedback to rising atmospheric CO2.

The biodegradation of soil organic nitrogen (SON) into forms available for plant uptake is an important control on nitrogen availability, plant growth, and carbon (C) sequestration in terrestrial ecosystems. Changes in biochemical composition during SON diagenesis were investigated by analyzing hydrolysable amino acids in the organic (L, F, and H) horizons and the surface mineral (B) horizon of soil profiles from the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT). The diagenetic continuum in the soil profiles provides insights about the mechanisms of SON transformations, and comparison of regions along the latitudinal transect provides insight on the effects of climate. The fraction of N accounted for as amino acids decreased with depth in the profile, indicating a preferential loss of amino acids compared to bulk N during diagenesis. The amino acid composition also changed consistently with depth, and was used to construct a degradation index for assessing the extent of SON diagenesis. The compositional changes were similar to those observed in marine and lake sediments, indicating leaching and biodegradation have similar effects on SON composition in all three systems. This was supported by the use of the amino acids hydroxproline and diaminopimelic acid as tracers of plant and bacterial remains, respectively. The contribution of microbial remains increased with depth to >34 % of amino acids and >19 % of total N in the B horizon, consistent with increasing microbial decomposition. The extent of SON diagenesis was similar in the three regions despite a younger mean age of the organic matter at the southernmost site, indicating a higher rate of N turnover with higher temperature and precipitation. The resulting increase in N availability likely contributes to higher productivity and thus the higher litter inputs observed in the southern region, maintaining similar C stocks despite more rapid decomposition.

Most of the carbon fixed in primary production is rapidly cycled and remin-eralized, leaving behind various forms of organic carbon that contribute to a vast reservoir of nonliving organic matter in seawater. Most of this carbon resides in dissolved molecules of varying bioavailability and reactivity, and aspects of the cycling of this carbon remain an enigma. The size-reactivity continuum model provides a conceptual framework for understanding the mechanisms governing the formation and mineralization of this carbon. In the seawater bioassay experiments that served as the original basis for this model, investigators observed that larger size classes of organic matter were more bioavailable and more rapidly remineralized by microbes than were smaller size classes. Studies of the chemical composition and radiocarbon content of marine organic matter have further indicated that the complexity and age of organic matter increase with decreasing molecular size. Biodegradation processes appear to shape the size distribution of organic matter and the nature of the small dissolved molecules that persist in the ocean.

