58 research outputs found
Hypersaline lakes harbor more active bacterial communities
ABSTRACT Extremophiles employ a diverse array of resistance strategies to thrive under harsh 18 environmental conditions but maintaining these adaptations comes at an energetic cost. If energy reserves to drop too low, extremophiles may enter a dormant state of reduced 20 metabolic activity to survive. Dormancy is frequently offered as a plausible explanation for the persistence of bacteria under suboptimal environmental conditions with the 22 prevalence of this mechanism only expected to rise as stressful conditions intensify. We estimated dormancy in ten hypersaline and freshwater lakes across the Western United 24 States. To our surprise, we found that extreme environmental conditions did not induce higher levels of bacterial dormancy. Based on our approach using rRNA:rDNA gene 26 ratios to estimate activity, halophilic and halotolerant bacteria were classified as inactive at a similar percentage as freshwater bacteria, and the proportion of the community 28 exhibiting dormancy was considerably lower (16%) in hypersaline than freshwater lakes across a range of cutoffs estimating activity. Of the multiple chemical characteristics we 30 evaluated, salinity and, to a lesser extent, total phosphorus concentrations influenced activity. But instead of dormancy being more common as stressful conditions intensified, 32 the percentage of the community residing in an inactive state decreased with increasing salinity in freshwater and hypersaline lakes, suggesting that salinity acts as a strong 34 environmental filter selecting for bacteria that persist and thrive under saltier conditions. Within the compositionally distinct and less diverse hypersaline communities, abundant 36 taxa were disproportionately active and localized in families Microbacteriacea
Whole Genome Pyrosequencing of Rare Hepatitis C Virus Genotypes Enhances Subtype Classification and Identification of Naturally Occurring Drug Resistance Variants
Background.âInfection with hepatitis C virus (HCV) is a burgeoning worldwide public health problem, with 170 million infected individuals and an estimated 20 million deaths in the coming decades. While 6 main genotypes generally distinguish the global geographic diversity of HCV, a multitude of closely related subtypes within these genotypes are poorly defined and may influence clinical outcome and treatment options. Unfortunately, the paucity of genetic data from many of these subtypes makes time-consuming primer walking the limiting step for sequencing understudied subtypes. Methods.âHere we combined long-range polymerase chain reaction amplification with pyrosequencing for a rapid approach to generate the complete viral coding region of 31 samples representing poorly defined HCV subtypes. Results.âPhylogenetic classification based on full genome sequences validated previously identified HCV subtypes, identified a recombinant sequence, and identified a new distinct subtype of genotype 4. Unlike conventional sequencing methods, use of deep sequencing also facilitated characterization of minor drug resistance variants within these uncommon or, in some cases, previously uncharacterized HCV subtypes. Conclusions.âThese data aid in the classification of uncommon HCV subtypes while also providing a high-resolution view of viral diversity within infected patients, which may be relevant to the development of therapeutic regimens to minimize drug resistanc
Spatially-Resolved O II Recombination Line Observations of the Ring Nebula, NGC 6720
We present spatially-resolved spectral of O II permitted lines and [O III]
forbidden lines in the Ring Nebula NGC 6720. We find significant differences in
the spatial distribution of the O II and [O III] lines. The [O III] emission
follows the H-beta emission measure; however, the O II emission peaks closer to
the central star. This suggests that radiative recombination may not be the
primary mechanism for producing the O II lines. O+2 abundances derived from O
II lines are 5-10 times larger than those derived from [O III] in the region
within 20" of the central star, but agree to within 0.2-0.3 dex outside this
region. The [O III] electron temperature rises smoothly from about 10,000 K in
the outer shell to about 12,000 K in the center; we see no evidence for a
temperature jump that would be associated with a shock. If temperature
fluctuations are responsible for the discrepancy in O+2 abundances, the average
temperature would have to be approximately 6,500 K in the He zone and
about 9,000 K in the outer shell in order to force the [O III]-derived
abundance to equal that derived from O II. We therefore argue that temperature
fluctuations can not explain the abundance discrepancy. The O II emission does
not peak at the locations of dusty knots, creating difficulties for models
which explain the O II - [O III] discrepancy by density fluctuations. We
examine the possibility high-temperature dielectronic recombination in a
central hot bubble enhances the O II line strengths in the central nebula.
However, comparison of recombination rates with collisional excitation rates
shows that the increase in recombination emission due to dielectronic
recombination at T ~ 10^5 K is not sufficient to overcome the increase in [O
III] emission. (Abridged)Comment: 33 pages, 11 postscript figures. Scheduled to appear in the 1 Sept
2001 Astrophysical Journa
A FUSE Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds
We describe a moderate-resolution FUSE survey of H2 along 70 sight lines to
the Small and Large Magellanic Clouds, using hot stars as background sources.
FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of
SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912
and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest column
densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced
H2 abundances in the Magellanic Clouds relative to the Milky Way, with average
molecular fractions = 0.010 (+0.005, -0.002) for the SMC and =
0.012 (+0.006, -0.003) for the LMC, compared with = 0.095 for the
Galactic disk over a similar range of reddening. The dominant uncertainty in
this measurement results from the systematic differences between 21 cm radio
emission and Lya in pencil-beam sight lines as measures of N(HI). These results
imply that the diffuse H2 masses of the LMC and SMC are 8 x 10^6 Msun and 2 x
10^6 Msun, respectively, 2% and 0.5% of the H I masses derived from 21 cm
emission measurements. The LMC and SMC abundance patterns can be reproduced in
ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x
10^-18 cm^3 s^-1, and incident radiation fields ranging from 10 - 100 times the
Galactic mean value. We find that these high-radiation, low-formation-rate
models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational
excitation ratios in the Clouds. We use H2 column densities in low rotational
states (J = 0 and 1) to derive a mean kinetic and/or rotational temperature
= 82 +/- 21 K for clouds with N(H2) >= 10^16 cm^-2, similar to Galactic
gas. We discuss the implications of this work for theories of star formation in
low-metallicity environments. [Abstract abridged]Comment: 30 pages emulateapj, 14 figures (7 color), 7 tables, accepted for
publication in the Astrophysical Journal, figures 11 and 12 compressed at
slight loss of quality, see http://casa.colorado.edu/~tumlinso/h2/ for full
version
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