165 research outputs found
Endosymbiosis before eukaryotes: mitochondrial establishment in protoeukaryotes
Endosymbiosis and organellogenesis are virtually unknown among prokaryotes. The single presumed example is the endosymbiogenetic origin of mitochondria, which is hidden behind the event horizon of the last eukaryotic common ancestor. While eukaryotes are monophyletic, it is unlikely that during billions of years, there were no other prokaryote–prokaryote endosymbioses as symbiosis is extremely common among prokaryotes, e.g., in biofilms. Therefore, it is even more precarious to draw conclusions about potentially existing (or once existing) prokaryotic endosymbioses based on a single example. It is yet unknown if the bacterial endosymbiont was captured by a prokaryote or by a (proto-)eukaryote, and if the process of internalization was parasitic infection, slow engulfment, or phagocytosis. In this review, we accordingly explore multiple mechanisms and processes that could drive the evolution of unicellular microbial symbioses with a special attention to prokaryote–prokaryote interactions and to the mitochondrion, possibly the single prokaryotic endosymbiosis that turned out to be a major evolutionary transition. We investigate the ecology and evolutionary stability of inter-species microbial interactions based on dependence, physical proximity, cost–benefit budget, and the types of benefits, investments, and controls. We identify challenges that had to be conquered for the mitochondrial host to establish a stable eukaryotic lineage. Any assumption about the initial interaction of the mitochondrial ancestor and its contemporary host based solely on their modern relationship is rather perilous. As a result, we warn against assuming an initial mutually beneficial interaction based on modern mitochondria–host cooperation. This assumption is twice fallacious: (i) endosymbioses are known to evolve from exploitative interactions and (ii) cooperativity does not necessarily lead to stable mutualism. We point out that the lack of evidence so far on the evolution of endosymbiosis from mutual syntrophy supports the idea that mitochondria emerged from an exploitative (parasitic or phagotrophic) interaction rather than from syntrophy
The Evolution of Microbial Facilitation: Sociogenesis, Symbiogenesis, and Transition in Individuality
Metabolic cooperation is widespread, and it seems to be a ubiquitous and easily evolvable interaction in the microbial domain. Mutual metabolic cooperation, like syntrophy, is thought to have a crucial role in stabilizing interactions and communities, for example biofilms. Furthermore, cooperation is expected to feed back positively to the community under higher-level selection. In certain cases, cooperation can lead to a transition in individuality, when freely reproducing, unrelated entities (genes, microbes, etc.) irreversibly integrate to form a new evolutionary unit. The textbook example is endosymbiosis, prevalent among eukaryotes but virtually lacking among prokaryotes. Concerning the ubiquity of syntrophic microbial communities, it is intriguing why evolution has not lead to more transitions in individuality in the microbial domain. We set out to distinguish syntrophy-specific aspects of major transitions, to investigate why a transition in individuality within a syntrophic pair or community is so rare. We review the field of metabolic communities to identify potential evolutionary trajectories that may lead to a transition. Community properties, like joint metabolic capacity, functional profile, guild composition, assembly and interaction patterns are important concepts that may not only persist stably but according to thought-provoking theories, may provide the heritable information at a higher level of selection. We explore these ideas, relating to concepts of multilevel selection and of informational replication, to assess their relevance in the debate whether microbial communities may inherit community-level information or not
Topological Excitations of One-Dimensional Correlated Electron Systems
Properties of low-energy excitations in one-dimensional superconductors and
density-wave systems are examined by the bosonization technique. In addition to
the usual spin and charge quantum numbers, a new, independently measurable
attribute is introduced to describe elementary, low-energy excitations. It can
be defined as a number w which determines, in multiple of , how many times
the phase of the order parameter winds as an excitation is transposed from far
left to far right. The winding number is zero for electrons and holes with
conventional quantum numbers, but it acquires a nontrivial value w=1 for
neutral spin-1/2 excitations and for spinless excitations with a unit electron
charge. It may even be irrational, if the charge is irrational. Thus, these
excitations are topological, and they can be viewed as composite particles made
of spin or charge degrees of freedom and dressed by kinks in the order
parameter.Comment: 5 pages. And we are not only splitting point
Transitions from small to large Fermi momenta in a one-dimensional Kondo lattice model
We study a one-dimensional system that consists of an electron gas coupled to
a spin-1/2 chain by Kondo interaction away from half-filling. We show that
zero-temperature transitions between phases with "small" and "large" Fermi
momenta can be continuous. Such a continuous but Fermi-momentum-changing
transition arises in the presence of spin anisotropy, from a Luttinger liquid
with a small Fermi momentum to a Kondo-dimer phase with a large Fermi momentum.
