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

摘要河流输运溶解有机质（DOM）是海洋陆相溶解有机碳的主要来源。河口发生的化学、光化学和微生物过程可以在河流DOM排入海洋之前显著改变其分子组成、反应性和光学性质。许多河流和河口是净异养的，正如CO2过饱和和外排所表明的那样，这反过来表明陆地上的DOM正以很高的速率被呼吸。高呼吸速率意味着陆地DOM经历了结构降解和源特异性的丧失。因此，如果没有更好地了解河口河流成分所经历的化学和光化学变化，我们可能会错误地解释最终排放到海岸带的DOM的化学特征（即来源特异性）和光学特性。在这项研究中，Old Dominion大学的研究人员将尝试量化河流和河口DOM的分子大小分布、分子水平特征、微生物可利用性和光学特性的变化，作为光照和初始DOM组成的函数。本实验将验证以下假设：(1)水生DOM光化学改变会导致分子量分布和官能团组成发生可测量的变化；移动的类型和幅度取决于初始DOM的化学特性。(2) DOM光化学变化会改变其生物利用度；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。多分子水平方法，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），将用于表征河口DOM分子水平成分和光介导的变化以及被认为对河口DOM有重要贡献的代表性源物质。DT-MS和PyGCMS将对从水样中分离的超滤粒度组分（UDOM）和粒度排除色谱（SEC）组分进行分析。光谱、色谱和质谱技术揭示的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）的变化有关。本研究将首次尝试详细评价光化学降解/改变过程在调节从河流和河口排放到海洋边缘的DOM的组成、微生物可利用性和光学性质方面的作用。通过结合质谱、FTIR、NMR和光学性质测量与微生物利用度的测定，研究人员希望确定影响DOM池光学性质和生物利用度变化的官能团和高阶结构。使用SEC和超滤来提供DOM尺寸的信息将表明在光降解过程中尺寸谱是否显著偏移，以及这种偏移是否影响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.