Fermentative Approaches to Hydrogen Production

A PowerPoint presentation given as part of the 2005 Hydrogen Program Review, May 23-26, 2005, in Washington, D.C

CO\u3csub\u3e2\u3c/sub\u3e-Fixing One-Carbon Metabolism in a Cellulose-Degrading Bacterium \u3cem\u3eClostridium thermocellum\u3c/em\u3e

Clostridium thermocellum can ferment cellulosic biomass to formate and other end products, including CO2. This organism lacks formate dehydrogenase (Fdh), which catalyzes the reduction of CO2 to formate. However, feeding the bacterium 13C-bicarbonate and cellobiose followed by NMR analysis showed the production of 13C-formate in C. thermocellum culture, indicating the presence of an uncharacterized pathway capable of converting CO2 to formate. Combining genomic and experimental data, we demonstrated that the conversion of CO2 to formate serves as a CO2 entry point into the reductive one-carbon (C1) metabolism, and internalizes CO2 via two biochemical reactions: the reversed pyruvate: ferredoxin oxidoreductase (rPFOR), which incorporates CO2 using acetyl-CoA as a substrate and generates pyruvate, and pyruvate- formate lyase (PFL) converting pyruvate to formate and acetyl-CoA. We analyzed the labeling patterns of proteinogenic amino acids in individual deletions of all five putative PFOR mutants and in a PFL deletion mutant. We identified two enzymes acting as rPFOR, confirmed the dual activities of rPFOR and PFL crucial for CO2 uptake, and provided physical evidence of a distinct in vivo “rPFOR-PFL shunt” to reduce CO2 to formate while circumventing the lack of Fdh. Such a pathway precedes CO2 fixation via the reductive C1 metabolic pathway in C. thermocellum. These findings demonstrated the metabolic versatility of C. thermocellum, which is thought of as primarily a cellulosic heterotroph but is shown here to be endowed with the ability to fix CO2 as well

Biological Systems for Hydrogen Photoproduction

Presentation on Biological Systems for Hydrogen Photoproduction for the 2005 Hydrogen, Fuel Cells & Infrastructure Technologies Program Annual Review held in Arlington, Virginia, May 23-26, 2005

The M Dwarf GJ 436 and its Neptune-Mass Planet

We determine stellar parameters for the M dwarf GJ 436 that hosts a Neptune-mass planet. We employ primarily spectral modeling at low and high resolution, examining the agreement between model and observed optical spectra of five comparison stars of type, M0-M3. Modeling high resolution optical spectra suffers from uncertainties in TiO transitions, affecting the predicted strengths of both atomic and molecular lines in M dwarfs. The determination of Teff, gravity, and metallicity from optical spectra remains at ~10%. As molecules provide opacity both in lines and as an effective continuum, determing molecular transition parameters remains a challenge facing models such as the PHOENIX series, best verified with high resolution and spectrophotometric spectra. Our analysis of GJ 436 yields an effective temperature of Teff = 3350 +/- 300 K and a mass of 0.44 Msun. New Doppler measurements for GJ 436 with a precision of 3 m/s taken during 6 years improve the Keplerian model of the planet, giving a minimum mass, M sin i = 0.0713 Mjup = 22.6 Mearth, period, P = 2.6439 d, and e = 0.16 +/- 0.02. The noncircular orbit contrasts with the tidally circularized orbits of all close-in exoplanets, implying either ongoing pumping of eccentricity by a more distant companion, or a higher Q value for this low-mass planet. The velocities indeed reveal a long term trend, indicating a possible distant companion.Comment: 27 pages, 7 figures, accepted to PAS

L1 Interaction with Ankyrin Regulates Mediolateral Topography in the Retinocollicular Projection