We have also added a frustrating next-nearest-neighbor interaction in the spin
chain to show the possibility of a similar Fermi-momentum-changing transition,
between the Kondo phase and a spin-Peierls phase, in the spin isotropic case.
This transition, however, appears to involve a region in which the two phases
coexist.Comment: The updated version clarifies the definitions of small and large
Fermi momenta, the role of anisotropy, and how Kondo interaction affects
Luttinger liquid phase. 12 pages, 5 figure
Non-perturbative approach to Luttinger's theorem in one dimension
The Lieb-Schultz-Mattis theorem for spin chains is generalized to a wide
range of models of interacting electrons and localized spins in one-dimensional
lattice. The existence of a low-energy state is generally proved except for
special commensurate fillings where a gap may occur. Moreover, the crystal
momentum of the constructed low-energy state is , where is the
Fermi momentum of the non-interacting model, corresponding to Luttinger's
theorem. For the Kondo lattice model, our result implies that must be
calculated by regarding the localized spins as additional electrons.Comment: Note added on the rigorous proof given by H. Tasaki; also added some
references; 5 pages, REVTEX (no figure
Localized charged states and phase separation near second order phase transition
Localized charged states and phase segregation are described in the framework
of the phenomenological Ginzburg-Landau theory of phase transitions. The
Coulomb interactions determines the charge distribution and the characteristic
length of the phase separated states. The phase separation with charge
segregation becomes possible because of the large dielectric constant and the
small density of extra charge in the range of charge localization. The phase
diagram is calculated and the energy gain of the phase separated state is
estimated. The role of the Coulomb interaction is elucidated
Relationship between incommensurability and superconductivity in Peierls distorted charge-density-wave systems
We study the pairing potential induced by fluctuations around a
charge-density wave (CDW) with scattering vector Q by means of the Froehlich
transformation. For general commensurability M, defined as |k+M*Q>=|k>, we find
that the intraband pair scattering within the M subbands scales with M whereas
the interband pair scattering becomes suppressed with increasing CDW order
parameter. As a consequence superconductivity is suppressed when the Fermi
energy is located between the subbands as it is usually the case for nesting
induced CDW's, but due to the vertex renormalization it can be substantially
enhanced when the chemical potential is shifted sufficiently inside one of the
subbands. The model can help to understand the experimentally observed
dependence of the superconducting transition temperature from the stripe phase
incommensurability in the lanthanum cuprates.Comment: 6 pages, 3 figure
Ordering of localized moments in Kondo lattice models
We describe the transition from a ferromagnetic phase, to a disordered para-
magnetic phase, which occurs in one-dimensional Kondo lattice models with
partial conduction band filling. The transition is the quantum order-disorder
transition of the transverse-field Ising chain, and reflects double-exchange
ordered regions of localized spins being gradually destroyed as the coupling to
the conduction electrons is reduced. For incommensurate conduction band
filling, the low-energy properties of the localized spins near the transition
are dominated by anomalous ordered (disordered) regions of localized spins
which survive into the paramagnetic (ferromagnetic) phase. Many interesting
properties follow, including a diverging susceptibility for a finite range of
couplings into the paramagnetic phase. Our critical line equation, together
with numerically determined transition points, are used to determine the range
of the double-exchange interaction. Models we consider are the spin 1/2 Kondo
lattices with antiferromagnetic (Kondo) coupling, with ferromagnetic (Hund's
rule) coupling, and the Kondo lattice with repulsive interactions between the
conduction electrons.Comment: 18 pages, 6 embedded eps figures. To appear in Phys Rev
Impurity correlations in dilute Kondo alloys
The single impurity Kondo model is often used to describe metals with dilute
concentrations (n_i) of magnetic impurities. Here we examine how dilute the
impurities must be for this to be valid by developing a virial expansion in
impurity density. The O(n_i^2) term is determined from results on the
2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial
fixed point of the 2-impurity problem could produce novel singularities in the
heat capacity of dilute alloys at O(n_i^2).Comment: 6 pages, no figure
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