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地球科学进展, 2019, 34(1): 20-33 doi: 10.11867/j.issn.1001-8166.2019.1.0020

脂类单体碳同位素在湖沼古环境和古生态重建中的研究进展

黄咸雨,1,2, 张一鸣1

1. 中国地质大学(武汉)流域关键带演化湖北省重点实验室,湖北 武汉 430074

2. 中国地质大学(武汉)生物地质与环境地质国家重点实验室,湖北 武汉 430074

An Overview of the Applications of Lipid Carbon Isotope Compositions in the Paleoenvironmental and Paleoecological Reconstructions in Lacustrine and Peat Deposits

Huang Xianyu,1,2, Zhang Yiming1

1. HuBei Key Laboratory of Critical Zone Evolution, China University of Geosciences, Wuhan 430074, China

2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China

收稿日期: 2018-11-27   修回日期: 2018-12-29   网络出版日期: 2019-02-26

基金资助: 国家自然科学基金项目“多种脂类碳同位素记录的泥炭地生态系统对百年尺度水位下降的生态响应”.  编号:41877317
生物地质与环境地质国家重点实验室自主研究课题项目“大九湖泥炭地碳循环与气候变化的耦合关系”.  编号:GBL11804

Received: 2018-11-27   Revised: 2018-12-29   Online: 2019-02-26

作者简介 About authors

黄咸雨(1981-),男,湖北阳新人,教授,主要从事泥炭脂类与全球变化研究.E-mail:xyhuang@cug.edu.cn , E-mail:xyhuang@cug.edu.cn

摘要

地质载体中保存的脂类来源于生物细胞膜、叶片蜡质层等,能够指示特定的生物分类学类群或微生物功能群,也能够记录生物生长或早期成岩过程中的生态环境信息,已经成为第四纪古环境和古生态研究的重要手段。除了化合物含量和分子组成,脂类的碳同位素组成也蕴含着重要的生态或环境信息。对于光能自养生物,这些信息来自光合作用和脂类的生物合成过程;对于异养生物,信息则来自摄食的底物和脂类的生物合成过程。总结了近些年来湖沼沉积中脂类单体碳同位素的研究进展,从长链正构烷烃、脂肪酸、陆源五环三萜等高等植物脂类和磷脂脂肪酸、藿类、四醚膜脂等微生物脂类等2个领域进行了系统阐述。在今后的发展中,需要重视实验技术,开发适合小样品量的分析方法,建立直接测试藿类和四醚类等分子量相对大的脂类碳同位素组成的新技术,加强单体放射性碳同位素的应用;可以考虑多种脂类碳同位素的联合、同一脂类单体碳同位素和单体氢同位素的联合;建议加强脂类单体碳同位素在生物地球化学过程,特别是微生物地球化学过程对过去全球变化的响应研究。

关键词: 脂类 ; 单体碳同位素 ; 湖沼沉积 ; 生物地球化学 ; 古生态

Abstract

Lipids preserved in geological materials mainly originate from cell membrane and leaf waxes, and have the potential to infer biological sources, metabolic pathways, and environmental information. Thus,lipid-based proxies have been widely applied to reconstruct paleoenvironment and paleoecology in the Quaternary. Except the concentration and molecular composition, the carbon isotope compositions of lipids are also a type of important signal sources. For photoautotrophs, the carbon isotope compositions of lipids are mainly mediated by the carbon isotope discrimination during the photosynthesis and lipid biosynthesis processes. In contrast, the carbon isotope compositions of heterotroph derived lipids are controlled by substrates and the carbon isotope fractionation during biosynthesis. In this review, we overview the advances of applications of lipid carbon isotope ratios in lacustrine and peat deposits. In the near future, more attention is suggested to pay to instrumental techniques, such as reduce the sample amount, direct analysis the carbon isotope compositions of molecules with relatively large molecular weight (e.g. BHPs, GDGTs), and widely application of compound-specific radiocarbon isotope analysis. In addition, combination of carbon isotope ratios from multi lipids, or the application of dual carbon and hydrogen isotope ratios of lipids, will shed more information on the response of ecological processes to climate changes. Furthermore, more works are worthy to investigate the relation between biogeochemical processes and paleoclimate changes in the Quaternary.

Keywords: Lipid ; Compound-specific carbon isotope ; Lacustrine ; Biogeochemistry ; Paleoecology.

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本文引用格式

黄咸雨, 张一鸣. 脂类单体碳同位素在湖沼古环境和古生态重建中的研究进展. 地球科学进展[J], 2019, 34(1): 20-33 doi:10.11867/j.issn.1001-8166.2019.1.0020

Huang Xianyu, Zhang Yiming. An Overview of the Applications of Lipid Carbon Isotope Compositions in the Paleoenvironmental and Paleoecological Reconstructions in Lacustrine and Peat Deposits. Advances in Earth Science[J], 2019, 34(1): 20-33 doi:10.11867/j.issn.1001-8166.2019.1.0020

