The non-biological oxidative degradation of dissolved xanthopterin and 2, 4, 6-trihydroxypteridine by the pH or salt content of seawater

The degree of chemical instability of xanthopterin and related pteridines in seawater was studied with a view to assessing its role in the ecological turnover of these compounds in the marine environment. Solutions of these compounds in natural and synthetic seawater, brines containing one or more component salts of seawater, and buffers covering a wide pH range were incubated aseptically at 22–25°C in complete darkness and the chemical changes were spectrophotometrically monitored.Pterin, lumazine, and isoxanthopterin were completely stable in seawater, while the other pteridines degraded in the order dioxylumazine < leucopterin < xanthopterin <<< oxylumazine (the trivial names oxylumazine and dioxylumazine are used to denote 2, 4, 6-trihydroxy- and 2, 4, 6, 7-tetrahydroxy-pteridine, respectively). Apart from oxylumazine, the chemical instability of the other pteridines was correlated with pH corresponding to that of seawater. The high instability of oxylumazine was shown to be due to the salt content of seawater and not its pH; this pteridine required minimal concentrations of salt and traces of heavy-metal ions (such as Cu²⁺) to show significant chemical change. When the salt present was NaCl or KCl only, oxylumazine showed 1:1 oxidative conversion to dioxylumazine, but with the total salts of seawater the conversion was 2:1 with half of the oxylumazine being degraded, apparently by ring-cleavage, to unidentified non-pteridine products; this latter degradation is attributed to the total complex of ions present in seawater. In contrast to oxylumazine, xanthopterin gave evidence of 1:1 oxidative degradation via leucopterin in seawater, and this degradation appeared to be independent of trace metal ions. Chemical mechanisms are suggested for the observed degradations, and the ecological implications of the latter are discussed.     

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

Pore water chemistry (total dissolved CO₂, NH₄, NO₃, NO₂, PO₄, Si(OH)₄, Ca, Mg, Fe, Mn, SO₄, H₂S and F, and titration alkalinity), solid phase chemistry (C_org, P_org, C_TOT, N_TOT, F, Si_OPAL and S_II), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20–40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C_org), very high porosity ( > 0.96 ml cm⁻³) and very low dry bulk density (< 0.1 g cm⁻³). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μM, orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation.

The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a “testable” model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites.     

Modification of sediment geochemistry by the hydrocarbon seep tubeworm Lamellibrachia luymesi: A combined empirical and modeling approach

The sulfide-oxidizing symbiotic tubeworm Lamellibrachia luymesi is a dominant member of deep-sea hydrocarbon seep ecosystems on the Gulf of Mexico seafloor. This tubeworm forms large aggregations that can live for centuries and provide habitat for an assortment of associated fauna. Previous studies have suggested that persistence of these tubeworms for such long time periods is contingent upon their ability to supply sediments with sulfate. To examine this hypothesis, we characterized the tubeworm’s geochemical environment using pore water peepers and compared the measured depth profiles with those predicted by a sulfur diffusion-reaction-supply model. We found a large range of sulfide concentrations in the tubeworm habitat, indicating that this species can live under conditions of both high and low sulfide availability. In sediments rich in hydrocarbons, we found compelling evidence that tubeworms enhance microbial sulfide production, likely through a combination of sulfide uptake and sulfate release through their root-like structures buried in the sediment. Our in situ empirical data combined with the results of the geochemical model corroborate previous physiological studies that indicate that tubeworms release between 70% and 90% of the sulfate produced during sulfide oxidation by their symbionts across their roots into the surrounding sediment. In sediments low in hydrocarbon content, sulfide production is hydrocarbon-limited rather than sulfate-limited, and our model predicts that tubeworm growth could be limited by low sulfide availability.     

The close approaches and coalescence in triple systems of gravitating masses,I

By computer simulations, the dynamical evolution of plane triple systems of gaseous protogalaxies and galaxies with zero initial velocities has been studied. Inside the regionD of initial configurations some subregions have been revealed corresponding to a coalescence of protogalaxies on the first double approach. The average spin momenta of mergers are approximately equal to those typical of disk galaxies. In triple galaxies, a coalescence on the first double approach does not occur. The presence of significant hidden mass makes the approaches wider and prevents the coalescence of bodies in the systems without a central object. A central pair in a group of galaxies aids to coalescence. Also the change during time of the virial coefficient has been investigated.