We designed a CO2-controlled cuvette and stripping system to trace a (CO2)-C-14 pulse-label from photosynthetic assimilation by wetland plants tin this study Orontium aquaticum L.) to its release as (CH4)-C-14 by microbial respiration. The system maintained cuvette CO2 concentrations to within +/-5 Pa of the set-point, and it allowed continuous recovery of (CO2)-C-14 and (CH4)-C-14 for 17 d without damage to the enclosed plant. The first emissions of (CH4)-C-14 mere detected <12 h after photosynthetic assimilation of the label. The (CH4)-C-14 flux increased linearly from 0.12 Bq min(-1) at 12 h to 3.0 Bq min(-1) at 5 d, then plateaued at approximate to 2 Bq min(-1). We could not distinguish between (CH4)-C-14 produced by aceticlastic methanogenesis vs, that produced by CO2 reduction. Radiocarbon activity in the soil dissolved inorganic C pool peaked on the first day then declined slowly. We did not detect radiocarbon activity in soil solution pools of several low molecular weight organic acids (acetate, formate, lactate, and propionate), but the label was detected in the bulk dissolved organic C pool. We argue that radiocarbon will be useful for investigating the contribution of root exudates to methanogenic metabolism, but data interpretation will require separation of the relative contributions of CO2 reduction and aceticlastic methanogenesis to overall (CH4)-C-14 emissions. Processes such as CH4 oxidation and acetogenesis must also be considered in quantitative estimates of photosynthetic support of methanogenesis.

The carbon isotopic composition of methane produced in anoxic marine sediment is controlled by four factors: (1) the pathway of methane formation, (2) the isotopic composition of the methanogenic precursors, (3) the isotope fractionation factors for methane production, and (4) the isotope fractionation associated with methane oxidation. The importance of each factor was evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Methane did not accumulate during the initial 42‐day period when sediment contained sulfate, indicating little methane production from “noncompetitive” substrates. Following sulfate depletion, methane accumulation proceeded in three distinct phases. First, CO2 reduction was the dominant methanogenic pathway and the isotopic composition of the methane produced ranged from −80 to −94‰. The acetate concentration increased during this phase, suggesting that acetoclastic methanogenic bacteria were unable to keep pace with acetate production. Second, acetate fermentation became the dominant methanogenic pathway as bacteria responded to elevated acetate concentrations. The methane produced during this phase was progressively enriched in 13C, reaching a maximum δ13C value of −42‰. Third, the acetate pool experienced a precipitous decline from >5 mM to <20 μM and methane production was again dominated by CO2 reduction. The δ13C of methane produced during this final phase ranged from −46 to −58‰. Methane oxidation concurrent with methane production was detected throughout the period of methane accumulation, at rates equivalent to 1 to 8% of the gross methane production rate. Thus methane oxidation was too slow to have significantly modified the isotopic signature of methane. A comparison of microcosm and field data suggests that similar microbial interactions may control seasonal variability in the isotopic composition of methane emitted from undisturbed Cape Lookout Bight sediment.

In the first experimental phase the divertor tokamak ASDEX was run with a closed SS-limiter. The cleaning procedure for the vessel consisted of baking to 120°C, carbon removal by glow discharge in hydrogen, and oxygen removal with a continuous low power 50 Hz AC discharge. Wall contact of the plasma was reduced by carefully positioning the plasma with a feedback system. Discharges with plasma currents up to 280 kA and a disruption-free duration of up to 1 s were reliably produced with a filling pressure of 5 × 10−5 mbar and a programmed current rise. No change in start-up conditions and discharge behaviour was observed in material limiter discharges with superimposed divertor field.