Effects of Photodegradation on the Composition, Optical Properties and Bioavailability of DOM in Estuaries

河流搬运的溶解有机物（DOM）是陆源溶解有机物的主要来源 海洋中的碳 发生在河口的化学、光化学和微生物过程， 在此之前，它会显著改变河流DOM的分子组成、反应性和光学性质， 被排入海洋。 许多河流和河口是净异养的，如CO2所示 过饱和和流出，这反过来又表明陆地DOM正在以高速率呼吸。 高呼吸速率意味着陆地DOM经历了结构退化和源的损失 的特异性因此，如果没有更好地了解化学和光化学变化， 河流成分在河口的作用，我们可能会错误地解释化学特征， (i.e.,源特异性）和最终排放到海岸带的DOM的光学特性。在这项研究中，老自治领大学的研究人员将试图量化河流和河口DOM的分子大小分布、分子水平特征、微生物可用性和光学特性的变化， 光暴露和初始DOM组成的函数。拟议的实验将检验以下假设： (1)水生DOM的光化学变化将导致可测量的分子量分布和官能团组成的变化，变化的类型和幅度将取决于初始DOM的化学特性。 (2)DOM的光化学改变将改变其生物利用度;生物利用度的增加或减少将取决于初始和光化学改变的材料的化学特性。 一个多分子水平的方法，包括直接温度分辨质谱（DT-MS），热解-气相色谱-质谱（PyGCMS），傅里叶变换红外光谱（FTIR），和核磁共振光谱（NMR），将被用来表征光介导的变化，在分子水平的组成在河口DOM和代表源材料被认为是贡献显着的河口DOM。 DT-MS和PyGCMS将在 对从水样品中分离的超滤尺寸级分（UDOM）和尺寸排阻色谱（SEC）级分两者进行。光谱、色谱和MS技术揭示的DOM结构、组成、官能团和氧化的变化将与光化学诱导的DOM生物利用度（掺入和呼吸）的变化相关。 这项研究将是第一次尝试，详细评估光化学降解/蚀变过程中的作用，调制的组成，微生物的可用性，和光学特性的DOM从河流和河口排放到海洋边缘。 通过将质谱、FTIR、NMR和光学性质测量与微生物可用性测定相结合，研究人员希望确定负责改变DOM池的光学性质和生物利用度的官能团和高阶结构。 使用SEC和超滤提供DOM大小的信息，将表明在光降解过程中的大小谱是否发生显著变化，以及这种变化是否会影响DOM的生物利用度。

ABSTRACTOCE-0241946River-transported dissolved organic matter (DOM) is the main source of terrestrial dissolved organic carbon in the sea. Chemical, photochemical and microbial processes that occur in estuaries can dramatically alter the molecular composition, reactivity and optical properties of riverine DOM before it is discharged into the oceans. Many rivers and estuaries are net heterotrophic as indicated by CO2 supersaturation and efflux, which in turn indicates that terrestrial DOM is being respired at high rates. High rates of respiration imply that terrestrial DOM undergoes structural degradation and loss of source specificity. Thus, without a better understanding of the chemical and photochemical alterations that riverine components undergo in estuaries, we may be incorrectly interpreting the chemical signatures (i.e., source specificity) and optical properties of DOM ultimately discharged to the coastal zone. In this study, researchers at the Old Dominion University will attempt to quantify variations in the molecular size distributions, molecular-level characteristics, microbial availability, and optical properties of riverine and estuarine DOM as a function of light exposure and initial DOM composition. The proposed experiments will test the following hypotheses: (1) Photochemical alteration of aquatic DOM will lead to measurable shifts in molecular-weight distributions and functional group composition; the types and magnitudes of the shifts will depend upon the chemical characteristics of the initial DOM. (2) Photochemical alterations of the DOM will change its bioavailability; whether the bioavailability increases or decreases will depend upon the chemical characteristics of the initial and photochemically altered material. A multiple molecular-level approach, including direct temperature-resolved mass spectrometry (DT- MS), pyrolysis-gas chromatography-mass spectrometry (PyGCMS), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR), will be used to characterize light-mediated changes in molecular-level composition within estuarine DOM and representative source materials believed to contribute significantly to estuarine DOM. DT-MS and PyGCMS will be performed on both ultrafiltered size fractions (UDOM) and size-exclusion chromatography (SEC) fractions isolated from the water samples. Changes in DOM structure, composition, functional groups and oxidation as revealed by spectroscopic, chromatographic, and MS techniques will then be related to photochemically induced changes in DOM bioavailability (incorporation and respiration). This study will be the first attempt to evaluate in detail the role of photochemical degradation/alteration processes on modulating the composition, microbial availability, and optical properties of DOM discharged from rivers and estuaries to ocean margins. By combining mass spectrometry, FTIR, NMR, and optical property measurements with microbial availability determinations, the investigators expect to identify the functional groups and higher order structures responsible for varying both the optical properties and the bioavailability of the DOM pool. The use of SEC and ultrafiltration to provide information on DOM size will indicate whether the size spectrum shifts significantly during photodegradation and whether this shift impacts DOM bioavailability.