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL072138 Key Points: • DOC in the deep Eurasian Basin appears to contain at least 8% bomb C • Modern DOC appears to be selectively lost in the Beaufort Sea and slope water • A relationship between DOC Δ C and total hydrolysable amino acids in the Beaufort suggests that chemical composition controls DOC cycling Supporting Information: • Supporting Information S1 Correspondence to: E. R. M. Druffel, edruffel@uci.edu Citation: Druffel, E. R. M., S. Griffin, C. S. Glynn, R. Benner, and B. D. Walker (2017), Radiocarbon in dissolved organic and inorganic carbon of the Arctic Ocean, Geophys. Res. Lett., 44, doi:10.1002/ 2016GL072138. Received 28 NOV 2016 Accepted 13 FEB 2017 Accepted article online 15 FEB 2017 Radiocarbon in dissolved organic and inorganic carbon of the Arctic Ocean E. R. M. Druffel 1 , S. Griffin 1 , C. S. Glynn 1 , R. Benner 2 , and B. D. Walker 1 Department of Earth System Science, University of California, Irvine, California, USA, 2 Marine Science Program, University of South Carolina, Columbia, South Carolina, USA Abstract Dissolved organic carbon (DOC) in the ocean is thousands of 14 C years old, yet a portion of the DOC cycles on much shorter time scales (days to decades). We present 14 C measurements of DOC in the Arctic Ocean and estimate that ≥8% of the DOC in the deep Eurasian Basin contains bomb 14 C. While this is a limited data set, there appears to be selective loss of modern DOC in the surface and halocline waters of the open Beaufort Sea versus the Beaufort slope. At one of the Beaufort Sea stations, there is a linear relationship between DOC Δ 14 C values and previously measured total hydrolysable amino acid concentrations as reported by Shen et al. (2012), indicating that deep DOC contains small amounts of bioavailable DOC. The 14 C data show that not all of the deep DOC is recalcitrant. 1. Introduction The Arctic Ocean has a large continental shelf region and the largest input of freshwater per unit volume of any ocean. Dissolved organic carbon (DOC) in surface waters of the Arctic is the highest of any ocean, due to the large flux of terrigenous DOC from Arctic rivers [Amon et al., 2012; Benner et al., 2005; Hansell et al., 2004]. This flux of DOC is expected to increase as the Earth continues to warm, caus- ing release of organic C from expansive Arctic permafrost watersheds [Anderson and Amon, 2015; Benner et al., 2004]. There are four main water masses in the Arctic Ocean: (1) polar surface water, (2) halocline water (30–200 m), (3) Atlantic derived water (200–800 m), and (4) deep Atlantic water. Polar surface water is cold and fresh, having been derived from Arctic rivers and sea ice melt. The halocline water is formed on the continental shelves from freezing and brine release and is laterally advected into the Arctic Basin [Aagaard et al., 1981]. Water from the Pacific Ocean flows into the Beaufort Sea between 75 and 100 m depth (Upper Halocline Water), while water from the Atlantic Ocean enters from the Fram Strait and Barents Sea (Lower Halocline Water). The Beaufort Sea (in the southern Canadian Basin) and Eurasian Basin are separated by the Lomonosov Ridge whose sill depth is about 1500 m, keeping the deep waters of the Canada Basin and Beaufort Sea relatively isolated [Ostlund et al., 1987] (Figure 1). Radiocarbon provides quantitative information on the time scale of DOC cycling in the global ocean. Though the existing data are sparse, the available 14 C ages of DOC reveal that the difference between the average, prebomb ( 1000 m depth) and slope (bottom depth from 100 to 1000 m) and the Eurasian Basin to evaluate the distribution and cycling of DOC in the Arctic Ocean. We estimate that ≥8% of the DOC in the Eurasian Basin is of postbomb origin. We find that DOC concentrations and Δ 14 C values in the upper Beaufort Sea and slope are linearly correlated with salinity. Samples from one station in the Beaufort Sea has total hydrolysable amino acid measurements [Shen et al., 2012] that are strongly correlated with DOC Δ 14 C values, which indicates that deep DOC contains little bioavailable DOC. RADIOCARBON IN THE ARCTIC OCEAN

Climate warming enhances multiple ecosystem C fluxes, but the net impact of changing C fluxes on soil organic carbon (SOC) stocks over decadal to centennial time scales remains unclear. We investigated the effects of climate on C fluxes and soil C stocks using space-for-time substitution along a boreal forest climate gradient encompassing spatially replicated sites at each of three latitudes. All regions had similar SOC concentrations and stocks (5.6 to 6.7 kg C m−2). The three lowest latitude forests exhibited the highest productivity across the transect, with tree biomass:age ratios and litterfall rates 300 and 125% higher than those in the highest latitude forests, respectively. Likewise, higher soil respiration rates (~55%) and dissolved organic C fluxes (~300%) were observed in the lowest latitude forests compared to those in the highest latitude forests. The mid-latitude forests exhibited intermediate values for these indices and fluxes. The mean radiocarbon content (Δ14C) of mineral-associated SOC (+9.6‰) was highest in the lowest latitude forests, indicating a more rapid turnover of soil C compared to the mid- and highest latitude soils (Δ14C of −35 and −30‰, respectively). Indicators of the extent of soil organic matter decomposition, including C:N, δ13C, and amino acid and alkyl-C:O-alkyl-C indices, revealed highly decomposed material across all regions. These data indicate that the lowest latitude forests experience accelerated C fluxes that maintain relatively young but highly decomposed SOC. Collectively, these observations of within-biome soil C responses to climate demonstrate that the enhanced rates of SOC loss that typically occur with warming can be balanced on decadal to centennial time scales by enhanced rates of C inputs.

