海洋酸化与光照耦合效应对海洋光合有机物光学与化学特性的影响

The optical-chemical characteristics of marine organic matter influence its photochemical reactivity and bioavailability, which could also be used to characterize the carbon sequestration efficiency of the marine biological pump. The enhanced atmospheric CO2 concentration by anthropogenic activities during the past decades has caused acidification in the upper ocean, which sequentially influences the marine primary production. Furthermore, the effect of ocean acidification is related with light intensity. Thus, more attention is needed to be paid on how the synergetic effects of ocean acidification and light intensity impacting on the chemical and optical characteristics of photosynthetic organic matter, and hence influencing the degradation reactivities of organic matter as well as the carbon sequestration efficiency. So, in this study, lab-based incubation of monocultures of Skeletonema costatum and Skeletonema dohrnii, isolated from Chinese coastal sea, will be conducted under enhanced CO2 level coupled with varied light intensities, as well as in situ incubation in the coastal East China Sea. The absorbance spectra of photosynthetic dissolved and particulate organic matter (DOM and POM) and the fluorescence spectra of DOM will be analyzed to evaluate the synergetic impacts of ocean acidification and light intensity on: (1) the stoichiometry (C:N) of POM and DOM and the ratio of the production rates of dissolved and particulate organic carbon (DOC and POC); (2) the characteristics of the absorption spectra of POM and DOM; (3) and the alteration of DOM molecular components. This project focusing on the optical-chemical characteristics of marine photosynthetic organic matter, will provide new thought and sight to better understand the potential effect of ocean acidification coupled with light intensity to the organic carbon and nitrogen cycling in the coastal seas in China.

有机物光学与化学特性是影响有机物在上层海洋光化学活性和微生物可利用性的内在因素，也是表征海洋生物泵碳埋藏效率的重要指标。人类活动导致大气CO2不断升高，引起海洋酸化现象，并进一步影响海洋初级生产力，而这一效应还与光照强度密切相关。海洋酸化与光照耦合如何改变光合有机产物的化学与光学特性，进而影响有机物的降解活性及碳埋藏效率，是需要深入研究的科学问题。因此，本课题拟以实验室（中肋骨条藻和多尼骨条藻）与现场（东海陆架海）培养相结合，采用吸收光谱与荧光光谱等技术，探讨海洋酸化与光照耦合对：（1）海洋光合有机物元素组成（C:N）及溶解有机碳（DOC）与颗粒有机碳（POC）生产速率比的影响；（2）颗粒有机物（POM）与溶解有机物（DOM）吸收光谱特性的影响；（3）DOM分子组成变化的影响。本研究从有机物光学与化学特性入手，可为认识酸化与光照耦合效应对我国近海有机碳、氮循环的潜在影响提供新的视角与思路。

近年来，海洋酸化越来越受到人们的重视，其对全球变化、近海水产养殖等均产生显著的影响。但在酸化条件下，浮游植物光合生产有机物的量和性质还了解较少。本项目选取了两种硅藻在室内培养，结果表明，酸化条件下光合生产颗粒有机物POM略有升高，但溶解有机物（DOM）丰度和光学特性均无显著改变。此外，连续两年的现场观测表明，秦皇岛外海底层海水夏季呈现显著的低氧、酸化状态，且自5月份开始，底层水体溶解氧（DO）浓度和pH逐渐降低，到8月下旬，底层水体DO和pH可降低至90 umol L-1和7.70。DO浓度和pH与时间呈显著的线性相关性，水体耗氧、酸化速率分别为2.18 umol L-1 d-1和0.0037 pH d-1。进一步分析表明，光合作用新生产的颗粒有机物（POM）沉降到底层水体后分解耗氧是引起水体低氧、酸化的主要生物化学控制因子，且该部分有机物大部分在水体中分解，进入沉积物后分解对该区域低氧、酸化的贡献<40%。在低氧、酸化演变过程中，DOM不是主控因子，且DOC浓度自5月至8月逐渐升高，说明POM分解过程中，部分发生不完全降解，产生新的DOM分子。对黄海区域DOM的研究表明，冷水团区域水体层化阻碍O2垂向输送的同时，还进一步阻碍上层水体DOM向底层的输送，但底层水体CDOM丰度较近岸海域高，说明该区域CDOM主要来自于微生物转化POM和无色DOM，且在该过程中不断耗氧，引起冷水团水体夏季O2不饱和与弱酸化状态。以上研究结果对进一步认识渤黄海夏季底层水体低氧、酸化机制提供了基本信息。

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