A methane surplus relative to the atmospheric equilibrium is a frequently observed feature of ocean surface water. Despite the common fact that biological processes are responsible for its origin, the formation of methane in aerobic surface water is still poorly understood. We report on methane production in the central Arctic Ocean, which was exclusively detected in Pacific derived water but not nearby in Atlantic derived water. The two water masses are distinguished by their different nitrate to phosphate ratios. We show that methane production occurs if nitrate is depleted but phosphate is available as a P source. Apparently the low N:P ratio enhances the ability of bacteria to compete for phosphate while the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) is utilized as a C source. This was verified by experimentally induced methane production in DMSP spiked Arctic sea water. Accordingly we propose that methylated compounds may serve as precursors for methane and thermodynamic calculations show that methylotrophic methanogenesis can provide energy in aerobic environments.

Methane concentration was analyzed within hours of sampling. The dissolved gas was extracted from water by vacuum-ultrasonic treatment and subsequently measured with a gas chromatograph (Chrompack 9003, GC) with flame ionization detector (FID). For gas chromatographic separation we used a packed column (Porapac Q 80/100 mesh). The GC oven was operated isothermally (60 °C) and the heated zone of the FID was held at a temperature of 250 °C. Two sets of standard gas mixtures (10 and 100 ppmv) were used for calibration. The standard deviation of duplicate analyses was 5%. This high overall error is almost exclusively due to the gas extraction procedure and not to GC precision, which had an error of only 1%. After GC analyses, the remainder of the gas was transferred into evacuated glass containers for analysis of the carbon isotopic signature on shore. The 13C-CH4 values were determined by a Delta XP plus, Finnigan mass spectrometer. The extracted gas was purged and trapped with PreCon equipment (Finnigan) to pre-concentrate the sample. Depending on the concentration of methane, the reproducibility derived from duplicates was 0.5-1per mil. Isotopic ratios are reported relative to the Pee Dee Belemnite (PDB) standard using conventional delta notation.DMSPtotal (DMSPt) samples were collected directly from Niskin sample bottles into 50 ml centrifuge tubes that contained 167 µl of 50% H2SO4. The tubes were sealed and the samples stored for later analysis on shore. DMSPt in the stored samples was analyzed as DMS after alkaline cleavage. A subsample of the solution was pipetted into a 14 ml serum vial and treated with 1 ml of 5N NaOH and quickly sealed. The released DMS was purged into a cryotrap and quantified with a gas chromatograph equipped with a Chromosil 330 column and a flame photometric detector. The oven temperature was 100 °C and Helium was used as the purge and carrier gas. The analytical system was calibrated for DMS with standards. The detection limit was 0.5 to 1 pmol DMS injected, which yielded DMSP detection limits of 0.17 to 0.33nM in a 3-mL sample.

Supplement to: Damm, Ellen; Helmke, Elisabeth; Thoms, Silke; Schauer, Ursula; Nöthig, Eva-Maria; Bakker, Karel; Kiene, Ronald P (2010): Methane production in aerobic oligotrophic surface water in the central Arctic Ocean. Biogeosciences, 7, 1099-1108