海洋中无色溶解有机物的光敏及生物降解的研究

The ocean harbors an enormous stock of dissolved organic matter (DOM) whose carbon component (i.e. dissolved organic carbon, DOC) is comparable to the atmospheric CO2 inventory. DOM can be divided into two classes: the colored DOM (CDOM), which absorbs ultra-violet (UV) and visible light， and the transparent (i.e. colorless) DOM (TDOM), which absorbs no light. CDOM is often the dominant fraction of vascular plant-originated (i.e. terrigenous) DOM while phytoplankton-derived DOM in freshwater and marine systems is mainly composed of TDOM. Terrigenous CDOM is generally resistant to biodegradation but is photochemically reactive. While the color of CDOM can be completely photobleached, only part of it can be photochemically mineralized and the rest (ca. 20-70%) is transformed to TDOM. TDOM is partially labile and thus can be degraded by microbes; a significant portion of TDOM is, however, refractory to biodegradation. Moreover, microbes can convert certain labile TDOM into refractory TDOM. TDOM cannot be directly photochemically degraded but in principle can be so in the presence of photosensitizers. The preferentially photochemical removal of CDOM reduces the color and enriches the 13C abundance of terrigenous DOM, making its optical properties and 13C isotopic compositions more resemble those of marine DOM. Consequently, it entails potentially large uncertainties to use optical and isotopic signatures as the tracers of terrigenous DOM in the ocean. Notwithstanding CDOM photooxidation has drawn much attention over the last several decades, photosensitized degradation of TDOM, potentially a large sink of DOM, remains essentially untouched. Biodegradation of DOM has also been extensively explored but its rates have been usually measured in dark incubations, though it is well recognized that solar UV affects microbial activity. The objectives of this project are aimed 1) to quantify the potential of photosensitized mineralization of TDOM; 2) to quantify the potential of microbial mineralization of TDOM in the presence of solar radiation with various spectral quality; 3) to evaluate the impact of photosensitized and microbial degradation of TDOM on organic carbon cycling in natural waters; 4) to assess the implication of photosensitized and microbial degradation of TDOM for the fate of terrigenous DOM in the ocean. To realize these objectives, water samples containing CDOM from contrasting origins (terrigenous, marine, and mixture of terrigenous and marine) will be completely photobleached to TDOM. The obtained TDOM will then be subjected to long-term (months to a year) solar-simulated irradiation with and without the presence of heterotrophic bacteria to evaluate the microbial and photosensitized degradation potentials of TDOM. This research has the potential of leading to groundbreaking advances in understanding organic carbon cycling in natural waters and in better constraining the fate of terrigenous DOM in the ocean.

海水中溶解有机物DOM的生物地球化学循环在全球碳循环中起到至关重要的作用。其中无色溶解有机物(TDOM)的光敏降解有可能是DOM潜在的一个重要的汇，但至今未见报道；同时，以往微生物对TDOM的降解多在黑暗中进行培养，而太阳紫外辐射对微生物降解TDOM的影响及其相互作用还知之甚少。另外，由于陆源的DOM光降解后在TDOM中富集了13C的含量，使得利用13C同位素作为海洋中陆源DOM的示踪剂带有较大的不确定性。陆源DOM在海洋中的最终去向，成为深入研究海洋碳循环的重要环节。 本课题拟以不同来源的TDOM水样为研究对象，量化TDOM的光敏矿化潜力；量化太阳辐射对微生物矿化TDOM的影响；评估微生物和光敏矿化在TDOM的降解过程中的相对贡献及相互作用；阐明光敏和微生物降解TDOM对海洋中陆源DOM归宿的意义。从而解决陆源DOM在海洋中的去向这一难题，为全球碳循环提供新的理论依据及信息平台。

本项目以实验室与现场结合的方式，主要进行了光敏剂稳定性测试、无色溶解有机碳（TDOC）相对含量推算、无色溶解有机物（TDOM）光化学与微生物降解动力学及其相互作用机制、长江口溶解有机物（DOM）光化学降解随温度、盐度和pH的变化以及光降解表观量子产率等研究工作。.研究结果表明，硝酸盐（NO3-）是一种合适的光敏剂，当NO3-浓度达到50 mol L-1时，能够显著催化降解有色溶解有机物（CDOM）和TDOM，而当NO3-浓度为200 mol L-1时，NO3-对TDOM（同TDOC）的催化效率达到最高值。这一结果也进一步揭示，目前以陆源为主的河口和陆架海区DOM光化学活性结果显著受到NO3-的影响，值得今后开展重构其与海源DOM相对活性的研究。在本项目资助下，还分别于2014年和2016年夏季分别对表层、叶绿素最大层和底层海水中DOM进行了光化学与微生物降解现场培养。培养结果表明，光化学过程主要引起CDOM降解，而微生物既可以摄食CDOM，又能摄食部分TDOM，因此当光化学过程和微生物过程同时存在时，二者存在复杂的相互作用，既相互促进又相互竞争，而在这一过程中，TDOM分解速率显著提高。此外，通过实验结果计算得到的TDOC相对含量在富里酸（代表陆源DOM）中最低，在外海海水（代表海源DOM）中较高，而在陆架海区处于中间值。在长江口，温度和pH能够显著影响CDOM光漂泊速率，而盐度的效应可以忽略不计；在长江口及邻近海域，CDOM光漂白速率约为DOC光降解速率的10倍。.本项目的研究结果，将有助于我们从全新的视角进一步了解海洋碳循环机制，尤其是海洋DOM降解方面，主要包括如下四点：1）在光敏剂催化和微生物作用下，TDOM可以同CDOM一样被分解；2）在河口和陆架海区，NO3-含量丰富，大大提高了DOM光化学降解速率；3）天然水体中，光化学过程和微生物摄食二者存在复杂的相互作用；4）在温带河口和陆架海区，环境因子效应导致DOM光降解具有显著的空间和季节变化。