Earth's millions of species influence a wide range of environmental processes, including elemental cycling, the stability of ecosystems, and the goods and services they provide. Naeem et al. (p. 1401) review recent advances in the young and evolving field of biodiversity and ecosystem functioning, explore the extent to which the field is becoming a predictive science, and indicate how the field needs to develop in order to aid worldwide efforts to achieve environmental sustainability in the face of rising rates of extinction.
Dating Cave Paintings
A number of caves in Europe contain exquisite ancient art. Most of the art has been thought to be produced during the time of last glaciation by recently arrived modern humans, but dating of the art has been problematic because the art contains only minimal amounts of carbon for radiocarbon dating. Pike et al. (p. 1409; see the cover; see the Perspective by Hellstrom) have now obtained U-series dates on the calcite crusts that formed over the art from 11 caves in northwestern Spain. The ages from three caves are older than 35,000 years ago, and one dates to nearly 41,000 years ago. The earliest art used primarily red and was relatively formless; animal depictions appeared later. This dating is near the time of the arrival of modern humans and, because Neandertals were also present, complicates identifying the artists.
Spin-orbit interactions present in many semiconductors are used to manipulate the spin of an electron in spin-based electronics. However, this same interaction is also responsible for the loss of information contained in a spin when it changes its direction during the scattering off of impurities. Sugimoto and Nagaosa (p. 1413) describe a new quantity, twisted spin, which, unlike regular spin, survives the scatterings; moreover, because the spin-orbit interaction is turned off adiabatically, the lost spin information is retrieved.
Expression of the tumor suppressor p53 is activated in response to cell stress. The dynamics of p53 activation can vary, depending on the stressor, resulting in either pulsatile or constant p53 levels; however, the functional consequence of these different dynamics is unclear. Purvis et al. (p. 1440) developed a method to control p53 dynamics in human cells. Pulsing p53 selectively activated genes involved in cell cycle arrest and DNA repair, allowing recovery from DNA damage. In contrast, sustained p53 promoted induction of terminal genes leading to cellular senescence. Thus, protein dynamics can affect cell fate decisions.
Dropping a Dimension?
In most magnetic materials, the exchange interaction causes the spins on the neighboring sites of a crystal lattice to align. In the absence of exchange interactions, dipolar interactions, which are highly orientation dependent, are also expected to be able to cause magnetism. Kraemer et al. (p. 1416) present evidence for antiferromagnetism in a dipolar-coupled material, LiErF4. Although a three-dimensional system, its critical behavior was more reminiscent of a two-dimensional material.
Alkynes contain carbon-carbon triple bonds and represent a diverse class of organic compounds. In principle, valence rules suggest that the boron analog of an alkyne, with a B-B triple bond, ought to be accessible by appending a two-electron donor to each B atom. Braunschweig et al. (p. 1420; see the Perspective by Frenking and Holzmann) now present the synthesis, isolation, and crystallization of a solid, triple-bonded diboryne, with N-heterocyclic carbenes as the terminal substituents, which contains the expected linear bonding geometry.
Porous materials are widely used in separation processes and catalysis. Seo and Hillmyer (p. 1422) used block copolymers that naturally separate into domains to achieve a continuous network of pores. A chain transfer agent was used to direct the copolymerization of the two materials in situ to generate a structure with a percolating porous structure with pore sizes of a few nanometers and with tunable control over the polymer properties and extent of cross-linking.
The Rocks That Hit the Moon
The cratered surface of the Moon bears witness to the numerous impacts it has suffered. Chemical signatures of these impacts have been detected indirectly. Now, Joy et al. (p. 1426, published online 17 May; see the Perspective by Rubin) report the detection and characterization of meteorite fragments preserved in ancient lunar regolith breccias from the Apollo 16 landing site. These meteoritic fragments represent direct samples of the population of small bodies traversing the inner solar system at around 3.4 billion years ago—the same time or just after the basin-forming epoch on the Moon.
Submarine seagrass meadows are critical to fisheries and coastline protection and provide feeding grounds for many endangered species, including dugongs and turtles, and serve as a nursery for coral reef fish. The persistence and maintenance of seagrass ecosystems have been mysterious, because accumulation of organic matter in the beds should rapidly lead to toxic sulphide levels in the sediment. Using a meta-analysis and a field study, van der Heide et al. (p. 1432) atttribute the 100-million-year success of seagrasses to a three-stage symbiosis. Seagrass beds worldwide contain high densities of small lucinid bivalves that have symbiotic sulphide-oxidizing bacteria in their gills. This association appears to relieve any sulphide stress for seagrasses, while the lucinids and their symbionts profit from the accumulation of degradable organic matter and oxygen release from seagrass roots.
