Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

More than 90% of the global ocean dissolved organic carbon (DOC) is refractory, has an average age of 4,000–6,000 years and a lifespan from months to millennia. The fraction of dissolved organic matter (DOM) that is resistant to degradation is a long-term buffer in the global carbon cycle but its chemical composition, structure, and biochemical formation and degradation mechanisms are still unresolved. We have compiled the most comprehensive molecular data set of 197 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses from solid-phase extracted marine DOM covering two major oceans, the Atlantic sector of the Southern Ocean and the East Atlantic Ocean (ranging from 50° N to 70° S). Molecular trends and radiocarbon dating of 34 DOM samples (comprising Δ14C values from -229 to -495‰) were combined to model an integrated degradation rate for bulk DOC resulting in a predicted age of >24 ka for the most persistent DOM fraction. First order kinetic degradation rates for 1,557 mass peaks indicate that numerous DOM molecules cycle on timescales much longer than the turnover of the bulk DOC pool (estimated residence times of >100 ka) and the range of validity of radiocarbon dating. Changes in elemental composition were determined by assigning molecular formulae to the detected mass peaks. The combination of residence times with molecular information enabled modelling of the average elemental composition of the slowest degrading fraction of the DOM pool. In our dataset, a group of 361 molecular formulae represented the most stable composition in the oceanic environment (“island of stability”). These most persistent compounds encompass only a narrow range of the elemental ratios H/C (average of 1.17 ± 0.13), and O/C (average of 0.52 ± 0.10) and molecular masses (360 ± 28 and 497 ± 51 Da). In the Weddell Sea DOC concentrations in the surface waters were low (46.3 ± 3.3 μM) while the organic radiocarbon was significantly more depleted than that of the East Atlantic, indicating average surface water DOM ages of 4,920 ± 180 a. These results are in accordance with a highly degraded DOM in the Weddell Sea surface water as also shown by the molecular degradation index IDEG obtained from FT-ICR MS data. Further, we identified 339 molecular formulae which probably contribute to an increased DOC concentration in the Southern Ocean and potentially reflect an accumulation or enhanced sequestration of refractory DOC in the Weddell Sea. These results will contribute to a better understanding of the persistent nature of marine DOM and its role as an oceanic carbon buffer in a changing climate

Molecular composition of dissolved organic matter in the Mediterranean Sea

ArticleThe molecular composition of marine dissolved organic matter (DOM) is still poorly understood, particularly in the Mediterranean Sea. In this work, DOM from the open Mediterranean Sea and the adjacent Northeast Atlantic Ocean was isolated by solid-phase extraction (SPE-DOM) and molecularly characterized using Fourier-transform ion cyclotron resonance mass spectrometry. We assessed the gradual reworking of the SPE-DOM transported by the shalloww overturning circulation of the Mediterranean Sea by following the increase in molecular weight (+20 Da), oxigenation (+5%), degradation index (Ideg +22%), and the proportional decrease of unsaturated aliphatic compounds (+34%) along the Levantine Intermediate Water. This reworked SPE-DOM that leaves the Mediterranean Sea through the Strait of Gibraltar strongly contrasts with the fresh material transported by the inflow of Atlantic water (Ideg -25%). In the deep eastern and western overturning cells, the molecular composition of the deep waters varied according to their area and/or time of formation. SPE-DOM of the waters formed in the Aegean Sea during the Earstern Mediterranean Transient (EMT) was more processed that the DOM in pre-EMT waters formed in the Adriatic Sea (molecular weight and the proportion of unsaturated aliphatic compounds were increased by 5 Da and 9%, respectively). Furthermore, pre-EMT waters contain more reworked SPE-DOM (Ideg +7%) than post-EMT waters formed also in the Adriatic Sea. In summary, our study shows that the Mediterranean Sea constitutes a laboratory basin where degradation processes and diagenetic transformations of DOM can be observed on close spatial and temporal scales.En prensa3,38

