Absence Of Isotope Fractionation Research Articles

Mass Transfer Limitation during Slow Anaerobic Biodegradation of 2-Methylnaphthalene.

While they are theoretically conceptualized to restrict biodegradation of organic contaminants, bioavailability limitations are challenging to observe directly. Here we explore the onset of mass transfer limitations during slow biodegradation of the polycyclic aromatic hydrocarbon 2-methylnaphthalene (2-MN) by the anaerobic, sulfate-reducing strain NaphS2. Carbon and hydrogen compound specific isotope fractionation was pronounced at high aqueous 2-MN concentrations (60 μM) (εcarbon = -2.1 ± 0.1‰/εhydrogen = -40 ± 7‰) in the absence of an oil phase but became significantly smaller (εcarbon = -0.9 ± 0.3‰/εhydrogen = -6 ± 3‰) or nondetectable when low aqueous concentrations (4 μM versus 0.5 μM) were in equilibrium with 80 or 10 mM 2-MN in hexadecane, respectively. This masking of isotope fractionation directly evidenced mass transfer limitations at (sub)micromolar substrate concentrations. Remarkably, oil-water mass transfer coefficients were 60-90 times greater in biotic experiments than in the absence of bacteria (korg-aq2-MN = 0.01 ± 0.003 cm h-1). The ability of isotope fractionation to identify mass transfer limitations may help study how microorganisms adapt and navigate at the brink of bioavailability at low concentrations. For field surveys our results imply that, at trace concentrations, the absence of isotope fractionation does not necessarily indicate the absence of biodegradation.

Magnesium Stable Isotope Fractionation on a Cellular Level Explored by Cyanobacteria and Black Fungi with Implications for Higher Plants.

In a controlled growth experiment we found that the cyanobacterium Nostoc punctiforme has a bulk cell 26Mg/24Mg ratio (expressed as δ26Mg) that is -0.27‰ lower than the growth solution at a pH of ca. 5.9. This contrasts with a recently published δ26Mg value that was 0.65‰ higher than growth solution for the black fungus Knufia petricola at similar laboratory conditions, interpreted to reflect loss of 24Mg during cell growth. By a mass balance model constrained by δ26Mg in chlorophyll extract we inferred the δ26 Mg value of the main Mg compartments in a cyanobacteria cell: free cytosolic Mg (-2.64‰), chlorophyll (1.85‰), and the nonchlorophyll-bonded Mg compartments like ATP and ribosomes (-0.64‰). The lower δ26Mg found in Nostoc punctiforme would thus result from the absence of significant Mg efflux during cell growth in combination with either (a) discrimination against 26Mg during uptake by desolvation of Mg or transport across protein channels or (b) discrimination against 24Mg in the membrane transporter during efflux. The model predicts the preferential incorporation of 26Mg in cells and plant organs low in Mg and the absence of isotope fractionation in those high in Mg, corroborated by a compilation of Mg isotope ratios from fungi, bacteria, and higher plants.

Molybdenum isotope variations in calc-alkaline lavas from the Banda arc, Indonesia: Assessing the effect of crystal fractionation in creating isotopically heavy continental crust

