Iron Promotes the Retention of Terrigenous Dissolved Organic Matter in Subtidal Permeable Sediments.

Marine permeable sediments are important sites for organic matter turnover in the coastal ocean. However, little is known about their role in trapping dissolved organic matter (DOM). Here, we examined DOM abundance and molecular compositions (9804 formulas identified) in subtidal permeable sediments along a near- to offshore gradient in the German North Sea. With the salinity increasing from 30.1 to 34.6 PSU, the DOM composition in bottom water shifts from relatively higher abundances of aromatic compounds to more highly unsaturated compounds. In the bulk sediment, DOM leached by ultrapure water (UPW) from the solid phase is 54 ± 20 times more abundant than DOM in porewater, with higher H/C ratios and a more terrigenous signature. With 0.5 M HCl, the amount of leached DOM (enriched in aromatic and oxygen-rich compounds) is doubled compared to UPW, mainly due to the dissolution of poorly crystalline Fe phases (e.g., ferrihydrite and Fe monosulfides). This suggests that poorly crystalline Fe phases promote DOM retention in permeable sediments, preferentially terrigenous, and aromatic fractions. Given the intense filtration of seawater through the permeable sediments, we posit that Fe can serve as an important intermediate storage for terrigenous organic matter and potentially accelerate organic matter burial in the coastal ocean.

ICBM-OCEAN: Processing Ultrahigh-Resolution Mass Spectrometry Data of Complex Molecular Mixtures.

Untargeted molecular analyses of complex mixtures are relevant for many fields of research, including geochemistry, pharmacology, and medicine. Ultrahigh-resolution mass spectrometry is one of the most powerful tools in this context. The availability of open scripts and online tools for specific data processing steps such as noise removal or molecular formula assignment is growing, but an integrative tool where all crucial steps are reproducibly evaluated and documented is lacking. We developed a novel, server-based tool (ICBM-OCEAN, Institute for Chemistry and Biology of the Marine Environment, Oldenburg-complex molecular mixtures, evaluation & analysis) that integrates published and novel approaches for standardized processing of ultrahigh-resolution mass spectrometry data of complex molecular mixtures. Different from published approaches, we offer diagnostic and validation tools for all relevant steps. Among other features, we included objective and reproducible reduction of noise and systematic errors, spectra recalibration and alignment, and identification of likeliest molecular formulas. With 15 chemical elements, the tool offers high flexibility in formula attribution. Alignment of mass spectra among different samples prior to molecular formula assignment improves mass error and facilitates molecular formula confirmation with the help of isotopologues. The online tool and the detailed instruction manual are freely accessible at www.icbm.de/icbm-ocean.

Molecular Fractionation of Dissolved Organic Matter in a Shallow Subterranean Estuary: The Role of the Iron Curtain.

Iron that precipitates under aerobic conditions in natural aquatic systems scavenges dissolved organic matter (DOM) from solution. Subterranean estuaries (STEs) are of major importance for land-ocean biogeochemical fluxes. Their specific redox boundaries, coined the "iron curtain" due to the abundance of precipitated iron(III) (oxy)hydroxides, are hot spots for the removal and redissolution of iron, associated nutrients, and DOM. We used ultra-high-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry to molecularly characterize the iron-coagulating fractions of 32 groundwater and seawater DOM samples along a salinity gradient from a shallow STE on Spiekeroog Island, North Sea, Germany, and linked our findings to trace metal and nutrient concentrations. We found systematic iron coagulation of large (>450 Da), oxygen-rich, and highly aromatic DOM molecules of terrestrial origin. The extent of coagulation increased with growing terrestrial influence along the salinity gradient. Our study is the first to show that the iron curtain may be capable of retaining terrigenous DOM fractions in marine sediments. We hypothesize that the iron curtain serves as an inorganic modulator for the supply of DOM from groundwaters to the sea, and that the STE has the potential to act as a temporal storage or even sink for terrigenous aromatic DOM compounds.

Cross-Shore and Depth Zonations in Bacterial Diversity Are Linked to Age and Source of Dissolved Organic Matter across the Intertidal Area of a Sandy Beach.

Microbial communities and dissolved organic matter (DOM) are intrinsically linked within the global carbon cycle. Demonstrating this link on a molecular level is hampered by the complexity of both counterparts. We have now investigated this connection within intertidal beach sediments, characterized by a runnel-ridge system and subterranean groundwater discharge. Using datasets generated by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and Ilumina-sequencing of 16S rRNA genes, we predicted metabolic functions and determined links between bacterial communities and DOM composition. Four bacterial clusters were defined, reflecting differences within the community compositions. Those were attributed to distinct areas, depths, or metabolic niches. Cluster I was found throughout all surface sediments, probably involved in algal-polymer degradation. In ridge and low water line samples, cluster III became prominent. Associated porewaters indicated an influence of terrestrial DOM and the release of aromatic compounds from reactive iron oxides. Cluster IV showed the highest seasonality and was associated with species previously reported from a subsurface bloom. Interestingly, Cluster II harbored several members of the candidate phyla radiation (CPR) and was related to highly degraded DOM. This may be one of the first geochemical proofs for the role of candidate phyla in the degradation of highly refractory DOM.

