河口-近海低氧区微生物不完全反硝化过程介导的N2O产生与释放机制

Nitrous oxide (N2O) ranks as the number one ozone-depleting chemical, which is an increasingly important greenhouse gas. Eutrophication of waters has been increasing in many estuarine and coastal areas, leading to very large hypoxia in the affected regions and accelerating the nitrogen cycle process. As a result, the flux of N2O emission was enhanced remarkably. At present, it has been recognized that estuarine and coastal zones were the source of N2O emission to atmosphere. As the marine N2O sources and sinks with complex spatial and temporal patterns of variation, it is not entirely clear microbiological mechanisms in the ocean up currently. Preliminary view that the microbial incomplete denitrification was an important process during reduction of NOx– to N2O by multiple types functional microorganisms. Increased N2O production is probably caused by the addition of anthropogenic nitrate and its subsequent incomplete denitrification, which is favored by hypoxia condition. Hypoxia may regulate the functional enzymes’ activity associated with the production and consumption of N2O in the process of denitrification, leading to an imbalance of source and sink of N2O. At last, N2O would be accumulated and emit to atmosphere. Under low oxygen condition, incomplete denitrification should contribute significantly to the N2O emission. But this process is ignored in the current biogeochemical model of near shore N2O emissions.In this study, we will investigate the microbial mechanism of N2O emission by incomplete denitrification and principle environmental factors impacting this emission in the Pearl River estuary and coastal zones using various methods combined geochemistry to molecular ecology. Firstly, we will analyze the contribution rate of iDNF process to N2O release flux on low oxygen gradient by means of isotope tracer combined with inhibitors. Then, we will explore the regulation mechanism of hypoxia on the production and release of N2O by iDNF process using molecular ecology methods. And furthermore, we will verify the effect of hypoxia on the production of N2O by iDNF process using microcosmic culture experiment. This study would improve the understanding for the fundamental scientific question of the production and release of N2O and building a new biogeochemical model of N2O in hypoxia estuary and coastal zones.

人类活动使得河口-近岸海域低氧区不断扩大，加速了氮素循环过程，显著提高了N2O的释放通量，导致河口近海成为释放N2O的重要来源。其中反硝化作用是近岸水域N2O排放的一个重要驱动力，但尚不明确其具体的作用机理。推测低氧可能通过调控与反硝化过程N2O产生与消耗相关的功能酶活性，导致N2O的源汇过程不平衡，造成N2O的积累和释放，形成不完全反硝化作用（iDNF）。在低氧条件下，由iDNF产生的N2O占有很大的比重，但目前有关河口-近岸N2O相关的生物地球化学分析中却忽视了这一过程。本项目以珠江口及近海为研究区域，通过氮同位素示踪的方法分析在低氧梯度上iDNF过程对N2O释放通量的贡献，运用分子生态学技术探索低氧对iDNF过程产生和释放N2O的调控机制，并进一步运用微宇宙培养实验验证低氧对促进iDNF过程产生N2O影响。本研究将阐明iDNF在河口近岸海域低氧区N2O释放的作用，揭示其低氧调控机制。

本项目围绕“微生物的不完全反硝化作用在河口-近海低氧区N2O释放中的作用和生物地球化学机制”这一核心可续问题，在珠江口海域和近海生态系统开展研究研究工作，取得如下主要结论：（i）珠江口水体具有高溶解性N2O浓度且呈从上游到下游减少的显著空间差异，上游河口表层沉积物反硝化过程是整个珠江口N2O的潜在的强源。低氧的条件是造成珠江口上游水体N2O积累的主要因素之一。珠江口表层沉积物潜在反硝化速率、潜在N2O产生速率均呈现出从上游到下游的显著空间分布特征：上游高值，中游递减，下游低值的分布模式，主要受溶氧、盐度和pH影响。（ii）微生物介导的不完全反硝化作用主导了珠江口表层沉积物N2O的释放。珠江口上下游表层沉积物nir与nosZ丰度的不平衡与潜在N2O产生速率有显著正相关关系。其中，上游河口表层沉积物基因丰度nirS高于nirK，下游河口nirK稍高。nosZ-Ⅱ高于nosZ-Ⅰ基因丰度1～2数量级；nirS，nosZ-Ⅰ和nosZ-Ⅱ基因丰度在珠江口上游到下游区域均呈现显著的空间差异特征，主要受DO和盐度影响。相关性结果表明nirS与nosZ-Ⅰ反硝化类群对珠江口表层沉积物N2O释放潜力具有显著影响。功能微生物群落结构从上游到下游均具有显著性差异，与底层潜在N2O产生速率和N2O/(N2O+N2)比值有显著相关关系。（iii）溶氧调控实验表明，低氧条件刺激珠江口表层沉积物不完全反硝化过程的发生，造成体系中N2O的积累。不同站位珠江口表层沉积物反硝化过程N2O释放潜能对溶氧具有不同的响应。在一定低氧阈值内，氧气的增加促进了珠江口表层沉积物N2O的产生与释放，形成不完全反硝化过程。研究揭示了珠江口表层沉积物微生物介导不完全反硝化N2O释放的机制，为认识不完全反硝化作用对近岸海域高N2O释放的重要贡献提供证据。项目共计发现学术论文25篇，其中SCI论文23篇，中文核心期刊论文2篇，部分研究成果获得海洋工程科学技术一等奖。