Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
6,469
result(s) for
"Op "
Sort by:
Several ways one goal—methanogenesis from unconventional substrates
Methane is the second most important greenhouse gas on earth. It is produced by methanogenic archaea, which play an important role in the global carbon cycle. Three main methanogenesis pathways are known: in the hydrogenotrophic pathway H2 and carbon dioxide are used for methane production, whereas in the methylotrophic pathway small methylated carbon compounds like methanol and methylated amines are used. In the aceticlastic pathway, acetate is disproportionated to methane and carbon dioxide. However, next to these conventional substrates, further methanogenic substrates and pathways have been discovered. Several phylogenetically distinct methanogenic lineages (Methanosphaera, Methanimicrococcus, Methanomassiliicoccus, Methanonatronarchaeum) have evolved hydrogen-dependent methylotrophic methanogenesis without the ability to perform either hydrogenotrophic or methylotrophic methanogenesis. Genome analysis of the deep branching Methanonatronarchaeum revealed an interesting membrane-bound hydrogenase complex affiliated with the hardly described class 4 g of multisubunit hydrogenases possibly providing reducing equivalents for anabolism. Furthermore, methylated sulfur compounds such as methanethiol, dimethyl sulfide, and methylmercaptopropionate were described to be converted into adapted methylotrophic methanogenesis pathways of Methanosarcinales strains. Moreover, recently it has been shown that the methanogen Methermicoccus shengliensis can use methoxylated aromatic compounds in methanogenesis. Also, tertiary amines like choline (N,N,N-trimethylethanolamine) or betaine (N,N,N-trimethylglycine) have been described as substrates for methane production in Methanococcoides and Methanolobus strains. This review article will provide in-depth information on genome-guided metabolic reconstructions, physiology, and biochemistry of these unusual methanogenesis pathways.Key points• Newly discovered methanogenic substrates and pathways are reviewed for the first time.• The review provides an in-depth analysis of unusual methanogenesis pathways.• The hydrogenase complex of the deep branching Methanonatronarchaeum is analyzed.
Journal Article
Complete nitrification by a single microorganism
Until now, the oxidation steps necessary for complete nitrification had always been observed to occur in two separate microorganisms in a cross-feeding interaction; here, together with the study by Daims
et al
., van Kessel
et al
. report the enrichment and characterization of
Nitrospira
species that encode all of the enzymes necessary to catalyse complete nitrification, a phenotype referred to as ‘comammox’ (for complete ammonia oxidation).
Time to rethink nitrification
Two groups this week report the enrichment and characterization of
Nitrospira
species that encode all of the enzymes necessary to catalyse complete nitrification, a phenotype referred to as 'comammox' (for complete ammonia oxidation). Until now, this two-step reaction was thought to involve two organisms in a cross-feeding interaction. Phylogenetic analyses suggest that comammox
Nitrospira
are present in a number of diverse environments, so these findings have the potential to fundamentally change our view of the nitrogen cycle and open a new frontier in nitrification research.
Nitrification is a two-step process where ammonia is first oxidized to nitrite by ammonia-oxidizing bacteria and/or archaea, and subsequently to nitrate by nitrite-oxidizing bacteria. Already described by Winogradsky in 1890
1
, this division of labour between the two functional groups is a generally accepted characteristic of the biogeochemical nitrogen cycle
2
. Complete oxidation of ammonia to nitrate in one organism (complete ammonia oxidation; comammox) is energetically feasible, and it was postulated that this process could occur under conditions selecting for species with lower growth rates but higher growth yields than canonical ammonia-oxidizing microorganisms
3
. Still, organisms catalysing this process have not yet been discovered. Here we report the enrichment and initial characterization of two
Nitrospira
species that encode all the enzymes necessary for ammonia oxidation via nitrite to nitrate in their genomes, and indeed completely oxidize ammonium to nitrate to conserve energy. Their ammonia monooxygenase (AMO) enzymes are phylogenetically distinct from currently identified AMOs, rendering recent acquisition by horizontal gene transfer from known ammonia-oxidizing microorganisms unlikely. We also found highly similar
amoA
sequences (encoding the AMO subunit A) in public sequence databases, which were apparently misclassified as methane monooxygenases. This recognition of a novel
amoA
sequence group will lead to an improved understanding of the environmental abundance and distribution of ammonia-oxidizing microorganisms. Furthermore, the discovery of the long-sought-after comammox process will change our perception of the nitrogen cycle.