Iron sulfide formation in young and rapidly-deposited permeable sands at the land-sea transition zone.

Organic-poor, permeable quartz sands are often present at land-sea transition zones in coastal regions. Yet, the biogeochemical cycles of carbon, sulfur, and iron are not well studied here. The aim of this work was, therefore, to improve our understanding regarding the chemical processes in these prominent coastal sediments. A 10 m core was collected at a dune base of the barrier island Spiekeroog, Germany, for this purpose. Additionally, groundwater was sampled from a multi-level well for one year to record seasonal hydrochemical variations. Methods included the analyses of geochemical (total carbon, total inorganic carbon, reactive iron, total sulfur, reduced inorganic sulfur) and hydrochemical parameters (field parameters, major ions, DOC, and molecular compositions of DOM), as well as stable sulfur isotopes (δ34S-sulfate, -sulfide, -total reduced inorganic sulfur). Moreover, optically stimulated luminescence (OSL) dating was applied. Results show that the core sediments are very young (<500 a) and were rapidly deposited. They are characterized by remarkably low contents of organic carbon (<0.1% dw.), reactive iron (~10 mmol/kg), and iron sulfides (<3 mmol/kg). Groundwater salinities were low in the top core sediments and increased at depth during most times of the year. However, the sampling site is subject to (seasonally) varying salinities, which could be linked to the biogeochemical cycles. For instance, the infiltration of seawater-derived labile DOM during inundation events drives microbial respiration besides sedimentary organic matter. Oxygen and nitrate were the dominant electron acceptors for the decomposition of organic matter in near-surface groundwater, while sulfate reduction was constrained to the lower brackish sediments. Here, authigenic pyrite formation was inferred based on the detection of dissolved sulfide, intact pyrite framboids, and matching stable sulfur isotope signatures of dissolved and solid sulfides. We concluded that the extremely low organic carbon contents limit pyrite formation in the organic-poor, permeable quartz sands.

Distribution patterns of chalcogens (S, Se, Te, and 210Po) in various tissues of a squid, Todarodes pacificus.

In order to elucidate the relationship between the chemically similar chalcogen elements S, Se, Te, and 210Po in marine invertebrates, we conducted a comparative study of the distribution patterns of these elements in a squid, Todarodes pacificus. Elemental concentrations of the four chalcogens were determined in (mantle) muscle, gill, stomach, and hepatopancreas tissues. No relationship between chalcogen concentrations and morphological parameters (mantle length, body weight, and sex) was evident. Gills showed slightly elevated levels of Se and 210Po, which may indicate absorption and uptake of these elements over the gill surface. All four chalcogens have their highest concentrations in the hepatopancreas and the lowest concentrations in the muscle tissue. However, concentration differences between tissues, revealed by (1) bioaccumulation values based on reference seawater values and (2) internal relative enrichment factors (IREF) based on enrichment of hepatopancreas compared to muscle tissue, were least pronounced for S, most distinct for 210Po, and moderate for Se and Te. Furthermore, no significant correlation for Se, Te, and 210Po with S within tissue concentrations, and only a slightly negative correlation between S and 210Po in the squid muscle and hepatopancreas tissues were found, which indicates either an antagonistic effect between, or a disconnection of the two elements through metabolic processing. Overall, the distribution patterns of Se and Te resemble those of essential trace elements, such as Zn and Cu, whereas 210Po is partitioned in a manner similar to toxic heavy metals, such as Cd and Ag.

An efficient and simple method for measuring (226)Ra using the scintillation cell in a delayed coincidence counting system (RaDeCC).

A delayed coincidence counter (RaDeCC), developed to determine ultra-low levels of (223)Ra (half life = 11.1 days) and (224)Ra (half life = 3.6 days) in seawater, was adapted to measure (226)Ra (half life = 1622 years). After pre-concentration of Ra from seawater onto MnO(2)-coated fiber we show in this study that the (226)Ra activity can be determined using the RaDeCC's ability to record alpha decay of its daughters as total counts. For sufficient ingrowth of (222)Rn, the Mn-fiber is hermetically sealed in a column for a few days. Then, the ingrown (222)Rn is circulated through the RaDeCC air-loop system followed by shutting down of the pump and closure of the scintillation cell for equilibration. Counting may be completed within a few hours for seawater samples. Sample measurements with this method agreed well with data obtained using gamma-ray spectrometry. This proves that a set of Ra isotopes ((223)Ra, (224)Ra, and (226)Ra), commonly used for geophysical studies such as mixing rates of different water masses and submarine groundwater discharge, can be efficiently and rapidly measured using the RaDeCC.