Extensive pre-Columbian populations inhabited the central and eastern Amazon basin, as evidenced by the clearing and modification of forests. McMichael et al. (p. 1429) examined how far inland such activities may have extended by sampling soils across western Amazonia, including river bluffs, which were heavily occupied downstream. Little evidence of human disturbance across a wide region was found by looking for charcoal layers (which would suggest use of fire) and phytoliths, which trace local plants and would indicate the presence of crops. Furthermore, no ceramics or tools were found. Thus, pre-Columbian human populations seem to have been sparse in western Amazonia.
COPI-coated vesicles are responsible for intracellular vesicular transport both within the Golgi and between the Golgi and endoplasmic reticulum. By applying subtomogram averaging from cryoelectron tomography data, Faini et al. (p. 1451, published online 24 May) were able to describe the complete three-dimensional structure for COPI-coated vesicles generated in a cell-free, membrane-budding reaction. The structures of multiple individual vesicles reveal assembly principles based on interactions that, unlike those for clathrin-coated vesicles, are not regular: The basic subunit can undergo significant conformational changes and assemble with different stoichiometries. This variability may allow the regulation of membrane curvature and vesicle size. Furthermore, forming a complete closed coat was not required to produce budded vesicles.
A direct connection exists between aboveground communities and belowground soil microbiota: Soil microbes break down detrital inputs from above. Generally, it has been assumed that this relationship is largely driven by the soil community and the quality of the unconsumed plant-matter that makes up the majority of detritus in most ecosystems. However, Hawlena et al. (p. 1434) found that grasshoppers exposed to the threat of spider predation have an altered carbon to nitrogen ratio. When these grasshopper carcasses were subsequently integrated into the plant litter, they significantly slowed the decomposition rate, although there was no impact on the rate of decomposition of the grasshoppers themselves. Thus, the mere presence of predators, and the stress they impose, can have cascading trophic impacts and even influence the process of decomposition.
Reading the Leaves
Excess inputs of nutrients—a type of pollution known as eutrophication—threatens biodiversity and water quality in rivers and streams. Woodward et al. (p. 1438; see the Perspective by Palmer and Febria) studied how one key ecosystem process—leaf-litter decomposition—responds to eutrophication across a large nutrient pollution gradient in 100 European streams. Leaf breakdown was stimulated by low to moderate nutrient concentrations but was inhibited at high rates of nutrient loading.
Spatial Memory Perturbation
The hippocampus is important for learning and memory. However, it is not clear which patterns of neural activity in the hippocampus support specific mnemonic functions. Jadhav et al. (p. 1454, published online 3 May) developed a real-time analysis system to detect and selectively interrupt a certain type of hippocampal neuronal network event—sharp-wave ripples—during learning. In awake animals, loss of sharp-wave ripples and associated memory replay activity caused a learning deficit specific to spatial working memory but had no effect on reference memory. This learning deficit was present despite the preservation of place-field representations and replay activity during rest.
To Silence or Not to Silence
Repressed genes commonly have methylated DNA, and/or covalent histone modifications associated with silent chromatin, and/or associated small interfering (si)RNAs. All three features are components of gene-silencing systems (see the Perspective by Jacob and Martienssen). In a screen for components of DNA methylation gene-silencing systems in the flowering plant, Moissiard et al. (p. 1448, published online 3 May) identified the genes AtMoRC1 and AtMORC6, which are homologs of the mouse Microrchidia1 gene. AtMORC1 and AtMORC6 are involved in silencing transposable elements and genes corresponding to DNA-methylated loci, and yet neither gene is required for maintenance of DNA methylation. Instead, AtMoRC1 and AtMORC6 are related to proteins that remodel chromatin superstructure, and they seem to control gene-silencing through the higher-order compaction of methylated and silent chromatin. Qian et al. (p. 1445) identified an Arabidopsis gene, IDM1 (increased DNA methylation 1), that is involved in regulating DNA methylation at loci enriched for repeats and multigene families containing highly homologous genes. IDM1 protects target genes from DNA silencing and recognizes both histone H3 and methylated DNA at target loci and is able to acetylate histone H3.
Controlling the Axon
The cellular mechanisms and circuits involved in gamma oscillations in the brain are not fully understood. Dugladze et al. (p. 1458) simultaneously performed patch-clamp recordings in the soma and axon of hippocampal pyramidal neurons during gamma oscillations in brain slices. Under these conditions, pyramidal cells were divided into two electrogenic compartments: the soma fired at low frequency, whereas, in the axon, ectopic action potentials were generated at higher frequencies. This functional separation was maintained by highly active axoaxonic interneurons. Powerful inhibition of the axon initial segment by these axoaxonic cells prevented the backpropagation of ectopic action potentials to the somatodendritic compartment. However, when the overall excitatory drive to pyramidal cells was high, normal orthodromic action potentials were generated.