Energy transfer in the inhomogeneously broadened core antenna of purple bacteria: a simultaneous fit of low-intensity picosecond absorption and fluorescence kinetics.

The excited state decay kinetics of chromatophores of the purple photosynthetic bacterium Rhodospirillum rubrum have been recorded at 77 K using picosecond absorption difference spectroscopy under strict annihilation free conditions. The kinetics are shown to be strongly detection wavelength dependent. A simultaneous kinetic modeling of these experiments together with earlier fluorescence kinetics by numerical integration of the appropriate master equation is performed. This model, which accounts for the spectral inhomogeneity of the core light-harvesting antenna of photosynthetic purple bacteria, reveals three qualitatively distinct stages of excitation transfer with different time scales. At first a fast transfer to a local energy minimum takes place (approximately 1 ps). This is followed by a much slower transfer between different energy minima (10-30 ps). The third component corresponds to the excitation transfer to the reaction center, which depends on its state (60 and 200 ps for open and closed, respectively) and seems also to be the bottleneck in the overall trapping time. An acceptable correspondence between theoretical and experimental decay kinetics is achieved at 77 K and at room temperature by assuming that the width of the inhomogeneous broadening is 10-15 nm and the mean residence time of the excitation in the antenna lattice site is 2-3 ps.     

Isolation, Growth, and Metabolism of an Obligately Anaerobic, Selenate-Respiring Bacterium, Strain SES-3

A gram-negative, strictly anaerobic, motile vibrio was isolated from a selenate-respiring enrichment culture. The isolate, designated strain SES-3, grew by coupling the oxidation of lactate to acetate plus CO₂ with the concomitant reduction of selenate to selenite or of nitrate to ammonium. No growth was observed on sulfate or selenite, but cell suspensions readily reduced selenite to elemental selenium (Se⁰). Hence, SES-3 can carry out a complete reduction of selenate to Se⁰. Washed cell suspensions of selenate-grown cells did not reduce nitrate, and nitrate-grown cells did not reduce selenate, indicating that these reductions are achieved by separate inducible enzyme systems. However, both nitrate-grown and selenate-grown cells have a constitutive ability to reduce selenite or nitrite. The oxidation of [¹⁴C]lactate to ¹⁴CO₂ coupled to the reduction of selenate or nitrate by cell suspensions was inhibited by CCCP (carbonyl cyanide m-chlorophenylhydrazone), cyanide, and azide. High concentrations of selenite (5 mM) were readily reduced to Se⁰ by selenate-grown cells, but selenite appeared to block the synthesis of pyruvate dehydrogenase. Tracer experiments with [⁷⁵Se]selenite indicated that cell suspensions could achieve a rapid and quantitative reduction of selenite to Se⁰. This reduction was totally inhibited by sulfite, partially inhibited by selenate or nitrite, but unaffected by sulfate or nitrate. Cell suspensions could reduce thiosulfate, but not sulfite, to sulfide. These results suggest that reduction of selenite to Se⁰ may proceed, in part, by some of the components of a dissimilatory system for sulfur oxyanions.     

Mathematical simulation of the diel O, S, and C biogeochemistry of a hypersaline microbial mat

The creation of a mathematical simulation model of photosynthetic microbial mats is important to our understanding of key biogeochemical cycles that may have altered the atmospheres and lithospheres of early Earth. A model is presented here as a tool to integrate empirical results from research on hypersaline mats from Baja California Sur (BCS), Mexico into a computational system that can be used to simulate biospheric inputs of trace gases to the atmosphere. The first version of our model, presented here, calculates fluxes and cycling of O(2), sulfide, and dissolved inorganic carbon (DIC) via abiotic components and via four major microbial guilds: cyanobacteria (CYA), sulfate reducing bacteria (SRB), purple sulfur bacteria (PSB) and colorless sulfur bacteria (CSB). We used generalized Monod-type equations that incorporate substrate and energy limits upon maximum rates of metabolic processes such as photosynthesis and sulfate reduction. We ran a simulation using temperature and irradiance inputs from data collected from a microbial mat in Guerrero Negro in BCS (Mexico). Model O(2), sulfide, and DIC concentration profiles and fluxes compared well with data collected in the field mats. There were some model-predicted features of biogeochemical cycling not observed in our actual measurements. For instance, large influxes and effluxes of DIC across the MBGC mat boundary may reveal previously unrecognized, but real, in situ limits on rates of biogeochemical processes. Some of the short-term variation in field-collected mat O(2) was not predicted by MBGC. This suggests a need both for more model sensitivity to small environmental fluctuations for the incorporation of a photorespiration function into the model.     

