A new method for the determination of the nitrogen isotopomers (intramolecular distribution of the nitrogen isotopes) of nitrous oxide has been developed. The method makes use of mass analyses of the molecular (N2O+) and fragment (NO+) ions of N2O on an isotope-ratio mass spectrometer equipped with a special ion collector system. The fragmentation of N2O in the electron impact ion source is fairly stable, and the precision of isotope ratio measurements of the fragment ion relative to the reference gas is better than 0.1‰ for pure N2O samples introduced from a conventional dual-inlet system. Although it is found that the observed isotope ratio of the fragment ion is affected by rearrangement reactions in the ion source, a correction can be applied using an experimentally determined rearrangement fraction. Calibration of the standard N2O for isotopomer measurements is performed by two procedures:  (1) preparation of an N2O standard by thermal decomposition of NH4NO3, (2) relative measurements with pure NO.

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Nitrous oxide (N2O) is an important trace gas in the atmosphere. It is an active greenhouse gas in the troposphere and it also controls ozone concentration in the stratosphere through nitric oxide production. One way to trace the geochemical cycle of N2O is by measuring the natural abundance of stable isotopes, namely 15N and 18O (refs 2-15). Here we report the intramolecular distribution of 15N within the linear NNO molecule, determined by measuring molecular and fragment ions of N2O on a modified mass spectrometer. This revealed a preference for 15N at the central N position, or alpha-site, within N2O isotopomers (isotope-containing molecules). Moreover, this preference varied significantly throughout the atmosphere. In the troposphere, low alpha-site preference indicates local emission of N2O from soils and fossil-fuel combustion, each with distinct isotopomer signatures, which then mixes with background N2O. In the stratosphere, on the other hand, loss of N2O is observed as enhanced alpha-site preference for 15N, due to fractionation during ultraviolet photolysis of N2O. We have constructed an atmospheric mass balance of N2O, incorporating isotopomer abundance, which shows that the intramolecular distribution of 15N is a parameter that has the potential to increase significantly the resolution with which sources and sinks of N2O can be identified and quantified in the atmosphere.

Hadal oceans at water depths below 6,000 m are the least-explored aquatic biosphere. The Challenger Deep, located in the western equatorial Pacific, with a water depth of ∼11 km, is the deepest ocean on Earth. Microbial communities associated with waters from the sea surface to the trench bottom (0 ∼10,257 m) in the Challenger Deep were analyzed, and unprecedented trench microbial communities were identified in the hadal waters (6,000 ∼10,257 m) that were distinct from the abyssal microbial communities. The potentially chemolithotrophic populations were less abundant in the hadal water than those in the upper abyssal waters. The emerging members of chemolithotrophic nitrifiers in the hadal water that likely adapt to the higher flux of electron donors were also different from those in the abyssal waters that adapt to the lower flux of electron donors. Species-level niche separation in most of the dominant taxa was also found between the hadal and abyssal microbial communities. Considering the geomorphology and the isolated hydrotopographical nature of the Mariana Trench, we hypothesized that the distinct hadal microbial ecosystem was driven by the endogenous recycling of organic matter in the hadal waters associated with the trench geomorphology.hadal | trench | niche separation | nitrification | Challenger Deep

[1] Stable isotopes of water are important climatic tracers used to understand atmospheric moisture cycling and to reconstruct paleoclimate. The combined use of hydrogen and oxygen isotopes in water provides an additional parameter, deuterium excess (d), which might reflect ocean surface conditions in moisture source regions for precipitation. The d records from polar ice cores covering glacial-interglacial cycles were used to reconstruct ocean surface temperatures at the moisture source, enabling elimination of source effects from the conventional isotope thermometer. However, observations of the essential relationship between d in vapor and ocean surface conditions are very limited. To date, theoretical values predicted using simple and atmospheric general circulation models (GCM) have not been validated against the data. Here, we show the isotope ratios of atmospheric water vapor near the ocean surface in middle and high latitudes of the Southern Ocean. Our observations show that d negatively correlates with relative humidity (h) above the ocean and correlates with sea surface temperature (SST). Despite the fact that the GCMs would underestimate the absolute value of observed d, the observations and simulation results are consistent for slopes between d versus h and d versus SST, suggesting that d is a reliable index to h and SST over the ocean surface.

