Because methane-oxidizing bacteria (MOB) are the only biological sink for the greenhouse gas methane, knowledge of the functioning of these bacteria in various ecosystems is needed to understand the dynamics observed in global methane emission. The activity of MOB is commonly assessed by methane oxidation assays. The resulting methane depletion curves often follow a biphasic pattern of initial and induced methane oxidation activity, often interpreted as representing the in situ active and total MOB community, respectively. The application of quantitative-PCR on soil incubations, which were stopped before, at and after the transition point in the methane-depletion curve, demonstrated that both pmoA-mRNA was produced as well as substantial cell growth took place already in the initial phase. In addition, type Ia and II MOB displayed markedly different behaviour, which can be interpreted as ecologically different strategies. For the correct interpretation of methane oxidation assays, the use of small time windows is recommended to calculate methane oxidation activities to avoid substantial cell growth.

21Next to carbon dioxide, methane is the most important greenhouse gas which predominantly 22 is released from natural wetlands and rice paddies. Climate change predictions indicate 23 enhanced methane emission from global ecosystems under elevated CO2 and temperature. 24 However, the extent of this positive feedback is far from clear and depends on factors 25 modulating microbial responses of microbes involved in methane cycling in various 26 ecosystems. Nitrogen input by atmospheric deposition or fertilizer additions, is such a factor 27 with a range of possible effects on microbial methane production and consumption. In this 28 paper we discuss the crucial lacks in knowledge preventing a better understanding and 29 predictions of climate change effects on global methane emissions. 30 31 Introduction 32 Climate change, processes, ecosystems 33 The recent, 5 th assessment report of the IPCC (Intergovernmental Panel on Climate Change) 34 [1] states that it is very likely that climate change phenomena observed during the last 35 decades are caused by human action. It is unequivocal that since the 1950s, the atmosphere 36 and the oceans have warmed, the amounts of snow and ice have diminished, sea level has 37 risen and concentrations of greenhouse gasses (GHGs) have increased in a way unprecedented 38 for decades to millennia. The main driving factors determining anthropogenic radiative 39 forcing are emitted greenhouse gases. CO2, CH4 and N2O together comprise more than 80% 40 of the total radiative forcing (i.e. global warming effect) [2] and their current concentrations 41 and rate of increase exceed what has been observed in the past 800.000 and 20.000 years, 42 respectively [2].While CO2 is by far the most abundant GHG in the atmosphere (390 ppm; not 43 taking H2O into account), CH4 (1.804 ppm) and N2O (0.324 ppm) have a much higher44 3 | P a g e warming potential which shifted research efforts and possible mitigation strategies towards 45 these non-CO2 GHGs[3]. 46 Methane, contributing 17% to radiative forcing is increasing again in concentration since 47 2007 after a period of stabilization, which is currently being attributed to climate-induced 48 changes in methane emissions from natural wetlands [2]. Although methane can be formed 49 thermogenically, the largest part of methane emitted to the atmosphere is produced 50 microbiologically (see next section) in anoxic habitats like wetland soils and sediments, lakes, 51 the digestive tract of ruminants and insects and in anthropogenically created habitats like 52 landfills and other waste systems (see Figure 1).The single largest source of atmospheric 53 methane is global wetlands, including rice cultivation, where anoxic conditions and the 54 availability of large amounts of plant-derived carbon promote microbial methane formation 55 which can escape through plant roots and stems into the atmosphere [4]. Global sinks for 56 methane are microbiological consumption (see next section) and chemical degradation by OH 57 radicals in the troposphere and stra...

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Microbes catalyze all major geochemical cycles on earth. However, the role of microbial traits and community composition in biogeochemical cycles is still poorly understood mainly due to the inability to assess the community members that are actually performing biogeochemical conversions in complex environmental samples. Here we applied a polyphasic approach to assess the role of microbial community composition in modulating methane emission from a riparian floodplain. We show that the dynamics and intensity of methane consumption in riparian wetlands coincide with relative abundance and activity of specific subgroups of methane-oxidizing bacteria (MOB), which can be considered as a minor component of the microbial community in this ecosystem. Microarray-based community composition analyses demonstrated linear relationships of MOB diversity parameters and in vitro methane consumption. Incubations using intact cores in combination with stable isotope labeling of lipids and proteins corroborated the correlative evidence from in vitro incubations demonstrating c-proteobacterial MOB subgroups to be responsible for methane oxidation. The results obtained within the riparian flooding gradient collectively demonstrate that niche partitioning of MOB within a community comprised of a very limited amount of active species modulates methane consumption and emission from this wetland. The implications of the results obtained for biodiversity-ecosystem functioning are discussed with special reference to the role of spatial and temporal heterogeneity and functional redundancy.

The relationship between phosphatase activity and the element limiting microbial activity (carbon [C], nitrogen [N], or phosphorus [P]) was studied experimentally in sediment from four stations in the Baltic Sea located along a depth transect from oxic to anoxic bottom waters. The role of extracellular phosphatases was assessed by determining the percentages of intact cells that could be labeled with an artificial substrate for phosphatases (i.e., enzyme-labeled fluorescence 97 phosphatase substrate [ELF]) using a flow cytometer. Phosphatase activity was detected in sediment slurries from all sites either with or without prior incubation under oxic or anoxic conditions. In addition, ELF-labeled cells were detected in all incubated sediments, indicating that intact cells bearing phosphatases contribute to the phosphatase activity. Phosphatase activities and percentages of ELF-labeled cells were lower for the anoxic than for the oxic slurry incubations. Phosphatases are likely used to relieve the limitation of microbial activity by utilizable C in these recently deposited, organic C-rich sediments in the Baltic Sea. In marine sediments overlain by anoxic bottom waters, the biological and chemical mechanisms of P retention are often less efficient than in oxic settings and the P released to relieve C limitation escapes to the overlying water. This explains the ongoing higher P fluxes from sediments overlain by anoxic bottom waters.