Persistence of dissolved organic matter explained by molecular changes during its passage through soil

Dissolved organic matter affects fundamental biogeochemical processes in the soil such as nutrient cycling and organic matter storage. The current paradigm is that processing of dissolved organic matter converges to recalcitrant molecules (those that resist degradation) of low molecular mass and high molecular diversity through biotic and abiotic processes. Here we demonstrate that the molecular composition and properties of dissolved organic matter continuously change during soil passage and propose that this reflects a continual shifting of its sources. Using ultrahigh-resolution mass spectrometry and nuclear magnetic resonance spectroscopy, we studied the molecular changes of dissolved organic matter from the soil surface to 60 cm depth in 20 temperate grassland communities in soil type Eutric Fluvisol. Applying a semi-quantitative approach, we observed that plant-derived molecules were first broken down into molecules containing a large proportion of low-molecular-mass compounds. These low-molecular-mass compounds became less abundant during soil passage, whereas larger molecules, depleted in plant-related ligno-cellulosic structures, became more abundant. These findings indicate that the small plant-derived molecules were preferentially consumed by microorganisms and transformed into larger microbial-derived molecules. This suggests that dissolved organic matter is not intrinsically recalcitrant but instead persists in soil as a result of simultaneous consumption, transformation and formation

Recovery of Gallium from Smartphones : Part II: Oxidative Alkaline Pressure Leaching of Gallium from Pyrolysis Residue

In this article, we examine the selective hydrometallurgical extraction of gallium from pyrolyzed smartphones. Gallium-enriched pyrolysis residue originating from pyrolyzed smartphones was leached using NaOH and gaseous oxygen at elevated temperatures and pressures. The high content of organic carbon in the material strongly influenced the leaching performance. Oxygen, which is indispensable for the dissolution of gallium, also oxidized the organic carbon in the feed so that CO2 was released, which had a neutralizing effect on the alkaline solution. As a result, the CO2 formation complicated the accurate process control as the leaching temperature increased. The highest gallium yield of 82% was obtained at 180 °C, 5 g/L NaOH and 5 bar oxygen pressure. Decreased temperatures, NaOH concentrations and oxygen pressures resulted in lower leaching yields but with a higher selectivity for Ga. Temperatures higher than 180 °C resulted in extensive carbon oxidation, NaOH consumption and the coextraction of Cu and Ag. We propose that those conditions also facilitated the formation of water-soluble organic compounds, which would also influence the metal dissolution

A molecular perspective on the ageing of marine dissolved organic matter

Dissolved organic matter (DOM) was extracted by solid-phase extraction (SPE) from 137 water samples from different climate zones and different depths along an eastern Atlantic Ocean transect. The extracts were analyzed with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with electrospray ionization (ESI). Δ14C analyses were performed on subsamples of the SPE-DOM. In addition, the amount of dissolved organic carbon was determined for all water and SPE-DOM samples as well as the yield of amino sugars for selected samples. Linear correlations were observed between the magnitudes of 43 % of the FT-ICR mass peaks and the extract Δ14C values. Decreasing SPE-DOM Δ14C values went along with a shift in the molecular composition to higher average masses (m/z) and lower hydrogen/carbon (H/C) ratios. The correlation was used to model the SPE-DOM Δ14C distribution for all 137 samples. Based on single mass peaks, a degradation index (IDEG) was developed to compare the degradation state of marine SPE-DOM samples analyzed with FT-ICR MS. A correlation between Δ14C, IDEG, DOC values and amino sugar yield supports that SPE-DOM analyzed with FT-ICR MS reflects trends of bulk DOM. DOM weighted normalized mass peak magnitudes were used to compare aged and recent SPE-DOM on a semi-quantitative molecular basis. The magnitude comparison showed a continuum of different degradation rates for the detected compounds. A high proportion of the compounds should persist, possibly modified by partial degradation, in the course of thermohaline circulation. Prokaryotic (bacterial) production, transformation and accumulation of this very stable DOM occur primarily in the upper ocean. This DOM is an important contribution to very old DOM, showing that production and degradation are dynamic processes

