Ian D. Hutcheon

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

Magnesium isotope heterogeneity of the isotopic standard SRM980 and new reference materials for magnesium-isotope-ratio measurements

Multicollector ICP-MS has been used for the precise measurement of variations in the isotopic composition of the isotopic standard of magnesium (SRM980) provided by the National Institute of Standards and Technology (Gaithersburg, MD, USA). The SRM980 consists of metal chips weighing between 1 and 50 mg and each unit delivered by the National Institute of Standards and Technology corresponds to a bottle containing about 0.3 g. Height units were analysed. Variations in sample 25Mg/24Mg, and 26Mg/24Mg ratios are expressed as δ25Mg and δ26Mg units, respectively, which are deviations in parts per 103 from the same ratio in a standard solution. The differences in δ25Mg and δ26Mg of the SRM980 are up to 4.20 and 8.19‰, respectively, while the long-term repeatability of δ25Mg and δ26Mg are 0.09 and 0.16‰, respectively, at 95% confidence. However, when plotted in a three-isotope diagram, all the data fall on a single mass fractionation line. Overall limits of error of the SRM980 reported here fall within the previously reported overall limits of error. The isotopic heterogeneity not only corresponds to differences among units but has been found at the chip-size level. This result, due to the precision of the MC-ICP-MS technique, makes the SRM980 inappropriate for the international isotopic standard of magnesium. The SRM980 can still be used to report the excess of 26Mg, which is defined by the deviation from the mass-dependent relationship between 25Mg/24Mg, and 26Mg/24Mg ratios. Two large batches (around 10 g of Mg in each) of pure Mg solutions (in 0.3 M HNO3) have been prepared and characterised. These 2 solutions (DSM3 and Cambridge 1) are suitable reference material because they are immune to heterogeneity. DSM3 and Cambridge 1 are isotopically different (by 1.3‰ per u) and are available upon request from the first author. In addition, DSM3 has an isotopic composition very similar to the Mg-isotopic composition of carbonaceous chondrites (Orgueil and Allende). Because of the lack of heterogeneity and the cosmochemical and geochemical significance of DSM3, we urge the use of DSM3 as the primary isotopic reference material to report Mg-isotopic variations.

Phase Separation of Lipid Membranes Analyzed with High-Resolution Secondary Ion Mass Spectrometry

Lateral variations in membrane composition are postulated to play a central role in many cellular events, but it has been difficult to probe membrane composition and organization on length scales of tens to hundreds of nanometers. We present a high-resolution imaging secondary ion mass spectrometry technique to reveal the lipid distribution within a phase-separated membrane with a lateral resolution of ∼100 nanometers. Quantitative information about the chemical composition within small lipid domains was obtained with the use of isotopic labels to identify each molecular species. Composition variations were detected within some domains.

Extracellular Proteins Limit the Dispersal of Biogenic Nanoparticles

High–spatial-resolution secondary ion microprobe spectrometry, synchrotron radiation–based Fourier-transform infrared spectroscopy, and polyacrylamide gel analysis demonstrated the intimate association of proteins with spheroidal aggregates of biogenic zinc sulfide nanocrystals, an example of extracellular biomineralization. Experiments involving synthetic zinc sulfide nanoparticles and representative amino acids indicated a driving role for cysteine in rapid nanoparticle aggregation. These findings suggest that microbially derived extracellular proteins can limit the dispersal of nanoparticulate metal-bearing phases, such as the mineral products of bioremediation, that may otherwise be transported away from their source by subsurface fluid flow.

Linking microbial phylogeny to metabolic activity at the single-cell level by using enhanced element labeling-catalyzed reporter deposition fluorescence in situ hybridization (EL-FISH) and NanoSIMS.

ABSTRACT To examine phylogenetic identity and metabolic activity of individual cells in complex microbial communities, we developed a method which combines rRNA-based in situ hybridization with stable isotope imaging based on nanometer-scale secondary-ion mass spectrometry (NanoSIMS). Fluorine or bromine atoms were introduced into cells via 16S rRNA-targeted probes, which enabled phylogenetic identification of individual cells by NanoSIMS imaging. To overcome the natural fluorine and bromine backgrounds, we modified the current catalyzed reporter deposition fluorescence in situ hybridization (FISH) technique by using halogen-containing fluorescently labeled tyramides as substrates for the enzymatic tyramide deposition. Thereby, we obtained an enhanced element labeling of microbial cells by FISH (EL-FISH). The relative cellular abundance of fluorine or bromine after EL-FISH exceeded natural background concentrations by up to 180-fold and allowed us to distinguish target from non-target cells in NanoSIMS fluorine or bromine images. The method was optimized on single cells of axenic Escherichia coli and Vibrio cholerae cultures. EL-FISH/NanoSIMS was then applied to study interrelationships in a dual-species consortium consisting of a filamentous cyanobacterium and a heterotrophic alphaproteobacterium. We also evaluated the method on complex microbial aggregates obtained from human oral biofilms. In both samples, we found evidence for metabolic interactions by visualizing the fate of substrates labeled with 13C-carbon and 15N-nitrogen, while individual cells were identified simultaneously by halogen labeling via EL-FISH. Our novel approach will facilitate further studies of the ecophysiology of known and uncultured microorganisms in complex environments and communities.

