Triggered contraction of self-assembled micron-scale DNA nanotube rings.

Contractile rings are formed from cytoskeletal filaments during cell division. Ring formation is induced by specific crosslinkers, while contraction is typically associated with motor protein activity. Here, we engineer DNA nanotubes and peptide-functionalized starPEG constructs as synthetic crosslinkers to mimic this process. The crosslinker induces bundling of ten to hundred DNA nanotubes into closed micron-scale rings in a one-pot self-assembly process yielding several thousand rings per microliter. Molecular dynamics simulations reproduce the detailed architectural properties of the DNA rings observed in electron microscopy. Theory and simulations predict DNA ring contraction - without motor proteins - providing mechanistic insights into the parameter space relevant for efficient nanotube sliding. In agreement between simulation and experiment, we obtain ring contraction to less than half of the initial ring diameter. DNA-based contractile rings hold promise for an artificial division machinery or contractile muscle-like materials.

Water level changes affect carbon turnover and microbial community composition in lake sediments.

Due to climate change, many lakes in Europe will be subject to higher variability of hydrological characteristics in their littoral zones. These different hydrological regimes might affect the use of allochthonous and autochthonous carbon sources. We used sandy sediment microcosms to examine the effects of different hydrological regimes (wet, desiccating, and wet-desiccation cycles) on carbon turnover. (13)C-labelled particulate organic carbon was used to trace and estimate carbon uptake into bacterial biomass (via phospholipid fatty acids) and respiration. Microbial community changes were monitored by combining DNA- and RNA-based real-time PCR quantification and terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA. The shifting hydrological regimes in the sediment primarily caused two linked microbial effects: changes in the use of available organic carbon and community composition changes. Drying sediments yielded the highest CO2 emission rates, whereas hydrological shifts increased the uptake of allochthonous organic carbon for respiration. T-RFLP patterns demonstrated that only the most extreme hydrological changes induced a significant shift in the active and total bacterial communities. As current scenarios of climate change predict an increase of drought events, frequent variations of the hydrological regimes of many lake littoral zones in central Europe are anticipated. Based on the results of our study, this phenomenon may increase the intensity and amplitude in rates of allochthonous organic carbon uptake and CO2 emissions.

Ageing behavior of phenanthrene and pyrene in soils: a study using sodium dodecylbenzenesulfonate extraction.

The surfactant SDBS has potential to be used as an extractant in advanced extraction methods. It was investigated if SDBS in concentrations of 10,000 mg L(-1) can be used to study the ageing behaviors of phenanthrene and pyrene in two soils. Compared to dichloromethane extraction, extraction with 10,000 mg L(-1) SDBS can improve the precision of the determination due to the low volatility of surfactant solution. In all cases tested, the amount of sorbed phenanthrene and pyrene in soils increased with contact time from 1 to 120 days, and can be applied well to both the three-domain model and the dual mode model. Redundancy analysis showed that time, organic carbon content and logK(ow) are major factors affecting the fitted Freundlich parameters and the sorption kinetics of phenanthrene and pyrene. The amount of sorbed phenanthrene and pyrene in low-concentrations samples varied more with time than the one in high concentrations, indicating that the sorption on soils is concentration-dependent. Also, the isotherms were operationally separated into a "fast" fraction and a "slow" fraction. The results imply that the adsorption sites are internal to the SOM matrix and unevenly distributed.