Journal Article
ميفي في المدرسة
تعد هذه القصة ميفي في المدرسة من قصص المنهج السلوكي التربوي الرائع والذي يعلم الطفل كيف يستخلص من مشكلاته وكيف يبني شخصيته بشكل مميز ويعطي المربي حلولا لحل مشكلات الأبناء وكيف تعلمه تجاوز الأزمة وإنهائها. وهي قصة مخصصة للأطفال تستهدف الطفولة المبكرة وتعمل على اسثمار الطفل في بناء المهارات المختلفة المرتبطة بالخيال والابتكار وقوة الشخصية والبحث عن حلول إبداعية ويستمد الطفل الكثير من العلم والمعرفة والمعلومات.
Book
Quantized Majorana conductance
Majorana zero-modes-a type of localized quasiparticle-hold great promise for topological quantum computing. Tunnelling spectroscopy in electrical transport is the primary tool for identifying the presence of Majorana zero-modes, for instance as a zero-bias peak in differential conductance. The height of the Majorana zero-bias peak is predicted to be quantized at the universal conductance value of 2e
/h at zero temperature (where e is the charge of an electron and h is the Planck constant), as a direct consequence of the famous Majorana symmetry in which a particle is its own antiparticle. The Majorana symmetry protects the quantization against disorder, interactions and variations in the tunnel coupling. Previous experiments, however, have mostly shown zero-bias peaks much smaller than 2e
/h, with a recent observation of a peak height close to 2e
/h. Here we report a quantized conductance plateau at 2e
/h in the zero-bias conductance measured in indium antimonide semiconductor nanowires covered with an aluminium superconducting shell. The height of our zero-bias peak remains constant despite changing parameters such as the magnetic field and tunnel coupling, indicating that it is a quantized conductance plateau. We distinguish this quantized Majorana peak from possible non-Majorana origins by investigating its robustness to electric and magnetic fields as well as its temperature dependence. The observation of a quantized conductance plateau strongly supports the existence of Majorana zero-modes in the system, consequently paving the way for future braiding experiments that could lead to topological quantum computing.
Journal Article
Molecular mechanism of anaerobic ammonium oxidation
Anammox pathway revealed
Anammox, or anaerobic ammonium oxidation, is one of two important microbial processes that recycle fixed nitrogen back to the atmosphere. Less well studied than the other process (denitrification), anammox converts ammonia and nitrite into dinitrogen (N
2
) gas. The complete biochemical and enzymatic mechanism of anammox has now been established. Nitric oxide and hydrazine are intermediate products in a pathway that involves three key enzymes — nitrite reductase, hydrazine synthase and hydrazine oxidoreductase.
Two distinct microbial processes, denitrification and anaerobic ammonium oxidation (anammox), are responsible for the release of fixed nitrogen as dinitrogen gas (N
2
) to the atmosphere
1
,
2
,
3
,
4
. Denitrification has been studied for over 100 years and its intermediates and enzymes are well known
5
. Even though anammox is a key biogeochemical process of equal importance, its molecular mechanism is unknown, but it was proposed to proceed through hydrazine (N
2
H
4
)
6
,
7
. Here we show that N
2
H
4
is produced from the anammox substrates ammonium and nitrite and that nitric oxide (NO) is the direct precursor of N
2
H
4
. We resolved the genes and proteins central to anammox metabolism and purified the key enzymes that catalyse N
2
H
4
synthesis and its oxidation to N
2
. These results present a new biochemical reaction forging an N–N bond and fill a lacuna in our understanding of the biochemical synthesis of the N
2
in the atmosphere. Furthermore, they reinforce the role of nitric oxide in the evolution of the nitrogen cycle.
Journal Article
Epitaxy of advanced nanowire quantum devices
Semiconductor nanowires are ideal for realizing various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasiparticles (such as anyons) can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought into contact with a superconductor. To exploit the potential of non-Abelian anyons-which are key elements of topological quantum computing-fully, they need to be exchanged in a well-controlled braiding operation. Essential hardware for braiding is a network of crystalline nanowires coupled to superconducting islands. Here we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks with a predefined number of superconducting islands. Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire 'hashtags' reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase-coherent system with strong spin-orbit coupling. In addition, a proximity-induced hard superconducting gap (with vanishing sub-gap conductance) is demonstrated in these hybrid superconductor-semiconductor nanowires, highlighting the successful materials development necessary for a first braiding experiment. Our approach opens up new avenues for the realization of epitaxial three-dimensional quantum architectures which have the potential to become key components of various quantum devices.
Journal Article