Iron is an essential micronutrient that limits primary production in up to 40% of the surface ocean and influences carbon dioxide uptake and climate change. Dissolved iron is mostly associated with loosely characterised organic molecules, called ligands, which define key aspects of the iron cycle such as its residence time, distribution and bioavailability to plankton. Models based on in situ ligand distributions and the behaviour of purified compounds include long-lived ligands in the deep ocean, bioreactive ligands in the surface ocean and photochemical processes as important components of the iron cycle. Herein, we further characterise biologically refractory ligands in dissolved organic matter (DOM) from the deep ocean and labile ligands in DOM from the surface ocean, and their photochemical and biological reactivities. Experimental results indicated that photodegradation of upwelled refractory iron-binding ligands can fuel iron remineralisation and its association with labile organic ligands, thus enhancing iron bioavailability in surface waters. These observations better elucidate the roles of biologically refractory and labile molecules and global overturning circulation in the ocean iron cycle, with implications for the initiation and sustainment of biological activity in iron-limited regions and the residence time of iron in the ocean.

Dissolved organic matter (DOM) in groundwater influences water quality and fuels microbial metabolism, but its origins, bioavailability and chemical composition are poorly understood. The origins and concentrations of dissolved organic carbon (DOC) and bioavailable DOM were monitored during a long-term (2-year) study of groundwater in a fractured-rock aquifer in the Carolina slate belt. Surface precipitation was significantly correlated with groundwater concentrations of DOC, bioavailable DOM and chromophoric DOM, indicating strong hydrological connections between surface and ground waters. The physicochemical and biological processes shaping the concentrations and compositions of DOM during its passage through the soil column to the saturated zone are conceptualized in the regional chromatography model. The model provides a framework for linking hydrology with the processes affecting the transformation, remineralization and microbial production of DOM during passage through the soil column. Lignin-derived phenols were relatively depleted in groundwater DOM indicating substantial removal in the unsaturated zone, and optical properties of chromophoric DOM indicated lower molecular weight DOM in groundwater relative to surface water. The prevalence of glycine, γ-aminobutyric acid, and d-enantiomers of amino acids indicated the DOM was highly diagenetically altered. Bioassay experiments were used to establish DOC-normalized yields of amino acids as molecular indicators of DOM bioavailability in groundwater. A relatively small fraction (8 ± 4 %) of DOC in groundwater was bioavailable. The relatively high yields of specific d-enantiomers of amino acids indicated a substantial fraction (15–34 %) of groundwater DOC was of bacterial origin.

Peatland vegetation is controlled primarily by the depth of the water table, making peat paleovegetation a useful climate archive. We applied a biochemical approach to quantitatively estimate the plant sources of peat carbon based on (1) neutral sugar compositions of Sphagnum, vascular plants, and lichens and (2) lignin phenol compositions of vascular plants. We used these biochemical indices to characterize vegetation change over the last 2000 years in four peat cores from the West Siberian Lowland (Russia) to investigate climate change during the Medieval Climate Anomaly and Little Ice Age. The vegetation was dominated by Sphagnum in all four cores, but was punctuated by several rapid but transient transitions to vascular plant dominance in the two cores from the southern West Siberian Lowland ( 60°N), possibly indicating permafrost development. However, there was no evidence for sustained vegetation change in response to either climatic event in cores from southern West Siberian Lowland. This suggests that these climatic events were relatively mild in the southern West Siberian Lowland, although the sensitivity of bog plant communities to climate change remains poorly understood.

The extent of peat decomposition was investigated in four cores collected along a latitudinal gradient from 56°N to 66°N in the West Siberian Lowland. The acid:aldehyde ratios of lignin phenols were significantly higher in the two northern cores compared with the two southern cores, indicating peats at the northern sites were more highly decomposed. Yields of hydroxyproline, an amino acid found in plant structural glycoproteins, were also significantly higher in northern cores compared with southern cores. Hydroxyproline-rich glycoproteins are not synthesized by microbes and are generally less reactive than bulk plant carbon, so elevated yields indicated that northern cores were more extensively decomposed than the southern cores. The southern cores experienced warmer temperatures, but were less decomposed, indicating that temperature was not the primary control of peat decomposition. The plant community oscillated between Sphagnum and vascular plant dominance in the southern cores, but vegetation type did not appear to affect the extent of decomposition. Oxygen exposure time appeared to be the strongest control of the extent of peat decomposition. The northern cores had lower accumulation rates and drier conditions, so these peats were exposed to oxic conditions for a longer time before burial in the catotelm, where anoxic conditions prevail and rates of decomposition are generally lower by an order of magnitude.

