What controls clumped isotopes? Stable isotopes of a molecule can clump together in several combinations, depending on their mass. Even for simple molecules such as O 2 , which can contain 16 O, 17 O, and 18 O in various combinations, clumped isotopes can potentially reveal the temperatures at which molecules form. Away from equilibrium, however, the pattern of clumped isotopes may reflect a complex array of processes. Using high-resolution gas-phase mass spectrometry, Yeung et al. found that biological factors influence the clumped isotope signature of oxygen produced during photosynthesis (see the Perspective by Passey). Similarly, Wang et al. showed that away from equilibrium, kinetic effects causing isotope clumping can lead to overestimation of the temperature at which microbially produced methane forms. Science , this issue p. 431; p. 428; see also p. 394

A deep sleep in coal beds Deep below the ocean floor, microorganisms from forest soils continue to thrive. Inagaki et al. analyzed the microbial communities in several drill cores off the coast of Japan, some sampling more than 2 km below the seafloor (see the Perspective by Huber). Although cell counts decreased with depth, deep coal beds harbored active communities of methanogenic bacteria. These communities were more similar to those found in forest soils than in other deep marine sediments. Science , this issue p. 420 ; see also p. 376

Continuous multiple sulfur isotope profiles from South African rocks pinpoint the Great Oxygenation Event in the geologic record.

Methane is an important energy resource and significant long-lived greenhouse gas. Carbon and hydrogen isotope ratios have been used to better constrain the sources of methane but interpretations based on these two parameters alone can often be inconclusive. The precise measurement of a doubly substituted methane isotopologue, 13CH3D, is expected to add a critical new dimension to source signatures by providing the apparent temperature at which methane was formed or thermally equilibrated. We have developed a new method to precisely determine the relative abundance of 13CH3D by using tunable infrared laser direct absorption spectroscopy (TILDAS). The TILDAS instrument houses two continuous wave quantum cascade lasers; one tuned at 8.6 μm to measure 13CH3D, 12CH3D, and 12CH4, and the other at 7.5 μm to measure 13CH4. With the use of an astigmatic Herriott cell with an effective path length of 76 m, a precision of 0.2‰ (2σ) was achieved for the measurement of 13CH3D abundance in ca. 10 mL STP (i.e., 0.42 mmol) pure methane samples. Smaller quantity samples (ca. 0.5 mL STP) can be measured at lower precision. The accuracy of the Δ13CH3D measurement is 0.7‰ (2σ), evaluated by thermally equilibrating methane with a range of δD values. The precision of ±0.2‰ corresponds to uncertainties of ±7 °C at 25 °C and ±20 °C at 200 °C for estimates of apparent equilibrium temperatures. The TILDAS instrument offers a simple and precise method to determine 13CH3D in natural methane samples to distinguish geological and biological sources of methane in the atmosphere, hydrosphere, and lithosphere.

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The potassium channel selectivity filter both discriminates between K(+) and sodium ions and contributes to gating of ion flow. Static structures of conducting (open) and nonconducting (inactivated) conformations of this filter are known; however, the sequence of protein rearrangements that connect these two states is not. We show that closure of the selectivity filter gate in the human K(v)11.1 K(+) channel (also known as hERG, for ether-a-go-go-related gene), a key regulator of the rhythm of the heartbeat, is initiated by K(+) exit, followed in sequence by conformational rearrangements of the pore domain outer helix, extracellular turret region, voltage sensor domain, intracellular domains and pore domain inner helix. In contrast to the simple wave-like sequence of events proposed for opening of ligand-gated ion channels, a complex spatial and temporal sequence of widespread domain motions connect the open and inactivated states of the K(v)11.1 K(+) channel.

23A series of experiments were carried out to determine the clumped ( 13 CH 3 D) 24 methane kinetic isotope effects during oxidation of methane by OH and Cl radicals, the 25 major sink reactions for atmospheric methane. Experiments were performed in a 100 L 26 quartz photochemical reactor, in which OH was produced from the reaction of O( 1 D) 27 (from O 3 photolysis) with H 2 O, and Cl was from photolysis of Cl 2 . Samples were taken 28 from the reaction cell and analyzed for methane ( 12 CH 4 , 12 CH 3 D, 13 CH 4 , 13 CH 3 D) 29 isotopologue ratios using tunable infrared differential laser absorption spectroscopy. 30Measured kinetic isotope effects for singly substituted species were consistent with 31 previous experimental studies. For doubly substituted methane, 13 CH 3 D, the observed 32 kinetic isotope effects closely follow the product of the kinetic isotope effects for the 13 C 33 and deuterium substituted species (i.e., 13,2 KIE = 13 KIE × 2 KIE). The deviation from this 34 relationship is 0.3‰ ± 1.2‰ and 3.5‰ ± 0.7‰ for OH and Cl oxidation, respectively. 35This is consistent with model calculations performed using quantum chemistry and 36 transition state theory. The OH and Cl reactions enrich the residual methane in the 37clumped isotopologue in open system reactions. In a closed system, however, this 38 effect is overtaken by the large D/H isotope effect, which causes the residual methane 39 to become anti-clumped relative to the initial methane. Based on these results, we 40 demonstrate that oxidation of methane by OH, the predominant oxidant for tropospheric 41 methane, will only have a minor (~0.3 ‰) impact on the clumped isotope signature 42 (Δ 13 CH 3 D, measured as a deviation from a stochastic distribution of isotopes) of 43 tropospheric methane. This paper shows that Δ 13 CH 3 D will provide constraints on 44 3 methane source strengths, and predicts that Δ 12 CH 2 D 2 can provide information on 45 methane sink strengths. 46 4

Microorganisms in marine subsurface sediments substantially contribute to global biomass. Sediments warmer than 40°C account for roughly half the marine sediment volume, but the processes mediated by microbial populations in these hard-to-access environments are poorly understood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hot sediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetative cells drop two orders of magnitude and endospores become more than 6000 times more abundant than vegetative cells. Methane is biologically produced and oxidized until sediments reach 80° to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrations demonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zones alternate with zones up to 192 meters thick where microbes were undetectable.