A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr(-1) since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (similar to 0.4 Pg C yr(-1)) or sequestered in sediments (similar to 0.5 Pg C yr(-1)) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of similar to 0.1 Pg C yr(-1) to the open ocean. According to our analysis, terrestrial ecosystems store similar to 0.9 Pg C yr(-1) at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr(-1) previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land-ocean aquatic continuum need to be included in global carbon dioxide budgets

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Recently, combined carbon and hydrogen isotope fractionation investigations have emerged as a powerful tool for the characterization of reaction mechanisms relevant for the removal of organic pollutants. Here, we applied this approach in order to differentiate benzene biodegradation pathways under oxic and anoxic conditions in laboratory experiments. Carbon and hydrogen isotope fractionation of benzene was studied with four different aerobic strains using a monooxygenase or a dioxygenase for the initial benzene attack, a facultative anaerobic chlorate-reducing strain as well as a sulfate-reducing mixed culture. Carbon and hydrogen enrichment factors (epsilon(C), epsilon(H)) varied for the specific pathways and degradation conditions, respectively, so that from the individual enrichment factors only limited information could be obtained for the identification of benzene biodegradation pathways. However, using the slope derived from hydrogen vs carbon isotope discriminations or the ratio of hydrogen to carbon enrichment factors (lambda = deltaH/ deltaC approximately epsilon(H)/epsilon(C)), benzene degradation mechanisms could be distinguished. Although experimentally determined lambda values partially overlapped, ranges could be determined for different benzene biodegradation pathways. Specific lambda values were < 2 for dihydroxylation, between 7 and 9 for monohydroxylation, and > 17 for anaerobic degradation. Moreover, variations in lambda values suggest that more than one reaction mechanism exists for monohydroxylation as well as for anaerobic benzene degradation under nitrate-reducing, sulfate-reducing, or methanogenic conditions. Our results show that the combined carbon and hydrogen isotope fractionation approach has potential to elucidate biodegradation pathways of pollutants in field and laboratory microcosm studies.

[1] The global-scale quantification of organic carbon (C org ) degradation pathways in marine sediments is difficult to achieve experimentally due to the limited availability of field data. In the present study, a numerical modeling approach is used as an alternative to quantify the major metabolic pathways of C org oxidation (C ox ) and associated fluxes of redox-sensitive species fluxes along a global ocean hypsometry, using the seafloor depth (SFD) as the master variable. The SFD dependency of the model parameters and forcing functions is extracted from existing empirical relationships or from the NOAA World Ocean Atlas. Results are in general agreement with estimates from the literature showing that the relative contribution of aerobic respiration to C ox increases from <10% at shallow SFD to >80% in deep-sea sediments. Sulfate reduction essentially follows an inversed SFD dependency, the other metabolic pathways (denitrification, Mn and Fe reduction) only adding minor contributions to the global-scale mineralization of C org . The hypsometric analysis allows the establishment of relationships between the individual terminal electron acceptor (TEA) fluxes across the sediment-water interface and their respective contributions to the C org decomposition process. On a global average, simulation results indicate that sulfate reduction is the dominant metabolic pathway and accounts for approximately 76% of the total C ox , which is higher than reported so far by other authors. The results also demonstrate the importance of bioirrigation for the assessment of global species fluxes. Especially at shallow SFD most of the TEAs enter the sediments via bioirrigation, which complicates the use of concentration profiles for the determination of total TEA fluxes by molecular diffusion. Furthermore, bioirrigation accounts for major losses of reduced species from the sediment to the water column prohibiting their reoxidation inside the sediment. As a result, the total carbon mineralization rate exceeds the total flux of oxygen into the sediment by a factor of 2 globally.