The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air-sea CO2 exchange in ocean margins. However, the extreme heterogeneity of river-influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations and thereby obscures the role of tDOC in biogeochemical cycles. Here a lignin-based optical proxy for tDOC and a shelf-wide mass balance approach were used to quantitatively assess the fate of tDOC discharged from the Mississippi-Atchafalaya River System (M-ARS) to the Louisiana shelf. The mass balance revealed that ~40% of the tDOC discharged by the M-ARS during March 2009–2010 was mineralized to CO2 on the Louisiana shelf, with two thirds of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates severalfold higher during summer than during winter. Independent assessments of specific mineralization processes indicated biomineralization accounted for ~94% of the tDOC mineralization on an annual basis and suggest that photochemical transformations of tDOC enhanced biomineralization by ~50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment directly confirms that ocean margins are major sinks of the tDOC discharged by rivers and indicates that tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO2.

Suspended particles from the Pearl River, and surface sediments from the northern South China Sea (NSCS) were analyzed for elemental composition (C, N), stable carbon isotopic composition (δ13C), lignin phenols, and total hydrolyzable amino acids (THAA). Organic matter (OM) in riverine particles was depleted in 13C (avg. − 28.3‰) and was derived primarily from plankton and soils, with minor contributions from C3 plant material. The δ13C of sedimentary OM became increasingly enriched in 13C (avg. − 21.3‰) from the inner shelf to the slope of the NSCS, and C/N ratios decreased from the inner shelf to the slope, indicating increasing contributions of marine organic carbon (OC). Lignin phenol yields in sedimentary OM decreased from the inner shelf to the slope, indicating declining contributions of terrestrial OC. Lignin phenol compositions in riverine particles and marine sediments reflected a predominately angiosperm source. Degradation indices (DI) of sedimentary amino acids indicated increasing diagenetic alteration of marine OM with water depth and distance offshore. Increasing acid:aldehyde ratios (Ad/Al) in lignin phenols and 3,5-dihydroxybenzoic acid:vanillyl phenols (3,5-BD/V) ratios also indicated a seaward increase in the diagenetic state of terrestrial OM, presumably due to hydrodynamic sorting and biogeochemical processing. A three end-member mixing model based on lignin phenol yields and stable C isotopic compositions was used to distinguish sources of OM. Marine OM accounted for an increasing fraction (62–82%) of sedimentary OC from the inner shelf to the slope, whereas soil-derived OM accounted for a decreasing fraction (39–18%) of sedimentary OC from inner shelf to the slope. The contribution of C3 plant OM was negligible. The significant contributions of terrestrial OM in NSCS sediments are ascribed to the relatively refractory nature of soil OM and the bioavailable nature of marine OM.

Much has been learned about the microbial decomposition of plant litter, but relatively little is known about microbial contributions to litter and soil chemistry. We conducted a 3-year litterbag experiment and measured hydrolyzable amino acids (AA) and amino sugars (AS) to gain insights about microbial contributions to the chemical characteristics of decomposing litter and soil. Microscopic observations of hyphae were used to estimate fungal contributions to litter. The carbon (C)-normalized yields of AA and AS increased during decomposition along with nitrogen (N), indicating a shift in chemical characteristics from C-rich plant-derived biopolymers to N-rich, microbially-derived biochemicals. The contributions of fungal biomass to C and N were minor, but necromass of fungi as melanized and clamp-bearing hyphae increased during litter decomposition. Yields of glucosamine and galactosamine in litter approached those in microorganisms, particularly bacteria, suggesting major contributions of bacterial residues to litter during decomposition. The microbial contributions to decomposing litter were consistent with those observed in organic and mineral soils. Microorganisms play important roles in the organization and stabilization of soil organic matter as well as N immobilization and organic C preservation.

