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

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

Molecular level investigation of reactions between dissolved organic matter and extraction solvents using FT-ICR MS

The investigation of marine dissolved organic matter (DOM) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) requires the concentration and extraction of the DOM. Reactions between extraction solvent and DOM might influence data evaluation parameters, which are used to compare DOM of different origin. Potential reactions between methanol and solid-phase extracted marine DOM (SPE-DOM) and their influence on FT-ICR MS data were studied on a molecular level. For elemental formula assignments 9 isotopes were included in the evaluation, and different strategies based on isotope ratios and mass peak intervals were applied to achieve data sets of single assignments. DOM was eluted from the sorbent with methanol, methanol-C-13-D-4 and acetonitrile and stored at 20 degrees C and -20 degrees C. Under both storage conditions HID exchanges were detected. In addition, esterifications were detected in the extract stored at 20 degrees C. but did not result in significant differences in the mass peak magnitudes. Hence, no changes of the weighted average H/C and O/C ratios or other values such as Kendrick mass defect (KMD) and double bond equivalents (DBE) were observed. Acetonitrile was tested as an alternative solvent. Both, methanol or acetonitrile can be used to extract marine DOM from sorbents if samples are immediately stored at -20 degrees C after the SPE process. However, FT-ICR MS data of DOM samples extracted with different solvents showed substantial differences as expected and should not be directly compared

Organic matter turnover by pelagic microorganisms under the impact of ocean acidification

Marine bacteria are the main consumers of the organic matter and largely control 2 and CO2 budgets in the ocean. The present increase in anthropogenic O2 emissions is constantly decreasing seawater pH. This ocean acidification is expected to affect enzymatic hydrolysis of organic compounds with so-far unknown consequences for microbial physiology, organic matter cycling and marine biogeochemistry. We studied the effect of ocean acidification on a natural plankton community during a large-scale mesocosm study in the Raunefjord (Norway). We determined the concentration and composition of dissolved organic matter, in particular combined carbohydrates and amino acid, as well as marine gel particles, bacterial growth and enzymatic rates of organic matter hydrolysis. Dissolved organic carbon concentrations were increasing over time in all treatments while dissolved organic nitrogen concentrations remained stable. We observed higher protein hydrolysis rates, gel particle concentrations, and bacterial cell numbers in the low pH treatments. We conclude that ocean acidification may favor bacterial growth and degradation activities and therewith potentially change organic matter composition and cycling in the future ocean

Dissolved organic matter in the Lena Delta in 2009

Large Arctic rivers discharge significant amounts of dissolved organic matter (DOM) into the Arctic Ocean. We sampled natural waters of the Lena River, the Buor-Khaya Bay (Laptev Sea), permafrost melt water creeks, ice complex melt water creeks and a lake. The goal of this study was to characterize the molecular DOM composition with respect to different water bodies within the Lena Delta. We aimed at an identification of source-specific DOM molecular markers and their relative contribution to DOM of different origin. The molecular characterization was performed for solid-phase extracted DOM by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Average dissolved organic carbon concentrations in the original samples were 490±75 µmol C/L for riverine and bay samples and 399±115 µmol C/L for permafrost melt water creeks. Average TDN concentrations were elevated in the permafrost melt waters (19.7±7.1 µmol N/L) in comparison to the river and the bay (both 13.2±2.6 µmol N/L). FT-ICR MS and statistical tools demonstrated that the origin of DOM in the Lena Delta was systematically reflected in its molecular composition. Magnitude weighted parameters calculated from MS data (O/Cwa, H/Cwa, C/Nwa) highlighted preliminary sample discrimination. The highest H/Cwa of 1.315 was found for DOM in melt water creeks in comparison to 1.281 for river and 1.230 for the bay samples. In the bay samples we observed a higher fraction of oxygen-rich components which was reflected in an O/Cwa ratio of 0.445 in comparison to 0.425 and 0.427 in the river and creeks, respectively. From the southernmost location to the bay a relative depletion of nitrogenous molecular markers and an enrichment of oxidized DOM components occurred. The highest contribution of nitrogenous components was indicative for creeks reflected in a C/Nwa of 104 in comparison to 143 and 176 in the river and bay, respectively. These observations were studied on a molecular formula level using principal component and indicator value analyses. The results showed systematic differences with respect to water origin and constitute an important basis for a better mechanistic understanding of DOM transformations in the changing Arctic rivers

Origin-specific molecular signatures of dissolved organic matter in the Lena Delta

Large Arctic rivers discharge significant amounts of dissolved organic matter (DOM) into the Arctic Ocean. We sampled natural waters of the Lena River, the Buor-Khaya Bay (Laptev Sea), permafrost melt water creeks, ice complex melt water creeks and a lake. The goal of this study was to characterize the molecular DOM composition with respect to different water bodies within the Lena Delta. We aimed at an identification of source-specific DOM molecular markers and their relative contribution to DOM of different origin. The molecular characterization was performed for solid-phase extracted DOM by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Average dissolved organic carbon concentrations in the original samples were 490 ± 75 µmol C L−1 for riverine and bay samples and 399 ± 115 µmol C L−1 for permafrost melt water creeks. Average TDN concentrations were elevated in the permafrost melt waters (19.7 ± 7.1 µmol N L−1) in comparison to the river and the bay (both 13.2 ± 2.6 µmol N L−1). FT-ICR MS and statistical tools demonstrated that the origin of DOM in the Lena Delta was systematically reflected in its molecular composition. Magnitude weighted parameters calculated from MS data (O/Cwa, H/Cwa, C/Nwa) highlighted preliminary sample discrimination. The highest H/Cwa of 1.315 was found for DOM in melt water creeks in comparison to 1.281 for river and 1.230 for the bay samples. In the bay samples we observed a higher fraction of oxygen-rich components which was reflected in an O/Cwa ratio of 0.445 in comparison to 0.425 and 0.427 in the river and creeks, respectively. From the southernmost location to the bay a relative depletion of nitrogenous molecular markers and an enrichment of oxidized DOM components occurred. The highest contribution of nitrogenous components was indicative for creeks reflected in a C/Nwa of 104 in comparison to 143 and 176 in the river and bay, respectively. These observations were studied on a molecular formula level using principal component and indicator value analyses. The results showed systematic differences with respect to water origin and constitute an important basis for a better mechanistic understanding of DOM transformations in the changing Arctic rivers