For many years, controversy has surrounded the use of biofilm models to describe the distribution of microbial biomass in natural or artificial porous media. This use is often advocated on the basis of the relative mathematical simplicity of the biofilm concept, and of the widespread availability of analytical solutions or numerical implementations. However, microscopic observations consistently point to a patchy, rather than homogeneous, distribution of microorganisms at the pore scale in many porous media of interest, even under conditions of severe bioclogging. Also, bioclogging models involving biofilms tend to underpredict the extent of permeability reductions in all be the coarse-textured materials. In this context, computer simulations described in the present article show that some of the limitations of biofilm models to describe the bioclogging of porous media are linked to the common constitutive assumption that biofilms are impermeable, that is, that nutrient transport occurs through the biofilms only by molecular diffusion. When this restriction is alleviated and liquid flow is allowed in the biofilms, the level of bioclogging achievable by a given biomass is very significantly increased and is comparable to that observed in experiments. In addition, the distribution of microorganisms becomes patchy and exhibits a self-organized periodic pattern with pores either entirely filled with biomass or without any biomass at all, again similar to published microscopic observations. These results suggest that biofilm models should not be ruled out a priori for the quantitative description of bioclogging in porous media, as long as biofilms are allowed to be permeable.

In recent years the analysis of stable isotope fractionation has increasingly been used for characterizing and quantifying biodegradation of contaminants in aquifers. The correlation of carbon and hydrogen isotope signatures of benzene in a BTEX-contaminated aquifer located in the area of a former hydrogenation plant gave indications that biodegradation mainly occurred under anoxic conditions. This finding was consistent with the investigation of hydrogeochemical conditions within the aquifer. Furthermore, the biodegradation of benzene was calculated by changes in carbon isotope signatures using the Rayleigh-equation-streamline approach. Since contaminant concentrations can be also affected by nonisotope-fractionating abiotic processes such as dilution, volatilization, or irreversible sorption to the aquifer matrix, the Rayleigh-equation-streamline approach was adjusted for scenarios assuming that biodegradation and abiotic processes occur either consecutively or simultaneously along a groundwater flow path between contaminant source and sampling well. The results of the scenarios differed significantly, indicating that an abiotic process (typically dilution) causes a decrease in benzene concentration within the investigated aquifer transect. This comparison of results derived from the different scenarios can help to identify whether biodegradation is the predominant process for decrease in contaminant concentration. However, for a proper quantification of biodegradation, the temporal sequence between biodegradation and dilution needs to be known. The uncertainty associated with the quantification of pollutant biodegradation by the Rayleigh-equation-streamline approach increases when nonisotope-fractionating abiotic processes cause a significant decrease in contaminant concentrations.

The microbial degradation of organic substrates often exhibits a fractionation of stable isotopes which leads to an enrichment of the heavier isotope in the remaining substrate. The use of this effect to quantify the amount of biodegraded substrate in contaminated aquifers requires that the isotope fractionation factor is constant in time and space. In many natural and engineered systems the bioavailable concentration at the location of the enzymes differs from the average bulk concentration of the substrate. When enzymatically driven substrate degradation is coupled to a preceding transport step controlling the bioavailability of the substrate, the observed isotope fractionation becomes a function of the bulk substrate concentration. The sensitivity of the observed isotope fractionation factor toward such substrate concentration changes depends on the ratio of bulk substrate concentration and Michaelis-Menten constant and on the ratio between the specific affinity of the microorganisms toward the substrate and the first order rate constant of the bioavailability limiting transport process. Highest sensitivities toward substrate concentration were found for combinations of high substrate concentration with low substrate bioavailability (i.e., high ratios of substrate concentration and Michaelis-Menten constant, and high ratios of specific affinity and transport rate constant). As a consequence, changes in concentration and isotopic composition of a bioavailability limited substrate in batch experiments should not exhibit a linear relation in a Rayleigh plot, and the slope of the Rayleigh plot should show a decreasing trend with concentration decrease. When using isotope fractionation to quantify biodegradation along groundwater flow paths, changes in observed isotope fractionation might occur while contaminant concentration decreases along a flow path.

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