Online Stable Isotope Analysis of Dissolved Organic Carbon Size Classes Using Size Exclusion Chromatography Coupled to an Isotope Ratio Mass Spectrometer

Stable isotopic content of dissolved organic carbon (δ¹³C-DOC) provides valuable information on its origin and fate. In an attempt to get additional insights into DOC cycling, we developed a method for δ¹³C measurement of DOC size classes by coupling high-performance liquid chromatography (HPLC)–size exclusion chromatography (SEC) to online isotope ratio mass spectrometry (IRMS). This represents a significant methodological contribution to DOC research. The interface was evaluated using various organic compounds, thoroughly tested with soil–water from a C3–C4 vegetation change experiment, and also applied to riverine and marine DOC. δ¹³C analysis of standard compounds resulted in excellent analytical precision (≤0.3‰). Chromatography resolved soil DOC into 3 fractions: high molecular weight (HMW; 0.4–10 kDa), low molecular weight (LMW; 50–400 Da), and retained (R) fraction. Sample reproducibility for measurement of δ¹³C-DOC size classes was ±0.25‰ for HMW fraction, ± 0.54‰ for LMW fraction, and ±1.3‰ for R fraction. The greater variance in δ¹³C values of the latter fractions was due to their lower concentrations. The limit of quantification (SD ≤0.6‰) for each size fraction measured as a peak is 200 ng C (2 mg C/L). δ¹³C-DOC values obtained in SEC mode correlated significantly with those obtained without column in the μEA mode (<i>p</i> < 0.001, intercept 0.17‰), which rules out SEC-associated isotopic effects or DOC loss. In the vegetation change experiment, fractions revealed a clear trend in plant contribution to DOC; those in deeper soils and smaller size fractions had less plant material. It was also demonstrated that the technique can be successfully applied to marine and riverine DOC without further sample pretreatment

Microbial activity promotes the enrichment of cobalt over nickel on hydrogenetic ferromanganese crusts

<p>The different mineral phases of the ferromanganese (Fe–Mn) crusts stem from the interaction of biotic and abiotic components. It is therefore vital to study the activity of these components to decipher their contribution to the enrichment/depletion of metals in the crust. Thus, the present study examined sorption and release of Co and Ni by Fe-Mn crusts with associated microbial communities in the presence and absence of the metabolic poison sodium azide (15 mM). The study was conducted in the presence (G⁺) and absence (G⁻) of added glucose (0.1%) at temperatures of 4 ± 1°C and 28 ± 2°C. Results showed that the microbial community had maximal sorption of Co of 66.12 µg g⁻¹ at 4 ± 1°C in the absence of added glucose and 479.75 µg g⁻¹ at 28 ± 2°C in the presence of added glucose. Maximum sorption of Ni in the absence of added glucose was 1.89 µg g⁻¹ at 4 ± 1°C and release of Ni was 51.28 µg g⁻¹ in the presence of added glucose. Under abiotic conditions with 15 mM sodium azide as a metabolic inhibitor, significant amounts of Co and Ni were released in the G⁺ medium. Total cell counts on the Fe-Mn crust in the presence of added glucose increased by an order of magnitude from 10⁶ to 10⁷ cells g⁻¹ and in the absence of added glucose remained within the order of 10⁶ cells g⁻¹ irrespective of temperature of incubation. Microscopic observation of the samples from biotic incubations showed numerous bacterial cells, exopolysaccharides, and structures resembling secondary minerals formed by bacteria. The results indicate that bacteria promote the enrichment of Co and Ni on the hydrogenetic Fe-Mn crusts by sorption processes and release of Ni by reductive dissolution of the oxides. The higher enrichment of Co than Ni is attributed to the way in which microbes interact with the metals.</p