Biogeochemical processes in anoxic sediment from Lake Plußsee, Germany

In anoxic environments, volatile methylated sulfides like methanethiol (MT) and dimethyl sulfide (DMS) link the pools of inorganic and organic carbon with the sulfur cycle. However, direct formation of methylated sulfides from reduction of dissolved inorganic carbon has previously not been demonstrated. When studying the effect of temperature on hydrogenotrophic microbial activity, we observed formation of DMS in anoxic sediment of Lake Plußsee at 55 °C. Subsequent experiments strongly suggested that the formation of DMS involves fixation of bicarbonate via a reductive pathway in analogy to methanogenesis and engages methylation of MT. DMS formation was enhanced by addition of bicarbonate and further increased when both bicarbonate and H2 were supplemented. Inhibition of DMS formation by 2-bromoethanesulfonate points to the involvement of methanogens. Compared to the accumulation of DMS, MT showed the opposite trend but there was no apparent 1:1 stoichiometric ratio between both compounds. Both DMS and MT had negative d13C values of -62 per mil and -55 per mil, respectively. Labeling with NaH**13CO3 showed more rapid incorporation of bicarbonate into DMS than into MT. The stable carbon isotopic evidence implies that bicarbonate was fixed via a reductive pathway of methanogenesis, and the generated methyl coenzyme M became the methyl donor for MT methylation. Neither DMS nor MT accumulation were stimulated by addition of the methyl-group donors methanol and syringic acid or by the methyl-group acceptor hydrogen sulphide. The source of MT was further investigated in a H2**35S labeling experiment, which demonstrated a microbially-mediated process of hydrogen sulfide methylation to MT that accounted for only <10% of the accumulation rates of DMS. Therefore, the major source of the 13C-depleted MT was neither bicarbonate nor methoxylated aromatic compounds. Other possibilities for isotopically depleted MT, such as other organic precursors like methionine, are discussed. This DMS-forming pathway may be relevant for anoxic environments such as hydrothermally influenced sediments and fluids and sulfate-methane transition zones in marine sediments.

Estimation of the normal boiling point of organic compounds via group contributions

The normal boiling point is a fundamental thermo-physical property, which is important in describing the transition between the vapor and liquid phases. Reliable method which can predict it is of great importance, especially for compounds where there are no experimental data available. In this work, an improved group contribution method, which is second order method, for determination of the normal boiling point of organic compounds based on the Joback functional first order groups with some changes and added some other functional groups was developed by using experimental data for 632 organic components. It could distinguish most of structural isomerism and stereoisomerism, which including the structural, cis- and trans- isomers of organic compounds. First and second order contributions for hydrocarbons and hydrocarbon derivatives containing carbon, hydrogen, oxygen, nitrogen, sulfur, fluorine, chlorine and bromine atoms, are given. The fminsearch mathematical approach from MATLAB software is used in this study to select an optimal collection of functional groups (65 functional groups) and subsequently to develop the model. This is a direct search method that uses the simplex search method of Lagarias et al. The results of the new method are compared to the several currently used methods and are shown to be far more accurate and reliable. The average absolute deviation of normal boiling point predictions for 632 organic compounds is 4.4350 K; and the average absolute relative deviation is 1.1047 %, which is of adequate accuracy for many practical applications.

Simulated Methyl iodide emission and concentration using the ocean biogeochemistry model MPIOM/HAMOCC forced by OMIP data

Production pathways of the prominent volatile organic halogen compound methyl iodide (CH3I) are not fully understood. Based on observations, production of CH3I via photochemical degradation of organic material or via phytoplankton production has been proposed. Additional insights could not be gained from correlations between observed biological and environmental variables or from biogeochemical modeling to identify unambiguously the source of methyl iodide. In this study, we aim to address this question of source mechanisms with a three-dimensional global ocean general circulation model including biogeochemistry (MPIOM-HAMOCC (MPIOM - Max Planck Institute Ocean Model HAMOCC - HAMburg Ocean Carbon Cycle model)) by carrying out a series of sensitivity experiments. The simulated fields are compared with a newly available global data set. Simulated distribution patterns and emissions of CH3I differ largely for the two different production pathways. The evaluation of our model results with observations shows that, on the global scale, observed surface concentrations of CH3I can be best explained by the photochemical production pathway. Our results further emphasize that correlations between CH3I and abiotic or biotic factors do not necessarily provide meaningful insights concerning the source of origin. Overall, we find a net global annual CH3I air-sea flux that ranges between 70 and 260 Gg/yr. On the global scale, the ocean acts as a net source of methyl iodide for the atmosphere, though in some regions in boreal winter, fluxes are of the opposite direction (from the atmosphere to the ocean).