Recent studies report a large Mo isotope variability of up to 1‰ (expressed in δ98/95MoNIST3134) in convergent margin lavas. These isotopic variations have been associated with subduction zone processes and ultimately may account for heavy and variable isotope signatures in evolved continental crust. Arc lavas show both lighter and heavier Mo isotopic values when compared to the chondritic average (δ98/95MoNIST3134 −0.16 ± 0.02‰), with a concentration weighted mean isotopic composition (δ98/95MoNIST3134 +0.03‰) distinctly heavier than that of the mantle (δ98/95MoNIST3134~−0.21 to −0.16‰). The absence of isotopic fractionation during tholeiitic igneous differentiation indicates that heavy Mo isotope signatures in convergent margin rocks are intrinsic to subduction zone processes, caused either by recycling of subducted components, by Mo isotope fractionation during magmatic differentiation, or by a combination thereof. In order to gain a more detailed understanding of the Mo isotope variability in arc lavas, we have studied Mo isotopes and concentrations in calc-alkaline lavas sampled along the Banda Arc in Indonesia, an intra-oceanic subduction zone well known for variable contributions of subducting continental meterial.Banda Arc lavas exhibit Mo isotopic compositions covering a large range from −0.48 to +0.24‰ in δ98/95MoNIST3134. We find that, combined with published data from other subduction zones, the majority of arc data are isotopically heavier compared to its presumed mantle wedge source or mid-ocean-ridge basalts. Furthermore, arc lava Mo isotope signatures show an apparent covariation between indices of amphibole-clinopyroxene fractionation from the melt. Generally, heavier Mo isotopic signatures are associated with higher degrees of REE fractionation expressed as λ1, λ2, and Dy*/Dy, high Ca/Al, and higher Sc contents. These observations can be best explained with an open system magma reservoir where fractional crystallisation, recharge and eruption are tightly linked. Notable exceptions in the Banda region are four samples with isotopically light Mo. Three of these isotopically light lavas indicate the assimilation of an isotopically light Mo reservoir with heavy δ18O and radiogenic 87Sr/86Sr-206Pb/204Pb, tentatively identified as lower arc crust. Together, our data demonstrate, in line with previous findings, that magmatic processes modify the Mo isotopic composition in arc lavas, thus adding complexity to the Mo isotope systematics inherent to subduction zones.

Optimisation of solid-state urea clathrate formation as a chemical separation method coupled to compound-specific stable carbon isotope analysis

ABSTRACTSolid-state urea clathrate formation (SSUCF) as a chemical separation method prior to stable carbon isotope fingerprinting of diesel fuel contaminations was studied. The stable carbon isotope ratios (δ13C) of n-alkanes in diesel fuel can be used to trace the origin of a contamination. The accurate measurement of the stable isotopic composition of individual compounds requires baseline separation from any other co-eluting compounds. For this purpose silica gel column chromatography (SGCC) and SSUCF were applied. Detailed optimisation of SSUCF was performed: different activators, clathrate formation temperatures, activator volumes, clathrate formation times and sample capacity were investigated. The main benefits of the developed method are reduced clathrate formation time and increased recoveries for lower molecular weight n-alkanes. The recoveries of the developed SSUCF method ranged between 63 and 100% for C10–C24 n-alkanes with relative standard deviation no more than 7%. The precision of the gas chromatography-isotope ratio mass spectrometry measurement was acceptable with a standard deviation of the δ13C values ranging between 0.08 and 0.15‰. The absence of isotopic fractionation was also investigated.The robustness of the method was tested within a model experiment. Nine different water samples including distilled water, tap water, river water, industrial wastewaters and groundwater samples were spiked with the same diesel fuel. The water samples were extracted with n-hexane and after purification with both SGCC and SSUCF n-alkanes were measured. The δ13C values of n-alkanes were found to be similar for all samples. The importance of sample purification prior to compound-specific isotope analysis (CSIA) was also demonstrated within this model experiment by analysing samples from different stages of the sample preparation.Our results show that the proposed method can remarkably improve the precision of compound-specific stable carbon isotope analysis of n-alkanes originating from diesel contamination of the aquatic environment.

Carbon isotopes of organic matter from the upper Jurassic oil shales from the Volgian-Pechora shale province and its accumulation mechanisms

The isotopic composition of carbon from the organic matter of late Jurassic oil shales from the Volgian-Pechora shale province is studied. The existence of a dependence between Corg content in the rock and the isotopic composition of kerogen carbon is ascertained. The content of the heavy carbon isotope increases with increasing Corg. This dependence is accounted for by the progressive accumulation of isotopically heavy hydrocarbons of the initial organic matter due to sulfurization. The data on the isotopic composition of individual n-alkanes of bitumen in the rocks and the data on the absence of isotopic fractionation between thermobitumen and the residual kerogen from oil shales from the Volgian-Pechora shale province obtained by treating shale in an autoclave in the presence of water are presented first in this paper.