A petrologic, chemical, and isotopic study of Monument Draw and comparison with other acapulcoites: Evidence for formation by incipient partial melting

Abstract We have conducted petrologic, chemical, and isotopic studies of acapulcoites (Acapulco, Monument Draw, Yamato 74063, ALH A77081, ALH A81261, ALH A81315, ALH 78230, ALH A81187 and ALH 84190) in an attempt to constrain their genesis. Acapulcoites have distinctly different oxygen isotopic compositions than silicate inclusions in IAB and IIICD irons, winonaites and ureilites and, thus, formed on a different parent body. Oxygen isotopic compositions, which are slightly heterogeneous within the group, overlap with lodranites, indicating a likely origin on a common parent body. These groups can be distinguished on the basis of mafic silicate grain size. All acapulcoites have mafic silicate compositions intermediate between E and H chondrites, roughly chondritic mineralogies, achondritic, equigranular textures, micrometer to centimeter sized veins of Fe,NiFeS which cross-cut silicate phases, rapid metallographic cooling rates at ∼600−400°C (103–105°C/Myr) and trapped noble gas abundances comparable to type 3–4 ordinary chondrites. They exhibit variable mafic silicate zoning, abundance of Fe,NiFeS veins, REE abundances and patterns and, possibly, cosmic ray exposure ages (∼5–7 Ma). Momument Draw and Yamato 74063 retain rare relict chondrules. Phosphates are associated with Fe,NiFeS veins or form separate veins in Monument Draw and Acapulco. Heating and cooling of acapulcoites occurred very early in the history of the Solar System, as evidenced by the 39Ar40Ar ages of ∼4.51 Ga. These ages appear distinctly younger than the likely formation time for Acapulco of 4.557 Ga, but are older than analogous 39Ar40Ar ages for most chondrites. Acapulcoites formed from a precursor chondrite which differs from known chondrites in mineral and oxygen isotopic compositions. Heating to ∼950–1000°C resulted in melting at the Fe,NiFeS cotectic, but silicates did not melt. Silicate textures resulted from extensive solid-state recrystallization. Heating was by noncollisional sources (e.g., 26Al, electromagnetic induction). Despite uncertainties owing to a lack of data, acapulcoites may have experienced a three-stage thermal history of slow cooling at high temperature, rapid cooling at intermediate temperatures, and slow cooling at low temperatures, possibly resulting from breakup and gravitational reassembly of the parent body. The complex thermal history is also reflected in disequilibrium REE abundances. One or at most two impact events (∼7 Ma and possibly ∼5 Ma ago) are consistent with the cosmic ray exposure ages of all four acapulcoites for which cosmogenic noble gas data exist.

Fixation and fate of C and N in the cyanobacterium Trichodesmium using nanometer-scale secondary ion mass spectrometry

The marine cyanobacterium Trichodesmium is ubiquitous in tropical and subtropical seas and is an important contributor to global N and C cycling. We sought to characterize metabolic uptake patterns in individual Trichodesmium IMS-101 cells by quantitatively imaging 13C and 15N uptake with high-resolution secondary ion mass spectrometry (NanoSIMS). Trichodesmium fix both CO2 and N2 concurrently during the day and are, thus, faced with a balancing act: the O2 evolved during photosynthesis inhibits nitrogenase, the key enzyme in N2 fixation. After performing correlated transmission electron microscopy (TEM) and NanoSIMS analysis on trichome thin-sections, we observed transient inclusion of 15N and 13C into discrete subcellular bodies identified as cyanophycin granules. We speculate that Trichodesmium uses these dynamic storage bodies to uncouple CO2 and N2 fixation from overall growth dynamics. We also directly quantified both CO2 and N2 fixation at the single cell level using NanoSIMS imaging of whole cells in multiple trichomes. Our results indicate maximal CO2 fixation rates in the morning, compared with maximal N2 fixation rates in the afternoon, bolstering the argument that segregation of CO2 and N2 fixation in Trichodesmium is regulated in part by temporal factors. Spatial separation of N2 and CO2 fixation may also have a role in metabolic segregation in Trichodesmium. Our approach in combining stable isotope labeling with NanoSIMS and TEM imaging can be extended to other physiologically relevant elements and processes in other important microbial systems.

