SGER: Exploration of Three-Dimensional Structure in Sedimentary Organic Matter

土壤和潮间带沉积物经常受到干湿循环的影响。大多数表层土壤夏季湿润，冬季干燥。由于涨潮和退潮，盐沼和潮间带沉积物每天都有干湿循环。在干燥条件下，脱水会改变土壤和沉积物的三维（3D）结构；因此，干燥的沉积物可能与未干燥的沉积物表现不同。例如，由于干燥，在许多类型的土壤中经常可以看到疏水性（或疏水性）。此外，干燥土壤有机质比未干燥土壤更易降解。同样，水果和蔬菜的有机基质在脱水后也会不可逆地瓦解。在探索性研究小额资助（SGER）的支持下，纽约州立大学石溪分校的研究人员将研究有机物的结构在其分解反应性中所起的作用。他们假设三维结构和有机质极性是控制盐沼沉积物分解的重要因素。包裹在矿物表面或以碎屑形式存在的有机疏水性基团暴露在海水中，具有高度水化的三维结构。三维结构中的有机分子具有亲水性和极性，允许其他极性有机化合物通过静电力被其三维结构吸收或分割。在干燥过程中，有机分子的极性官能团被迫相互作用或与矿物表面相互作用，从而导致极性部分向外翻转，暴露出更多的疏水性基团。干燥过程使有机物改变构象，体积缩小，疏水性增强。再湿的有机物比未干燥的材料更疏水，因此对非极性有机化合物的吸附能力比原始有机物强。在干湿循环的环境中，疏水或亲水化合物在溶液相和颗粒相之间的分布可能因此受到有机物的三维结构和极性的影响。这将极大地影响有机质再矿化的速率。研究人员将进一步探索一些初步发现，即有机质的三维结构对沿海沉积物的吸收行为至关重要。具体来说，他们将研究(1)陆地与海洋有机质在这种三维结构中的重要性，以及(2)三维性质对有机质再矿化率的重要性。作为一个明显的更广泛的影响，该项目具有社会意义，有助于更好地理解有机物质再矿化的机制，从而促进全球碳循环。如果能对海洋对二氧化碳浓度上升的反应有更深入的了解，那么在各种政治选择之间就有可能做出更合理的选择。沿海沉积物包含了世界海洋中大部分的沉积有机碳。

Soils and intertidal sediments are often subjected to wet-dry cycles. Most surface soils are wetted in summer and dried in winter. Salt marsh and intertidal sediments can have daily wet-dry cycles due to the rising and falling tide. Under dry conditions, dehydration can alter the three dimensional (3D) structure of soils and sediments; thus dried sediments may behave differently than never-dried sediments. For example, water repellency (or hydrophobicity) is often seen in many types of soils as a result of drying. Furthermore, dried soil organic matter is more easily degraded than non-dried soil. Similarly, the organic matrix of fruits and vegetables can collapse irreversibly after dehydration. With support from this Small Grant for Exploratory Research (SGER), researchers at the State University of New York at Stony Brook will investigate the role play by the structure of organic matter in its decompositional reactivity. They hypothesize that the three-dimensional structure and the polarity of organic matter are important factors controlling decomposition in salt marsh sediment. Hydrophobic groups of organic matter coating mineral surfaces or existing as detritus are exposed to seawater, and have a highly hydrated 3D structure. The organic molecules in the 3D structures are hydrophilic and polar, allowing other polar organic compounds to be absorbed by or partitioned into its 3D structure via electrostatic forces. During the drying process, polar functional groups of organic molecules are forced to interact with each other or with mineral surfaces, thus causing the polar moieties to turn inside out and expose more hydrophobic groups. The drying process makes organic matter change its conformation, shrink its volume, and become more hydrophobic. Rewetted organic matter is more hydrophobic than never-dried material and thus has a stronger sorption capacity for nonpolar organic compounds than the original organic matter. In environments with wet and dry cycles, the distribution of hydrophobic or hydrophilic compounds between solution and particulate phases could thus be influenced by the 3D structure and polarity of organic matter. This should greatly affect the rate of remineralization of the organic matter. The investigators will further explore some preliminary findings that the three-dimensional architecture of organic matter is critically important to sorption behavior of coastal sediments. Specifically, they will investigate (1) the importance of terrestrial versus marine organic matter in this 3D structure, and (2) the importance of the 3D nature to remineralization rates of organic matter. As a clear broader impact, this project has societal implications by contributing to a better mechanistic understanding of organic matter remineralization and thus the global carbon cycle. If a more sophisticated understanding of the ocean's response to increased levels of carbon dioxide can be developed, then more reasonable choices between political alternatives are possible. Coastal sediments contain much of the sedimentary organic carbon in the world oceans.