The stable hydrogen isotope composition (δD) of major plant biochemicals is variable. We present δD values for cellulose, hemicelluloses and lignin of six plant species. The δD value for lignin is consistently lower than that of bulk tissue (by ∼50‰) and cellulose (by ∼100‰). We show that these differences can be used to assess the extent of degradation of organic matter from a single source. A decrease in the δDbulk of decomposing Spartina alterniflora roots and rhizomes from −72‰ to −87‰ was observed over 18 months, reflecting a relative enrichment of lignin content due to the preferential removal of polysaccharides from the detrital material. Similar changes in δ13C were observed previously during the degradation of these plant tissues. These findings indicate that the extent of organic matter degradation should be considered when using stable isotope approaches to assess possible sources of organic matter in soils and sediments. We show that the change in δDbulk of plant detritus is best described by an exponential equation, which is simpler than the multiple exponential decay (multi-G) model which best describes the change in δ13Cbulk of plant detritus. Therefore correcting for isotopic shifts caused by decomposition may be more easily accomplished using δD.

The amino acid hydroxyproline (Hyp) has potential for tracing plant N because Hyp-rich glycoproteins (HRGPs), structural proteins in the cell walls of plants, are its only major biochemical source in soil. The reactivity of Hyp in tissue from three vascular plants (black mangrove leaves, smooth cordgrass and cypress needles) was investigated in a 4 yr decomposition study to test its suitability as a tracer of plant N. The reactivity was similar to that of bulk plant N in cordgrass and cypress needles, but Hyp was more resistant to decomposition in mangrove leaves. This was likely due to increased cross liking between peptides in HRGPs from dicotyledons vs. other plants. These cross links involve stable diphenyl ether and biphenyl linkages that may protect the HRGP from degradation. The results suggest that Hyp can provide a quantitative estimate of the contribution of plant residues to the detrital N pool in systems dominated by gymnosperm and monocotyledon inputs. A mixing model using litter input representative of boreal forest vegetation demonstrated the potential of Hyp for tracing plant N in soil.

The present study demonstrates that the spectral slope coefficient of chromophoric dissolved organic matter (CDOM) between 275 nm and 295 nm (S275–295) can be used as a tracer of the percent terrigenous dissolved organic carbon (%tDOC) in river-influenced ocean margins, where rivers exert an important control on carbon dynamics and CO2 fluxes. Absorption coefficients of CDOM and concentrations of dissolved organic carbon (DOC) and dissolved lignin were measured on a seasonal basis in the Mississippi and Atchafalaya rivers and in surface waters of the northern Gulf of Mexico (NGoM). A strong, linear relationship between lignin concentrations and CDOM absorption coefficients indicated lignin is an important chromophore in this environment. The dual nature of lignin as an important chromophore in CDOM and as a terrigenous component of DOC facilitated development of the tracer. The applicability of the tracer relies on the existence of a strong, nonlinear relationship between S275–295 and the DOC-normalized lignin yield in rivers and along the freshwater– marine continuum in the NGoM. Physical mixing and the effects of photodegradation on S275–295 and dissolved lignin were largely responsible for maintaining this relationship, suggesting the tracer is applicable to surface waters of most river-influenced ocean margins. The spectral slope coefficient (S275–295) provides new capabilities to trace tDOC on synoptic scales of relevance to ocean margins and represents an important tool for improving ocean carbon budgets. The key processes controlling carbon transformations in ocean margins remain poorly quantified, thereby limiting our understanding of how the coastal ocean affects the ocean carbon cycle and atmospheric CO2. Ocean margins account for , 10% of the global ocean surface area but