Dynamic modulation of adhesion provided by anchorage of axonal receptors with the cytoskeleton contributes to attractant or repellent responses that guide axons to topographic targets in the brain. The neural cell adhesion molecule L1 engages the spectrin-actin cytoskeleton through reversible linkage of its cytoplasmic domain to ankyrin. To investigate a role for L1 association with the cytoskeleton in topographic guidance of retinal axons to the superior colliculus, a novel mouse strain was generated by genetic knock-in that expresses an L1 point mutation (Tyr1229His) abolishing ankyrin binding. Axon tracing revealed a striking mistargeting of mutant ganglion cell axons from the ventral retina, which express high levels of ephrinB receptors, to abnormally lateral sites in the contralateral superior colliculus, where they formed multiple ectopic arborizations. These axons were compromised in extending interstitial branches in the medial direction, a normal response to the high medial to low lateral SC gradient of ephrinB1. Furthermore, ventral but not dorsal L1(Y1229H) retinal cells were impaired for ephrinB1-stimulated adhesion through beta1 integrins in culture. The retinocollicular phenotype of the L1(Tyr1229His) mutant provides the first evidence that L1 regulates topographic mapping of retinal axons through adhesion mediated by linkage to the actin cytoskeleton and functional interaction with the ephrinB/EphB targeting system

Evidence for a Long-Standing Top-Heavy IMF in the Central Parsec of the Galaxy

We classify 329 late-type giants within 1 parsec of Sgr A*, using the adaptive optics integral field spectrometer SINFONI on the VLT. These observations represent the deepest spectroscopic data set so far obtained for the Galactic Center, reaching a 50% completeness threshold at the approximate magnitude of the helium-burning red clump (Ks ~ 15.5 mag.). Combining our spectroscopic results with NaCo H and Ks photometry, we construct an observed Hertzsprung-Russell diagram, which we quantitatively compare to theoretical distributions of various star formation histories of the inner Galaxy, using a chi-squared analysis. Our best-fit model corresponds to continuous star formation over the last 12 Gyr with a top-heavy initial mass function (IMF). The similarity of this IMF to the IMF observed for the most recent epoch of star formation is intriguing and perhaps suggests a connection between recent star formation and the stars formed throughout the history of the Galactic Center.Comment: 18 pages, 10 figures, Accepted to ApJ: 15 July 200

The nuclear star cluster of the Milky Way

The nuclear star cluster of the Milky Way is a unique target in the Universe. Contrary to extragalactic nuclear star clusters, using current technology it can be resolved into tens of thousands of individual stars. This allows us to study in detail its spatial and velocity structure as well as the different stellar populations that make up the cluster. Moreover, the Milky Way is one of the very few cases where we have firm evidence for the co-existence of a nuclear star cluster with a central supermassive black hole, Sagittarius A*. The number density of stars in the Galactic center nuclear star cluster can be well described, at distances $\gtrsim1$ pc from Sagittarius A*, by a power-law of the form $\rho(r)\propto r^{-\gamma}$ with an index of $\gamma\approx1.8$. In the central parsec the index of the power-law becomes much flatter and decreases to $\gamma\approx1.2$. We present proper motions for more than 6000 stars within 1 pc in projection from the central black hole. The cluster appears isotropic at projected distances $\gtrsim0.5$ pc from Sagittarius A*. Outside of 0.5 pc and out to 1.0 pc the velocity dispersion appears to stay constant. A robust result of our Jeans modeling of the data is the required presence of $0.5-2.0\times10^{6} M_{\odot}$ of extended (stellar) mass in the central parsec of the Galaxy.Comment: To appear in the proceedings of "The Universe under the Microscope - Astrophysics at High Angular Resolution", Journal of Physics:Conference Series (IOP; http://www.iop.org/EJ/conf) This version has been slightly modified (e.g. double-log plot in right hand panel of Figure 5

Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b

The nearby extrasolar planet GJ 436b--which has been labelled as a 'hot Neptune'--reveals itself by the dimming of light as it crosses in front of and behind its parent star as seen from Earth. Respectively known as the primary transit and secondary eclipse, the former constrains the planet's radius and mass, and the latter constrains the planet's temperature and, with measurements at multiple wavelengths, its atmospheric composition. Previous work using transmission spectroscopy failed to detect the 1.4-\mu m water vapour band, leaving the planet's atmospheric composition poorly constrained. Here we report the detection of planetary thermal emission from the dayside of GJ 436b at multiple infrared wavelengths during the secondary eclipse. The best-fit compositional models contain a high CO abundance and a substantial methane (CH4) deficiency relative to thermochemical equilibrium models for the predicted hydrogen-dominated atmosphere. Moreover, we report the presence of some H2O and traces of CO2. Because CH4 is expected to be the dominant carbon-bearing species, disequilibrium processes such as vertical mixing and polymerization of methane into substances such as ethylene may be required to explain the hot Neptune's small CH4-to-CO ratio, which is at least 10^5 times smaller than predicted

Simultaneous Multi-Wavelength Observations of Sgr A* during 2007 April 1-11

We report the detection of variable emission from Sgr A* in almost all wavelength bands (i.e. centimeter, millimeter, submillimeter, near-IR and X-rays) during a multi-wavelength observing campaign. Three new moderate flares are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray to near-IR flux in the flares is consistent with inverse Compton scattering of near-IR photons by submillimeter emitting relativistic particles which follow scaling relations obtained from size measurements of Sgr A*. We also find that the flare statistics in near-IR wavelengths is consistent with the probability of flare emission being inversely proportional to the flux. At millimeter wavelengths, the presence of flare emission at 43 GHz (7mm) using VLBA with milli-arcsecond spatial resolution indicates the first direct evidence that hourly time scale flares are localized within the inner 30$\times$70 Schwarzschild radii of Sgr A*. We also show several cross correlation plots between near-IR, millimeter and submillimeter light curves that collectively demonstrate the presence of time delays between the peaks of emission up to three hours. The evidence for time delays at millimeter and submillimeter wavelengths are consistent with the source of emission being optically thick initially followed by a transition to an optically thin regime. In particular, there is an intriguing correlation between the optically thin near-IR and X-ray flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4. This would be the first evidence of a radio flare emission at 43 GHz delayed with respect to the near-IR and X-ray flare emission.Comment: replaced with revised version 57 pages, 28 figures, ApJ (in press

Dissecting the Moth: Discovery of an off-centered ring in the HD 61005 debris disk with high-resolution imaging

The debris disk known as "The Moth" is named after its unusually asymmetric surface brightness distribution. It is located around the ~90 Myr old G8V star HD 61005 at 34.5 pc and has previously been imaged by the HST at 1.1 and 0.6 microns. Polarimetric observations suggested that the circumstellar material consists of two distinct components, a nearly edge-on disk or ring, and a swept-back feature, the result of interaction with the interstellar medium. We resolve both components at unprecedented resolution with VLT/NACO H-band imaging. Using optimized angular differential imaging techniques to remove the light of the star, we reveal the disk component as a distinct narrow ring at inclination i=84.3 \pm 1.0{\deg}. We determine a semi-major axis of a=61.25 \pm 0.85 AU and an eccentricity of e=0.045 \pm 0.015, assuming that periastron is located along the apparent disk major axis. Therefore, the ring center is offset from the star by at least 2.75 \pm 0.85 AU. The offset, together with a relatively steep inner rim, could indicate a planetary companion that perturbs the remnant planetesimal belt. From our imaging data we set upper mass limits for companions that exclude any object above the deuterium-burning limit for separations down to 0.3". The ring shows a strong brightness asymmetry along both the major and minor axis. A brighter front side could indicate forward-scattering grains, while the brightness difference between the NE and SW components can be only partly explained by the ring center offset, suggesting additional density enhancements on one side of the ring. The swept-back component appears as two streamers originating near the NE and SW edges of the debris ring.Comment: 6 pages, 6 figures. Accepted to Astronomy and Astrophysics letter