Methanogenic microbes may be one of the most primitive organisms, although it is uncertain when methanogens first appeared on Earth. During the Archaean era (before 2.5 Gyr ago), methanogens may have been important in regulating climate, because they could have provided sufficient amounts of the greenhouse gas methane to mitigate a severely frozen condition that could have resulted from lower solar luminosity during these times. Nevertheless, no direct geological evidence has hitherto been available in support of the existence of methanogens in the Archaean period, although circumstantial evidence is available in the form of approximately 2.8-Gyr-old carbon-isotope-depleted kerogen. Here we report crushing extraction and carbon isotope analysis of methane-bearing fluid inclusions in approximately 3.5-Gyr-old hydrothermal precipitates from Pilbara craton, Australia. Our results indicate that the extracted fluids contain microbial methane with carbon isotopic compositions of less than -56 per thousand included within original precipitates. This provides the oldest evidence of methanogen (> 3.46 Gyr ago), pre-dating previous geochemical evidence by about 700 million years.

[1] Nitrous oxide (N 2 O) is an important atmospheric greenhouse gas and is involved in stratospheric ozone depletion. Analysis of the isotopomer ratios of N 2 O (i.e., the intramolecular distribution of 15 N within the linear NNO molecule and the conventional N and O isotope ratios) can elucidate the mechanisms of N 2 O production and destruction. We analyzed the isotopomer ratios of dissolved N 2 O at a site in the eastern tropical North Pacific (ETNP) and a site in the Gulf of California (GOC). At these sites, the flux of N 2 O to the atmosphere is extremely high but denitrification activity in the oxygen minimum zone (OMZ) also reduces N 2 O to N 2 . We estimated the isotopomeric enrichment factors for N 2 O reduction by denitrification. The factor was À11.6 ± 1.0% for the bulk (average) N, À19.8 ± 2.3% for the center N (a-site nitrogen), À3.4 ± 0.3% for the end N (b-site nitrogen), and À30.5 ± 3.2% for the 18 O of N 2 O. Isotopomer analysis of N 2 O suggests that nitrifiers should contribute to N 2 O production more than denitrifiers at the oxycline above the OMZs in the ETNP (50-80 m) and in the GOC (80-300 m). In contrast, denitrifiers should largely contribute to the N 2 O production and consumption in the OMZs both in the ETNP (120-130 m) and in the GOC (600-800 m). The N 2 O isotopomer analysis will be a useful tool for resolving the distribution of water masses that carry a signal of N loss by denitrification.

N2O is a powerful greenhouse gas contributing both to global warming and ozone depletion. While fungi have been identified as a putative source of N2O, little is known about their production of this greenhouse gas. Here we investigated the N2O-producing ability of a collection of 207 fungal isolates. Seventy strains producing N2O in pure culture were identified. They were mostly species from the order Hypocreales order—particularly Fusarium oxysporum and Trichoderma spp.—and to a lesser extent species from the orders Eurotiales, Sordariales, and Chaetosphaeriales. The N2O 15N site preference (SP) values of the fungal strains ranged from 15.8‰ to 36.7‰, and we observed a significant taxa effect, with Penicillium strains displaying lower SP values than the other fungal genera. Inoculation of 15 N2O-producing strains into pre-sterilized arable, forest and grassland soils confirmed the ability of the strains to produce N2O in soil with a significant strain-by-soil effect. The copper-containing nitrite reductase gene (nirK) was amplified from 45 N2O-producing strains, and its genetic variability showed a strong congruence with the ITS phylogeny, indicating vertical inheritance of this trait. Taken together, this comprehensive set of findings should enhance our knowledge of fungi as a source of N2O in the environment.