Photochemistry of dissolved organic matter (DOM) plays an important role in marine biogeochemical cycles, including the regeneration of inorganic nutrients. DOM photochemistry affects nitrogen cycling by converting bio-refractory dissolved organic nitrogen to labile inorganic nitrogen, mainly ammonium (NH4+). During the August 2009 Mackenzie Light and Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of NH4+ photoproduction (i.e. photoammonification) were determined using water samples from the SE Beaufort Sea, including the Mackenzie River estuary, shelf, and Canada Basin. The photoammonification efficiency decreased with increasing wavelength across the ultraviolet and visible regimes and was higher in offshore waters than in shelf and estuarine waters. The efficiency was positively correlated with the molar nitrogen:carbon ratio of DOM and negatively correlated with the absorption coefficient of chromophoric DOM (CDOM). Combined with collateral measurements of CO2 and CO photoproduction, this study revealed a stoichiometry of DOM photochemistry with a CO2 : CO : NH4+ molar ratio of 165 : 11 : 1 in the estuary, 60 : 3 : 1 on the shelf, and 18 : 2 : 1 in the Canada Basin. The NH4+ efficiency spectra, along with solar photon fluxes, CDOM absorption coefficients and sea ice concentrations, were used to model the monthly surface and depth-integrated photoammonification rates in 2009. The summertime (June–August) rates at the surface reached 6.6 nmol l−1 d−1 on the Mackenzie Shelf and 3.7 nmol l−1 d−1 further offshore; the depth-integrated rates were correspondingly 8.8 μmol m−2 d−1 and 11.3 μmol m−2 d−1. The offshore depth-integrated rate in August (8.0 μmol m−2 d−1) was comparable to the missing dissolved inorganic nitrogen (DIN) source required to support the observed primary production in the upper 10-m layer of that area. The yearly NH4+ photoproduction in the entire study area was estimated to be 1.4 × 108 moles, with 85% of it being generated in summer when riverine DIN input is low. Photoammonification could mineralize 4% of the annual dissolved organic nitrogen (DON) exported from the Mackenzie River and provide a DIN source corresponding to 7% of the riverine DIN discharge and 1400 times the riverine NH4+ flux. Under a climate warming-induced ice-free scenario, these quantities could increase correspondingly to 6%, 11%, and 2100 times. Photoammonification is thus a significant nitrogen cycling term and may fuel previously unrecognized autotrophic and heterotrophic production pathways in the surface SE Beaufort Sea.

Current methods are not generally suitable for the quantitative analysis of nucleobases in cellular material and more complex matrices that are often encountered with environmental samples. Simple and reliable quantitative methods for nucleobase analysis in purified ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), microorganisms, and environmental samples are presented. Novel microwave-assisted vapor phase and liquid phase hydrolyses with HCl (6 mol L−1) were developed for the analysis of nucleobases in soluble and particulate samples, respectively. Optimal conditions for the release of purines (130°C, 10 min) and pyrimidines (160°C, 40 min) from DNA and RNA were applicable to both the vapor phase and liquid phase hydrolysis. Residual hydrochloric acid in hydrolysate can be removed easily by drying under a stream of nitrogen gas. Nucleobases were separated and quantified using high performance liquid chromatography with diode array detection (HPLC-DAD). The limit of detection for primary nucleobases ranged from 20 to 32 nmol L−1. Total hydrolysable nucleobases were analyzed in salmon testes DNA, yeast RNA, cultures of marine microalgae and cyanobacteria, and suspended particles from an estuary. The RNA and DNA contents and RNA/DNA ratios in cellular material were estimated from analyses of hydrolysable nucleobases.

Comparisons of the concentrations and compositions of dissolved organic matter (DOM) between the lower Mississippi River (MR) and its distributary, the Atchafalaya River (AR), indicated a strong influence of floodplains on DOM in the AR. Concentrations of dissolved organic carbon (DOC), lignin phenols, amino acids, neutral sugars, and chromophoric DOM (CDOM) absorption coefficients were higher in the AR than in the MR. Significantly lower syringyl to vanillyl phenol ratios (S:V) in the AR indicated substantial contributions from gymnosperms (e.g., Taxodium distichum). The lowest S:V values in the AR were measured during maximal litterfall in October‐November when yields of lignin phenols were highest. Lower spectral slope coefficients (S275‐295 )i n the AR were indicative of relatively high-molecular-weight CDOM. Yields and compositions of amino acids and neutral sugars indicated DOM in the AR was more bioavailable than in the MR. Nitrogen removal in the floodplain was in part responsible for the significantly lower concentrations of total dissolved nitrogen (TDN) in the AR. Seasonal variability in DOM composition was greater in the AR. About 35% of the DOC and . 44% of the lignin phenols, amino acids, and neutral sugars exported from the Mississippi‐Atchafalaya River system was carried by the AR, thereby revealing the important role of the AR in DOM export from the river system. DOC export from rivers was predominantly controlled by water discharge. The long-term (1996‐2010) average DOC export from the Mississippi‐Atchafalaya River system was 2.70 Tg yr21 (MR: 1.75 Tg yr21 ;A R: 0.95 Tg yr21), accounting for 0.8‐1.1% of global riverine DOC export.