Assimilation of seawater-derived components in an oceanic volcano: evidence from matrix glasses and glass inclusions from Loihi seamount, Hawaii

We report major element, H_(2)O, Cl, B, and Be analyses of matrix glass and olivine-hosted glass inclusions from two pillow lava samples dredged from 4200 m on the southern rift zone of Loihi seamount, Hawaii. Matrix glasses (MgO∼9 wt.%) have H_(2)O, Cl, and B contents considerably in excess of the values expected from mantle melting or fractional crystallization of parental Loihi magmas. Glass inclusions have H_(2)O, Cl, and B contents ranging from the high values of the matrix glasses to lower concentrations that are more typical of Hawaiian magmas. Concentrations of other incompatible elements (e.g., K_(2)O, P_(2)O_(5), and Be) in matrix glasses and glass inclusions are uncorrelated with their H_(2)O, Cl, and B contents. Glass inclusions show considerable scatter in major element compositions compared to matrix glasses, but except for H_(2)O, Cl, and B, the average glass inclusion composition corresponds well to the matrix glass compositions. We propose that the glass inclusions represent compositionally diverse liquids present within the magmatic plumbing system at Loihi that were mixed and homogenized to produce the liquid that quenched to the matrix glass on eruption. This range of liquid compositions present at depth was trapped by crystallizing olivine prior to blending and homogenizing and therefore preserves a compositional diversity not present in erupted whole rocks. The high H_(2)O, Cl, and B contents of matrix glasses and some glass inclusions, and the range of H_(2)O, Cl, and B concentrations in glass inclusions, are best explained by variable extents of assimilation by Loihi magmas of H2O–Cl–B-rich, seawater-derived components prior to eruption. The required assimilants range from material similar in composition to seawater to materials with Cl/H_(2)O and B/H_(2)O ratios much higher than seawater. Our preferred explanation (similar to that suggested for MORB by Michael and Schilling, 1989) [Michael, P.J., Schilling, J.-G., 1989. Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components. Geochim. Cosmochim. Acta, 53, pp. 3131–3143.] is that most of the assimilated materials were brines (or rocks containing brines in inclusions or along grain boundaries) enriched in Cl by high temperature phase separation of seawater in sub-sea-floor hydrothermal circulation systems. Addition of ∼1.0 wt.% of a 15 wt.% NaCl brine can explain the H_(2)O and Cl contents of the matrix glasses. Addition of altered basalt cannot readily account for the Cl and H_(2)O contents of matrix glasses and glass inclusions, but may be required to account for their elevated B contents. The enrichment in Cl and contamination with atmospheric noble gases observed in other samples from Loihi could also result from assimilation of Cl-enriched, seawater-derived components.

Direct chemical evidence for sphingolipid domains in the plasma membranes of fibroblasts

Sphingolipids play important roles in plasma membrane structure and cell signaling. However, their lateral distribution in the plasma membrane is poorly understood. Here we quantitatively analyzed the sphingolipid organization on the entire dorsal surface of intact cells by mapping the distribution of 15N-enriched ions from metabolically labeled 15N-sphingolipids in the plasma membrane, using high-resolution imaging mass spectrometry. Many types of control experiments (internal, positive, negative, and fixation temperature), along with parallel experiments involving the imaging of fluorescent sphingolipids—both in living cells and during fixation of living cells—exclude potential artifacts. Micrometer-scale sphingolipid patches consisting of numerous 15N-sphingolipid microdomains with mean diameters of ∼200 nm are always present in the plasma membrane. Depletion of 30% of the cellular cholesterol did not eliminate the sphingolipid domains, but did reduce their abundance and long-range organization in the plasma membrane. In contrast, disruption of the cytoskeleton eliminated the sphingolipid domains. These results indicate that these sphingolipid assemblages are not lipid rafts and are instead a distinctly different type of sphingolipid-enriched plasma membrane domain that depends upon cortical actin.

Widespread assimilation of a seawater-derived component at Loihi Seamount, Hawaii

Many tholeiitic and transitional pillow-rim and fragmental glasses from Loihi seamount, Hawaii, have high Cl contents and Cl/K_2O ratios (and ratios of Cl to other incompatible components, such as P_2O_5, H_2O, etc.) relative to other Hawaiian subaerial volcanoes (e.g., Mauna Loa, Mauna Kea, and Kilauea). We suggest that this results from widespread contamination of Loihi magmas by a Cl-rich, seawater-derived component. Assimilation of high-Cl phases such as saline brine or Cl-rich minerals (halite or iron–hydroxychlorides) with high Cl/H_2O ratios can explain the range and magnitude of Cl contents in Loihi glasses, as well as the variations in the ratios of Cl to other incompatible elements. Brines and Cl-rich minerals are thought to form from seawater within the hydrothermal systems associated with submarine volcanoes, and Loihi magmas could plausibly have assimilated such materials from the hydrothermal envelope adjacent to the magma chamber. Our model can also explain semiquantitatively the observed contamination of Loihi glasses with atmospheric-derived noble gases, provided the assimilant has concentrations of Ne and Ar comparable to or slightly less than seawater. This is more likely for brines than for Cl-rich minerals, leading us to favor brines as the major assimilant. Cl/Br ratios for a limited number of Loihi samples are also seawater-like, and show no indication of the higher values expected to be associated with the assimilation of Cl-rich hydrothermal minerals. Although Cl enrichment is a common feature of lavas from Loihi, submarine glasses from other Hawaiian volcanoes show little (Kilauea) or no (Mauna Loa, Mauna Kea) evidence of this process, suggesting that assimilation of seawater-derived components is more likely to occur in the early stages of growth of oceanic volcanoes. Summit collapse events such as the one observed at Loihi in October 1996 provide a ready mechanism for depositing brine-bearing rocks from the volcanic edifice into the top of a submarine summit magma chamber.