湖泊外源溶解有机碳的光化学降解及其对细菌碳源利用的影响

Allochthonous (terrestrial) dissolved organic carbon (DOC) is important component of carbon pool of lakes. However, allochthonous DOC is mostly humus substance with high molecular weight, which makes it difficult to be directly utilized by aquatic bacteria. Photochemical degradation of allochthonous DOC can influence bacterial carbon source utilization, and thus allochthonous DOC could be an important carbon source of bacteria in high mountain lakes with strong solar (especially UV) radiation. We will investigate the properties of DOC concentration and bacteria density, and their spatial and temporal distribution in a high mountain lake (Lake Tiancai). We will measure carbon stable isotope of bacteria, autochthonous (phytoplankton) and allochthonous DOC to assess the contribution ratio of allochthonous DOC to bacterial carbon source. To elucidate the pattern of allochthonous DOC photodegradation, we will investigate the influence of environmental factors on photochemical reaction rate through the combined in-situ and controlled experiments. Moreover, we intend to analyze the molecular structure and molecular weight distribution of allochthonous DOC. The allochthonous DOC solutions being irradiated at different time will be used to culture bacteria in controlled experiment. Then by comparing the bacterial growth efficiency, density, phospholipid fatty acids biomarkers and their carbon stable isotope, we will unveil the effect of the photochemical degradation of allochthonous DOC on bacterial carbon source utilization. This work is valuable for revealing the environmental transformation pathways of allochthonous DOC in lacustrine ecosystem. Furthermore, it will be helpful to enhance understanding the relation between terrestrial and aquatic ecosystems and carbon cycling.

外（陆）源溶解有机碳（DOC）是湖泊碳库的重要组成部分，但进入水体的外源DOC多为高分子量物质，难以被细菌直接利用，而光化学降解可能促进细菌利用外源DOC。因此，在日光辐照较强的高山湖泊，外源DOC可能是细菌的重要碳源。本项目以外源DOC丰富的高山湖泊为对象，以外源DOC的光化学降解对细菌碳源利用的影响为核心，调查湖泊DOC浓度与细菌密度的时空分布特征；通过测定细菌、内源（浮游植物源）和外源DOC的碳稳定同位素，估算不同来源DOC对细菌碳源构成的贡献比例；结合原位实验与室内实验，考察光照条件对外源DOC光化学反应的影响，分析光降解产物分子结构与分子量分布，探明其光化学降解规律；通过添加不同光解程度的外源DOC培养细菌，比较添加前后细菌的密度、生长效率和碳稳定同位素的变化，揭示细菌利用外源DOC的机理。研究结果有助于阐明外源DOC的转化途径，为开展水-陆生态系统间的相互作用研究增加科学积累。

为了阐明光化学降解作用对外源溶解有机碳（DOC）—细菌碳源贡献—细菌生长效率之间关系的影响，揭示外源DOC的光化学降解过程对细菌碳源利用的影响机理，本研究于2014年6月进行了外源有色可溶性有机物（CDOM）原位光降解实验。结果显示，其光吸收系数（a250）随辐照时间增加而不断降低，表明CDOM浓度在降低。光降解动力学常数为石英组：0.096 ± 0.011 d-1，玻璃组0.079 ± 0.009 d-1，对照组0.003 ± 0.001 d-1。石英组与玻璃组经光辐照后6天后， a250/a365比值较初始时明显升高，表明溪流输入到天才湖的外源CDOM经光降解作用后平均分子粒径变小。类色氨酸荧光Fn280与类腐殖质Fn355随光照时间的增加，均呈现波动状降低的趋势，尤其是Fn280的降解趋势更为显著。表明类色氨酸荧光的总体光降解速率高于类腐殖质荧光。用未光降解的CDOM和光降解8天后的CDOM原位培养细菌，后者所培养的细菌丰度与生长速率远高于前者，且比生长速率达到了0.090 h-1。.为了进一步证实原位试验的结果，采集了天才湖岸边土壤中提取腐殖酸。选择不同功率的紫外灯（对照组0W，20W，40W）进行光照实验。对照组、20W组和40W组的吸收系数a440的下降速率分别为6.73%、33.63%和39.52%，表明随着光照强度的增加，腐殖酸的光降解速率也随之增加。CDOM溶液在465nm和665nm处吸收系数的比值（E4/E6）分别为：对照组为6.83-5.34；20W组为6.83-3.68；40W组为6.83-2.76。这一结果表明光照强度越强，溶液的芳香化程度就越高，腐殖酸的降解程度也越大。用光降解后的腐殖酸溶液来培养细菌，培养13天后，对照组细菌丰度达到峰值1.82×107cells/ml，而处理组峰值分别为3.74×107cells/ml（20W组）、4.77×107cells/ml（40W组）；说明腐殖酸的光降解程度不同对细菌丰度峰值有显著影响，光降解程度越大，其细菌丰度的峰值就越大。同时，对培养的细菌样品进行了高通量测序。结果表明，丰富度指数Chaol和多样性指数Shannon都是呈不断下降的趋势。对照组（0w）的丰富度指数Chaol和多样性指数Shannon最大，其次是20w组，最小的是40w组。由Alpha多样性指数可知，物种丰富度与多样性随时间