This study evaluated the relationship between concentrations of operationally defined potentially bioavailable organic carbon (PBOC) and hydrolyzable amino acids (HAAs) in sediments collected from a diverse range of chloroethene- contaminated sites. Concentrations of PBOC an d HAA were measured using aquifer sediment samples collected at six selected study sites. Average concentrations of total HAA and PBOC ranged from 1.96 ± 1.53 to 20.1 ± 25.6 mg/kg and 4.72 ± 0.72 to 443 ± 65.4 mg/kg, respectively. Results demonstrated a statistically significant positive relationship between concentrations of PBOC and total HAA present in the aquifer sediment ( p < 0.05). Higher levels of HAA were consistently observed at sites with greater levels of PBOC and first-order decay rates. Because amino acids are known to be readily biodegradable carbon compounds, this relationship suggests that the sequential chemical extraction procedure used to measure PBOC is a useful indicator of bioavailable carbon in aquifer sediments. This, in turn, is consistent with the interpretation that PBOC measure-ments can be used for estimating the amount of natural organic carbon available for driving the reductive dechlorination of chloroethenes in groundwater systems.

Carbon-normalized yields of amino acids in size-fractionated seawater samples and bacterial cultures were used in a novel application to estimate the carbon content of heterotrophic marine bacteria. The estimated carbon content (6.3 ± 1.6 fg C cell -1 ) of open ocean bacteria derived using this approach was lower than most values estimated in previous studies. Based on these values, living heterotrophic bacteria accounted for 6.8 ± 1.1 % of suspended particulate organic carbon (POC) in surface waters at Stn ALOHA in the North Pacific gyre. Living cyanobacteria accounted for 13.3 ± 2.2 % of suspended POC in surface waters. Carbon-normalized yields of muramic acid, D -alanine and D -glutamic acid, unique biomarkers of bacteria, were used to estimate that bacterial detritus accounted for 4.3 ± 2.9 % of suspended POC in surface waters. The relative contribution of bacterial detritus to suspended POC increased dramatically with increasing depth, indicating that some components of bacterial detritus are resistant to decomposition in the deep ocean. The total living and detrital bacterial contribution to suspended POC in surface waters was -25 %.

The biochemical composition of dissolved organic phosphorus (DOP) in the ocean is dominated by phosphoesters (C-O-P and C-O-P-O-C bonds), which are hydrolyzed by a diverse group of alkaline phosphatases (PhoA, PhoD, PhoX), and by phosphonates (C-P bond), which are degraded by C-P lyases and hydrolases. We designed a bioinformatics pipeline and a statistical approach to recover and analyze the alkaline phosphatase and phosphonate utilization genes from a metagenomic database derived from water samples collected from 7 depths (between 10 and 4000 m) in the oligotrophic North Pacific Subtropical Gyre. The alkaline phosphatase genes phoD and phoX were more abundant than phoA in the euphotic zone (10-130 m) and in deep waters (500-4000 m). The C-P lyase genes were most abundant in the euphotic zone at 70 m and were rare in deep water (≥500 m) where phosphate concentrations were relatively high. These observations indicate that phosphonates are utilized primarily as a phosphorus source by bacterial C-P lyases; this is consistent with the observation that C-P lyase genes are part of the pho regulon which is expressed upon phos- phorus limitation. In contrast, C-P hydrolase and alkaline phosphatase genes were often more abun- dant in deep waters, indicating that DOP serves mainly as a carbon and energy source in phosphate- rich deep waters which are depleted in bioavailable dissolved organic matter (DOM). The observed differences in depth distributions and presumed functions of C-P lyase and hydrolase genes indicate variability in the chemical composition of phosphonates between the euphotic zone and deep waters.

The biological pump is a process whereby CO(2) in the upper ocean is fixed by primary producers and transported to the deep ocean as sinking biogenic particles or as dissolved organic matter. The fate of most of this exported material is remineralization to CO(2), which accumulates in deep waters until it is eventually ventilated again at the sea surface. However, a proportion of the fixed carbon is not mineralized but is instead stored for millennia as recalcitrant dissolved organic matter. The processes and mechanisms involved in the generation of this large carbon reservoir are poorly understood. Here, we propose the microbial carbon pump as a conceptual framework to address this important, multifaceted biogeochemical problem.