细菌霍多醇记录长江口外低氧演变历史的潜力初探

Reconstructing evolutionary history of hypoxia in estuaries is beneficial to extend the record, figure out the natural factors in the process of its formation and development, and deal with the critical problem about how it is developed and what will happen in the future. Nevertheless, there was much less research about the evolutionary history of estuary hypoxia because of absence of effective proxies. Recently, 35-aminobacteriohopanepolyols and bacteriohopanetetrols isomer are considered to be possible biomarkers recording oceanic anoxic events, with higher concentration in particles corresponding to stronger hypoxia. However, hypoxia off Yangtze estuary was seasonal and influenced obviously by terrestrial nutrient and freshwater input, whether concentrations of 35-amino BHP and BHT isomer in water column with low oxygen were higher than those in oxygen-rich layers and can record the historic evolution of estuary hypoxia are unclear. This research will focus on the vertical distribution of these two compounds’ concentration in the particles of the Yangtze estuary hypoxia zone and aerobic region to discuss their environmental implications in detail. Meanwhile, the possibility of recording hypoxia evolution for these proxies will be further definitely discussed through comparing records from 35-amino BHP, BHT isomer and redox sensitive elements in sediment cores located in this hypoxic zone. This research can not only contribute to the scientific explanation of sediment records and historical reconstruction of estuary hypoxia but also lay a foundation for the effective management of human’s activities in this area.

重建河口低氧历史有助于延长毗邻海域低氧现象的历史记录，解析出其形成和发展过程中的自然因素，回答其如何发展而来，又将如何发展下去这一关键科学问题。但由于缺乏可靠的代用指标，关于河口低氧演变历史的研究并不多。35-氨基细菌霍多醇和细菌霍四醇的同分异构体最近被认为可能能够记录海洋低氧历史，其含量的升高和降低可能对应着水体缺氧程度的增加和减弱。但长江口外低氧区为季节性缺氧区，35-氨基细菌霍多醇和细菌霍四醇同分异构体能否记录低氧演变历史并不清楚。本研究拟通过分析长江口外缺氧区和非缺氧区悬浮颗粒物中35-氨基细菌霍多醇和细菌霍四醇同分异构体的垂直分布规律，详细探讨两种物质的环境指示意义，并通过比较沉积柱中35-氨基细菌霍多醇和细菌霍四醇同分异构体与氧化还原敏感元素记录的差异，进一步明确其记录低氧演变历史的可能性，为该区域沉积记录的科学解释和低氧历史重建及毗邻海域人类活动的有效管理奠定科学基础。

长江口外低氧区的存在和演化一直是近年来研究较多的河口海洋环境问题。目前主要观点认为该低氧区的存在主要归因于强烈的人类活动导致水体富营养化，因此产生的大量有机质的降解又过度消耗了水体中的溶解氧，而夏季水体层化又阻碍了溶解氧的补给。然而沉积记录表明该低氧区在历史时期一直存在，因此重建该海域的低氧记录有助于解开低氧区与人类活动之间的关系。本项目通过分析分层水体颗粒物和表层沉积物中细菌霍四醇的同分异构体（BHT’）和氨基细菌霍四醇的分布，发现低氧区表层沉积物中BHT’和氨基细菌霍四醇的含量确实与非低氧区有所不同，有可能与不同水体环境中的微生物活动有关，但用来进行重建时存在含量较低、难以检出的问题。由于底边界层氧化还原状况与pH值存在一定关系，我们分析了细菌合成的另一类重要物质，即支链四醚类脂物（brGDGTs），以便进一步了解东海低氧环境与细菌合成生物标志物之间的关系。通过分析东海和南海表层水体颗粒物及表层沉积物中brGDGTs的分布，我们发现海洋环境中自生brGDGTs的分布与陆源不同，四甲基brGDGTs的环化指数#Ringstetra很可能反映了底层水体和沉积物界面的pH值，并利用东海沉积柱尝试重建了近百年来底边界层pH的变化。上述研究内容补充了我国在BHPs这类化合物研究方面的不足，研究成果对于我们继续开展此类化合物的研究具有重要的借鉴意义。