蓮華寺池と西湖 : 石野雲嶺の風景

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern1. Multi-year droughts during the instrumental period2 and decadal-length droughts of the past two millennia1, 3 were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming4. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies5 to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ~2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase

Ptychography of Organic Thin Films at Soft X-ray Energies

Organic photovoltaics (OPV) are a sustainable and inexpensive source of energy comprised of a mixture of an electron-donating and an electron-accepting material (often a polymer and fullerene derivative, respectively) to create a bulk heterojunction (BHJ). However, these devices are held back by low efficiencies. Improving the efficiency in a systematic way requires a comprehensive understanding of the device function and microstructure, but directly imaging the subsurface active layer structure is challenging. Here we demonstrate that ptychography may be used to image the OPV active layer with good film stability and materials contrast in both polymer:fullerene and all-polymer BHJ films. This is possible using soft X-ray ptychography available at the Advanced Light Source beamline 5.3.2.1. We achieve a resolution of 30-50 nm with no discernible radiation damage and discuss the potential for further improvements. This characterization technique could complement widely used electron microscopy and soft X-ray scattering

Protein motions, dynamic effects and thermal stability in dihydrofolate reductase from the hyperthermophile thermotoga maritima

Dihydrofolate reductase (DHFR) has long been used as a model system in studies of the relationship between enzyme structure and catalysis. DHFR from the hyperthermophilic bacterium Thermotoga maritima (TmDHFR) is substantially different to other chromosomal DHFRs. It is dimeric where most others are monomeric, it lacks the conformational behaviour of monomeric DHFRs, and the kinetics of the catalysed reaction are significantly different. Experimental and computational studies of TmDHFR and comparison to other DHFRs have yielded deep insights into the role of enzyme motions and dynamics in catalysis. Mutational studies and formation of hybrids between TmDHFR and a monomeric homologue have demonstrated that dimerisation is required for extreme thermostability, but also leads to an inability to adequately close the active site with detrimental effects for the speed of the catalysed reaction. However, in common with other DHFRs there is no involvement of large-scale enzyme motions in the chemical reaction itself and dynamic coupling to the reaction coordinate is efficiently minimised. Studies of DHFRs from hyperthermophilic organisms and comparisons to their mesophilic counterparts remain a rich source of information on the fundamental nature of enzyme catalysis