Dissolved organic matter in sea spray: a transfer study from marine surface water to aerosols

Atmospheric aerosols impose direct and indirect effects on the climate system, for example, by absorption of radiation in relation to cloud droplets size, on chemical and organic composition and cloud dynamics. The first step in the formation of Organic primary aerosols, i.e. the transfer of dissolved organic matter from the marine surface into the atmosphere, was studied. We present a molecular level description of this phenomenon using the high resolution analytical tools of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and nuclear magnetic resonance spectroscopy (NMR). Our experiments confirm the chemoselective transfer of natural organic molecules, especially of aliphatic compounds from the surface water into the atmosphere via bubble bursting processes. Transfer from marine surface water to the atmosphere involves a chemical gradient governed by the physicochemical properties of the involved molecules when comparing elemental compositions and differentiating CHO, CHNO, CHOS and CHNOS bearing compounds. Typical chemical fingerprints of compounds enriched in the aerosol phase were CHO and CHOS molecular series, smaller molecules of higher aliphaticity and lower oxygen content, and typical surfactants. A non-targeted metabolomics analysis demonstrated that many of these molecules corresponded to homologous series of oxo-, hydroxy-, methoxy-, branched fatty acids and mono-, di- and tricarboxylic acids as well as monoterpenes and sugars. These surface active biomolecules were preferentially transferred from surface water into the atmosphere via bubble bursting processes to form a significant fraction of primary organic aerosols. This way of sea spray production leaves a selective biological signature of the surface water in the corresponding aerosol that may be transported into higher altitudes up to the lower atmosphere, thus contributing to the formation of secondary organic aerosol on a global scale or transported laterally with possible deposition in the context of global biogeocycling

Biogeochemistry of dissolved organic matter in the Arctic permafrost and the Lena delta

Permafrost soils have accumulated vast stores of organic carbon recently estimated to be ~1600 Pg C. During the next century, near-surface permafrost across the circumpolar Arctic is expected to degrade significantly. Large amounts, especially of dissolved organic matter (DOM), may be released, transported to the oceans and either mineralized or convected down to the deep Arctic Ocean. Although DOC in Arctic rivers often shows relatively conservative characteristics, it has been recently shown that terrestrial DOC removal is an active but probably slow process. The major discharge of freshwater to the Arctic Ocean occurs within a very short period of time when ice is melting in late spring /early summer. All involved processes are characterized by high spatial and temporal variability but in their entirety they are likely to have critical implications for primary production and carbon cycling on Arctic shelves and in the Arctic Ocean. DOC in the Lena River has quite similar concentrations over long distances with a high carbon to nitrogen ratio. The classical relation between DOC and salinity in the coastal regions implies a conservative mixing, however, a strong modification of DOM must occur during transition from freshwater to seawater. C:N ratio strongly decrease in the estuarine and near shore regions. The discharge of nutrients is low except for silicate. The major aim is to identify and quantify the flux of soil-derived DOM and nutrients from the permafrost into the Lena estuary and the Arctic shelf-ocean system. Field sampling and experiments were carried out in August 2009 and 2010. Samples were taken from the main channels in the Lena delta and some near-shore coastal stations in the Buor-Khaya Bay as well as from run-off water from different ice complexes and permafrost lakes and soils. These samples with very different characteristics had highly variable DOC concentrations that are in the range from 450 to 650 µM for Lena River water, 800 to 10,800 µM for ice complex meltwater and of about 800 µM for the ice of ice complexes. Most of the freshwater and DOM is discharged via the eastern and north-eastern channels of the Lena River. Bacterial and photo-degradation batch experiments have shown that a considerable amount of terrestrial DOM of ice complex meltwater is mineralized or degraded. Ultra high resolution mass spectrometry is used to identify markers to trace the fate of DOM. Our goal is to implement a molecular classification system for the bioavailability of permafrost-derived DOM