【土壤表面细菌细胞相互作用】Cell-to-cell bacterial interactions promoted by drier conditions on soil surfaces. PNAS, doi: 10.1073/pnas.1808274115
Abstract
Bacterial cell-to-cell interactions are in the core of evolutionary and ecological processes in soil and other environments. Under most conditions, natural soils are unsaturated where the fragmented aqueous habitats and thin liquid films confine bacterial cells within small volumes and close proximity for prolonged periods. We report effects of a range of hydration conditions on bacterial cell-level interactions that are marked by plasmid transfer between donor and recipient cells within populations of the soil bacterium Pseudomonas putida. Using hydration-controlled sand microcosms, we demonstrate that the frequency of cell-to-cell contacts under prescribed hydration increases with lowering water potential values (i.e., under drier conditions where the aqueous phase shrinks and fragments). These observations were supported using a mechanistic individual-based model for linking macroscopic soil water potential to microscopic distribution of liquid phase and explicit bacterial cell interactions in a simplified porous medium. Model results are in good agreement with observations and inspire confidence in the underlying mechanisms. The study highlights important physical factors that control short-range bacterial cell interactions in soil and on surfaces, specifically, the central role of the aqueous phase in mediating bacterial interactions and conditions that promote genetic information transfer in support of soil microbial diversity.
【树木丧失何时不算毁林：绘测全球林木流失】Philip G. Curtis1,*, Christy M. Slay1, Nancy L. Harris2, Alexandra Tyukavina3, Matthew C. Hansen3. Classifying drivers of global forest loss. Science 14 Sep 2018: Vol. 361, Issue 6407, pp. 1108-1111 DOI: 10.1126/science.aau3445
(来源：EurekAlert!) 据一项新的基于地图的研究，由商品驱动的森林总体丧失速度（被定义成为生产商品而永久性地改变土地使用）占了全球范围内森林砍伐的四分之一以上；尽管国际上各国政府、公司和保护主义者在努力减少这些森林损失，但这种情况自2001年以来一直没有变化。这些结果不仅揭示了全球森林丧失的主要肇因以及在哪些地方正在发生森林丧失，而且它们还提示，并非所有的森林丧失是因为永久性毁林造成的。森林丧失是由几种不同因素引起的。有些森林丧失是永久性的，例如，因商品驱动的毁林造田或采矿。有些则并非永久性的森林丧失——例如，农业轮垦、林业或野火，后者常常接着会出现林木再生。然而，显示全球毁林的数据（如在广泛使用的全球森林观察平台上公开可得的数据）并不能区别永久性森林丧失与其它暂时性的森林受到干扰的形式。据Phillip Curtis和同事披露，它限制了将这些数据集用于确认来自公司的商品驱动性毁林的风险和责任；在这些公司中，有近450家致力于到2020年时在其供应链中实现零毁林。为了在林木覆盖丧失的背景中论述这一需要，Curtis等人研发了一种模型，旨在表征和绘制全球林木所受的干扰；他们使用的是2001年至2015年间的高分辨率谷歌地球卫星成像。他们的结果表明，约有27%的全球森林丧失可归因于商品驱动的森林砍伐。更重要的是，作者发现，商品驱动的森林砍伐率（每年约5百万公顷）在为期15年的时期中一直保持稳定。其他林木丧失的驱动因子可归因于：林业规划（26%）、游耕农业（24%）、野火（23%）及城市化（0.6%）。这些结果提示，零毁林政策目前的贯彻速度还不足以实现2020年时的目标。
