Impact of bacterial biomass on the surface wettability of soil particles under varying moisture conditions

颗粒润湿性是土壤中许多过程的重要控制因素，包括流体的运动和分布，胶体和有机分子的运输和吸附，以及生物活性。土壤颗粒的低润湿性（土壤疏水性）通常被认为是由于来自真菌菌丝或植物材料的吸附有机化合物的存在。最近的研究揭示了细菌生物量在土壤中疏水性发展中的潜在作用的第一个证据。然而，到目前为止，尚不清楚细菌细胞及其片段在多大程度上促成了土壤拒水性的发生和持续，也不清楚细菌对水和盐胁迫的适应是否可能解释在干湿事件中经常观察到的拒水性变化。我们的项目旨在通过一系列密切相关的联合实验(1)调查导致细菌表面疏水性发生的因素和条件，(2)分析细菌表面特性如何通过细胞-矿物结合和土壤材料的表面特性来反映，(3)评估细胞/碎片（坏死块）-矿物结合的生物和物理稳定性，(4)测试土壤颗粒润湿性对细菌及其表面特性的潜在反馈。从人工系统的实验开始，纯细菌培养将暴露于不同种类的压力（渗透或基质压力），分离的细胞和细胞片段随后与矿物颗粒混合。结合接触角测量和表面自由能计算，结合x射线光电子能谱和原子力显微镜获得的表面化学结构和纳米力学特性信息，对细菌细胞、矿物及其关联的表面特性进行表征。结合使用这些技术将有助于探索细菌表面特性变化的潜在机制，并将允许确定矿物质和细菌细胞之间重要的相互作用机制，作为评估观察到的效果的持久性的先决条件。对不同干旱程度土壤的磷脂脂肪酸进行分析，将提供有关干旱引起的土壤微生物群落变化和对改变土壤颗粒湿润特性特别有效的细菌的第一手信息。识别与细胞表面特性有密切关系的生物标志物将有可能在复杂系统（如天然土壤）中评估细胞润湿性。最终，综合本项目获得的数据将大大提高对微生物介导的土壤拒水发育和动态过程和控制因素的机制理解。

Particle wettability is an important controlling factor for a multitude of processes in soil, including fluid movement and distribution, transport and adsorption of colloids and organic molecules, as well as biological activity. Low wettability of soil particles (soil water repellency) generally is considered to be attributable to the presence of adsorbed organic compounds derived from fungal hyphae or plant material. Recent research revealed first evidence for a potential role of bacterial biomass in the development of water repellency in soil. However, until now it is unclear to which extent bacterial cells and their fragments contribute to the occurrence and persistence of soil water repellency and whether bacterial adaptation to water and salt stress can possibly explain the frequently observed variation of water repellency in response to wetting and drying events. Our project aims at unraveling these questions in a series of closely interlinked joint experiments by (1) investigating the factors and conditions that contribute to the occurrence of bacterial surface hydrophobicity, (2) analyzing how bacterial surface properties are reflected by the surface properties of cell-mineral associations and soil material, (3) evaluating the biological and physical stability of cell/fragment (necromass)-mineral associations, and (4) testing the potential feedback of soil particle wettability on bacteria and their surface properties. Starting from experiments in artificial systems, pure bacterial cultures will be exposed to different kinds of stress (osmotic or matric stress) and the isolated cells and cell fragments subsequently intermixed with mineral particles. Surface properties of bacterial cells, minerals and their associations will be characterized by combining contact angle measurements and surface free energy calculations with information on surface chemical structure and nanomechanical properties obtained by X-ray photoelectron spectroscopy and atomic force microscopy. Using the combination of these techniques will help to explore the underlying mechanisms of changes in bacterial surface properties and will allow identifying important interaction mechanisms between minerals and bacterial cells as a prerequisite for evaluating the persistence of the observed effects. Analysis of phospholipid fatty acids in soils exposed to different drought levels will provide first information on drought-induced shifts in the soil microbial community and on bacteria which are particularly effective in changing the wetting properties of soil particles. Identification of biomarkers with strong relationship to cell surface properties will potentially allow to estimate cell wettability even in complex systems such as natural soil. Ultimately, the synthesis of data obtained in this project will greatly enhance the mechanistic understanding of processes and controlling factors involved in the microbially-mediated development and dynamics of soil water repellency.