Linkage disequilibrium in the domesticated pig

This study investigated the extent of linkage disequilibrium (LD) in two genomic regions (on chromosomes 4 and 7) in five populations of domesticated pigs. LD was measured with D' and tested for significance with the Fisher exact test. Effects of genetic (linkage) distance, chromosome, population, and their interactions on D' were tested both through a linear model analysis of covariance and by a theoretical nonlinear model. The overall result was that (1) the distance explained most of the variability of D', (2) the effect of chromosome was significant, and (3) the effect of population was significant. The significance of the chromosome effect may have resulted from selection and the significance of the population effect illustrates the effects of population structures and effective population sizes on LD. These results suggest that mapping methods based on LD may be valuable even with only moderately dense marker spacing in pigs.     

Changes in stomatal frequency and size during elongation of Tsuga heterophylla needles

BACKGROUND AND AIMS: The inverse relationship between the number of stomata and atmospheric CO2 levels observed in different plant species is increasingly used for reconstructions of past CO2 concentrations. To validate this relationship, the potential influence of other environmental conditions and ontogenetical development stage on stomatal densities must be investigated as well. Quantitative data on the changes in stomatal density of conifers in relation to leaf development is reported. METHODS: Stomatal frequency and epidermal cells of Tsuga heterophylla needles during different stages of budburst were measured using computerized image analysis systems on light microscope slides. KEY RESULTS: Stomata first appear in the apical region and subsequently spread basipetally towards the needle base during development. The number of stomatal rows on a needle does not change during ontogeny, but stomatal density decreases nonlinearly with increasing needle area, until about 50 % of the final needle area. The total number of stomata on the needle increases during the entire developmental period, indicating that stomatal and epidermal cell formation continues until the needle has matured completely. CONCLUSIONS: Epidermal characteristics in developing conifer needles appear to be fundamentally different from angiosperm dicot leaves, where in general leaf expansion in the final stages is due to cell expansion rather than cell formation. The lack of further change in either stomatal density or stomatal density per millimetre needle length (the stomatal characteristic most sensitive to CO2 in conifers) in the final stages of leaf growth indicates that in conifers the stage of leaf maturation would not influence CO2 reconstructions based on stomatal density.     

Vibration Signal Modulates the Behavior of House-hunting Honey Bees (Apis mellifera)

Honey bees ( Apis mellifera ) in house-hunting swarms perform vibration signals (dorsoventral abdominal vibration (DVAV)) of 18.05 ± 0.45 Hz for 1.36 ± 0.23 s throughout the house selection process. These signals are performed by a specialized subset of bees, most of whom never perform recruitment dances to nest sites. Individuals repeatedly vibrate others. The patterns of vibration signal performance are consistent with the hypothesis that it serves to activate bees for take-off, but may also activate bees to scout for nest sites.     

Polypeptide-assisted oligomerization of analogs in dilute aqueous solution

Poly (Leu-Lys) was shown to assist the oligomerization of activated nucleotide diphosphates. Short oligomers of pdGp are formed in dilute solution. Activated oligomers can complex to the polypeptide and polymerize to form longer oligomers. Oligomers up to the 18-mer can be obtained under these conditions from 1 × 10^–3 M ImpdGpIm in the absence of a preformed polynucleotide template. The total yield of polymerization remains limited although 50% more pyrophosphate bonds are formed in the presence of polypeptide. However, the elongation effect is more significant since the yield of oligomers longer than the decamer is increased by a factor of 60. The possible prebiotic implications of these experiments are discussed.Abbreviations pdGp the 3,5-bisphosphate of dG - ImpdGpIm the 3,5-bisphosphoimidazolide of dG - KEDTA ethylenediamine tetraacetic acid, potassium salt - Bis-Tris bis(2-hydroxymethyl)iminotris(hydroxymethyl)methane Correspondence to: B. Barbier     

Methylated sulfur compounds in microbial mats: in situ concentrations and metabolism by a colorless sulfur bacterium

The concentrations of the volatile organic sulfur compounds methanethiol, dimethyl disulfide, and dimethyl sulfide (DMS) and the viable population capable of DMS utilization in laminated microbial ecosystems were evaluated. Significant levels of DMS and dimethyl disulfide (maximum concentrations of 220 and 24 nmol cm3 of sediment-1, respectively) could be detected only at the top 20 mm of the microbial mat, whereas methanethiol was found only at depth horizons from 20 to 50 mm (maximum concentration of 42 nmol cm3 of sediment-1). DMS concentrations in the surface layer doubled after cold hydrolysis of its precursor, dimethylsulfoniopropionate. Most-probable-number counts revealed 2.2 x 10(5) cells cm3 of sediment-1, in the 0- to 5-mm depth horizon, capable of growth on DMS as the sole source of energy. An obligately chemolithoautotrophic bacillus designated strain T5 was isolated from the top layer of the marine sediment. Continuous culture studies in which DMS was the growth-limiting substrate revealed a maximum specific growth rate of 0.10 h-1 and a saturation constant of 90 mumol liter-1 for aerobic growth on this substrate.