Methane production controls in a young thermokarst lake formed by abrupt permafrost thaw.

Methane (CH4 ) release to the atmosphere from thawing permafrost contributes significantly to global CH4 emissions. However, constraining the effects of thaw that control the production and emission of CH4 is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH4 production and cycling in a region of rapidly degrading permafrost. Big Trail Lake, located in central Alaska, is a young, actively expanding thermokarst lake. The lake was investigated by taking two 1 m cores of sediment from different regions. Two independent methods of measuring microbial CH4  production, long term (CH4 accumulation) and short term (14 C tracer), produced similar average rates of 11 ± 3.5 and 9 ± 3.6 nmol cm-3  d-1 , respectively. The rates had small variations between the different lithological units, indicating homogeneous CH4 production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH4 production, the CH4 production rates were multiplied through the 10-15 m deep talik (thaw bulb). This estimate suggests that CH4  production is higher than emission by a maximum factor of ~2, which is less than previous estimates. Stable and radioactive carbon isotope measurements showed that 50% of dissolved CH4 in the first meter was produced further below. Interestingly, labeled 14 C incubations with 2-14 C acetate and 14 C CO2 indicate that variations in the pathway used by microbes to produce CH4 depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH4  was produced. This study demonstrates that at least half of the CH4 produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH4 is produced in the deep sediment.

The reduction of environmentally abundant iron oxides by the methanogen Methanosarcina barkeri.

Microbial dissimilatory iron reduction is a fundamental respiratory process that began early in evolution and is performed in diverse habitats including aquatic anoxic sediments. In many of these sediments microbial iron reduction is not only observed in its classical upper zone, but also in the methane production zone, where low-reactive iron oxide minerals are present. Previous studies in aquatic sediments have shown the potential role of the archaeal methanogen Methanosarcinales in this reduction process, and their use of methanophenazines was suggested as an advantage in reducing iron over other iron-reducing bacteria. Here we tested the capability of the methanogenic archaeon Methanosarcina barkeri to reduce three naturally abundant iron oxides in the methanogenic zone: the low-reactive iron minerals hematite and magnetite, and the high-reactive amorphous iron oxide. We also examined the potential role of their methanophenazines in promoting the reduction. Pure cultures were grown close to natural conditions existing in the methanogenic zone (under nitrogen atmosphere, N2:CO2, 80:20), in the presence of these iron oxides and different electron shuttles. Iron reduction by M. barkeri was observed in all iron oxide types within 10 days. The reduction during that time was most notable for amorphous iron, then magnetite, and finally hematite. Importantly, the reduction of iron inhibited archaeal methane production. When hematite was added inside cryogenic vials, thereby preventing direct contact with M. barkeri, no iron reduction was observed, and methanogenesis was not inhibited. This suggests a potential role of methanophenazines, which are strongly associated with the membrane, in transferring electrons from the cell to the minerals. Indeed, adding dissolved phenazines as electron shuttles to the media with iron oxides increased iron reduction and inhibited methanogenesis almost completely. When M. barkeri was incubated with hematite and the phenazines together, there was a change in the amounts (but not the type) of specific metabolites, indicating a difference in the ratio of metabolic pathways. Taken together, the results show the potential role of methanogens in reducing naturally abundant iron minerals in methanogenic sediments under natural energy and substrate limitations and shed new insights into the coupling of microbial iron reduction and the important greenhouse gas methane.

The role of highly sratified nutrient-rich small estuaries as a source of dissolved inorganic nitrogen to coastal seawater, the Qishon (SE Mediterranean) case.

We studied the role of small, highly stratified, sulfate and nutrient enriched estuaries, as a source or sink of inorganic nitrogen species, using the Qishon estuary at the Mediterranean coast of Israel, as a case study. Measurements of nutrient concentrations, δ(15)N and δ(18)O of nitrate+nitrite, δ(13)CDIC and δ(18)OH2O were performed during 2008-2009 along the upper-fresh and lower-saline water masses, as well as sediment porewater depth-profiles. Such estuaries are characterized by relatively low removal flux of NO3(-) (via sedimentary denitrification) and enhanced (×3) upward flux of NH4(+) (via sulfate reduction), attributed to the penetration of seawater of low NO3(-) and high dissolved oxygen and sulfate concentrations. The role of such small estuaries in releasing dissolved inorganic nitrogen, especially in sensitive oligotrophic areas as the Levantine basin and in the long-term, as a result of enhanced seawater penetration due to the expected sea level rise, has important environmental policy implications.