Small phonon contribution to the photoemission kink in the copper oxide superconductors

Despite over two decades of intense research efforts, the origin of high-temperature superconductivity in the copper oxides remains elusive. Angle-resolved photoemission spectroscopy experiments have revealed a kink in the dispersion relations (energy versus wavevector) of electronic states in the copper oxides at binding energies of 50-80 meV, raising the hope that this anomaly could be a key to understanding high-temperature superconductivity. The kink is often interpreted in terms of interactions between the electrons and a bosonic field. Although there is no consensus on the nature of the bosons (or even whether a boson model is appropriate), both phonons and spin fluctuations have been proposed as possible candidates. Here we report first-principles calculations of the role of phonons and the electron-phonon interaction in the photoemission spectra of La(2 - x)Sr(x)CuO4. Our calculations within the standard formalism demonstrate that the phonon-induced renormalization of the electron energies and the Fermi velocity is almost one order of magnitude smaller than the effect observed in photoemission experiments. Therefore, our result rules out electron-phonon interaction in bulk La(2 - x)Sr(x)CuO4 as the exclusive origin of the measured kink. Our conclusions are consistent with those reached independently in a recent study of the related compound YBa2Cu3O7.     

The genome of Laccaria bicolor provides insights into mycorrhizal symbiosis

Mycorrhizal symbioses--the union of roots and soil fungi--are universal in terrestrial ecosystems and may have been fundamental to land colonization by plants. Boreal, temperate and montane forests all depend on ectomycorrhizae. Identification of the primary factors that regulate symbiotic development and metabolic activity will therefore open the door to understanding the role of ectomycorrhizae in plant development and physiology, allowing the full ecological significance of this symbiosis to be explored. Here we report the genome sequence of the ectomycorrhizal basidiomycete Laccaria bicolor (Fig. 1) and highlight gene sets involved in rhizosphere colonization and symbiosis. This 65-megabase genome assembly contains approximately 20,000 predicted protein-encoding genes and a very large number of transposons and repeated sequences. We detected unexpected genomic features, most notably a battery of effector-type small secreted proteins (SSPs) with unknown function, several of which are only expressed in symbiotic tissues. The most highly expressed SSP accumulates in the proliferating hyphae colonizing the host root. The ectomycorrhizae-specific SSPs probably have a decisive role in the establishment of the symbiosis. The unexpected observation that the genome of L. bicolor lacks carbohydrate-active enzymes involved in degradation of plant cell walls, but maintains the ability to degrade non-plant cell wall polysaccharides, reveals the dual saprotrophic and biotrophic lifestyle of the mycorrhizal fungus that enables it to grow within both soil and living plant roots. The predicted gene inventory of the L. bicolor genome, therefore, points to previously unknown mechanisms of symbiosis operating in biotrophic mycorrhizal fungi. The availability of this genome provides an unparalleled opportunity to develop a deeper understanding of the processes by which symbionts interact with plants within their ecosystem to perform vital functions in the carbon and nitrogen cycles that are fundamental to sustainable plant productivity.     

Climate change decreases aquatic ecosystem productivity of Lake Tanganyika,Africa

Although the effects of climate warming on the chemical and physical properties of lakes have been documented, biotic and ecosystem-scale responses to climate change have been only estimated or predicted by manipulations and models. Here we present evidence that climate warming is diminishing productivity in Lake Tanganyika, East Africa. This lake has historically supported a highly productive pelagic fishery that currently provides 25-40% of the animal protein supply for the populations of the surrounding countries. In parallel with regional warming patterns since the beginning of the twentieth century, a rise in surface-water temperature has increased the stability of the water column. A regional decrease in wind velocity has contributed to reduced mixing, decreasing deep-water nutrient upwelling and entrainment into surface waters. Carbon isotope records in sediment cores suggest that primary productivity may have decreased by about 20%, implying a roughly 30% decrease in fish yields. Our study provides evidence that the impact of regional effects of global climate change on aquatic ecosystem functions and services can be larger than that of local anthropogenic activity or overfishing.