脂类(lipid)是指由生物合成的、不溶于水而溶于甲醇、氯仿、二氯甲烷等非极性溶剂的生物化学成分[1]。脂类来自生物体,主要分布于细胞膜、叶片蜡质层等部位。脂类化合物种类众多,有正构脂类,也有异构脂类和环状脂类。某些特征的脂类专属于特定的生物分类学类群或微生物功能群[2,3],可以充当这些生物类群或代谢途径的标志物。例如,2-甲基藿类主要来自蓝细菌,长链烯酮来源于颗石藻,非常负偏的碳同位素组成(<-50‰)可以指示甲烷氧化过程。由于能够记录生物源、代谢途径、生物生长或早期成岩转化过程中的生态环境信息,脂类已经成为生物—有机地球化学、新生代古气候与古生态研究等领域的重要工具。已有较多的文章综述了脂类在古环境和古生态方面的研究进展[4,5,6]

湖泊和泥炭沼泽是重要的陆地沉积场所,其沉积物中蕴含的脂类分子是开展陆地古环境和古生态研究的重要工具。除了化合物含量和分子组成,脂类的碳同位素组成也蕴含着重要的生态或环境信息。对于光能自养生物,这些信息来自光合作用和脂类的生物合成过程;对于异养生物,信息则来自摄食的底物和脂类的生物合成过程。本文对近年来湖沼脂类单体碳同位素的研究进展进行总结,并对未来的发展趋势进行展望。

1 单体碳同位素分析技术简介

受益于已故美国科学院院士John M. Hayes教授课题组的开创性贡献[7],以及后续技术改进,单体碳同位素分析技术已经成为古生物地球化学过程研究的重要技术。脂类单体碳同位素分析包括几个重要的环节:化合物分离,在线燃烧转化为CO2,CO2碳同位素比值测定,以及同位素结果的校正[8,9,10]。在这些环节中,化合物的分离纯化非常关键。对于新鲜的植物样品或有机质含量比较高的泥炭或湖相沉积物样品,长链正构烷烃的含量通常比较高,且其他组分的干扰比较弱,因而硅胶柱层析后得到的烷烃组分可以直接上机进行测试。在一些情况下,植物或土壤样品中含有较高含量的长链烯烃,可以在柱层析环节添加硝酸银来去除烯烃[11]。对于其他的样品,需要考虑色谱峰之间的干扰,尤其是鼓包(unresolved complex mixture),在很多情况下需要进行分子筛络合或尿素络合[12,13],消除异构烷烃和环烷烃的干扰。对于一些非常关键的化合物,还可以采用制备液相色谱法来进行单个化合物的纯化[14,15]。极性脂类如脂肪酸和脂肪醇等要进行衍生化后才能上机测试,常规的衍生化方法有硅烷化和甲酯化。这些衍生化过程会加入额外的碳原子,需要对最终的测试结果进行校正[16]。一方面,尽量少改变原化合物的碳原子数量;另一方面,需要测定衍生化过程中加入的碳原子的碳同位素组成。

除了分离纯化,单体碳分析技术还有一个很重要的环节,就是同位素结果的校正[8]。通过色谱分离后的脂类在氧化炉中被在线燃烧转化为CO2气体。这个过程需要CuO或CuO-NiO释放氧气,因此需要定期通氧来保证反应器的供氧能力。由于燃烧过程是在高温条件(900~1 000 ℃)下完成,如此高的温度有可能造成碳同位素分馏,因而需要在样品分析中加入碳同位素值已知的内标化合物来校正燃烧过程对碳同位素的影响。同时,还需要定期运行碳同位素值已知的标样来定期检验仪器的状态。目前,大部分实验室采用的是美国Indiana大学Arndt Schimmelmann博士实验室配置的单个正构烷烃或混合标样。最近Schimmelmann博士联合多家国际实验室,建立了多种新的同位素参考物质,其中包括正构碳十六烷(编号为USGS67, USGS68和USGS69,分别对应于不同负偏的碳同位素值)[17]

在单体碳同位素分析方面,还需要提及常规气相色谱仪难以气化的脂类,主要是藿类和四醚类。传统上,需要将具有多种官能团的藿类经氢碘酸/硼氢化钠反应后,降解成藿醇,然后进行单体碳同位素分析[18,19]。四醚类化合物也需要断裂醚键,转变为支链烷烃来进行单体碳同位素分析[20,21,22]。最近,Pearson等[23]建立了一种能够直接测试完整四醚膜脂(Glycerol Dialkyl Glycerol Tetraether Lipids,GDGTs)碳同位素组成的新方法。该方法已被用于格陵兰湖泊沉积物中完整GDGTs单体碳同位素组成的研究[24]。Hemingway等[25]建立了一种可以直接测试完整的细菌藿多醇(Bacteriohopanepolyols,BHPs)单体碳同位素组成的新方法。

2 高等植物脂类

高等植物能够合成种类众多的脂类,包括长链正构烷烃、正构脂肪酸和脂肪醇,非藿五环三萜类(下文中简称为三萜类),以及一些C29甾醇[26,27,28]。其中长链正构脂类(包括烷烃,脂肪酸和脂肪醇,碳链长度>C21)主要分布于叶片的表皮蜡层,因而常常被称为叶蜡脂类。实际上,长链正构脂类也少量地分布于其他器官的表皮中[29,30]。三萜类主要分布于常绿阔叶树的叶片和树皮等中[31]。前期已有文章总结了高等植物脂类组成和含量在生物体中的分布特征[32,33,34]

高等植物脂类中的碳原子来源于植物光合作用固定的CO2,它们的碳同位素组成会继承光合作用过程中的碳同位素分馏值(通常被记做Δleaf[33]和脂类生物合成中的同位素分馏值(εlipid[33],因而能够记录相应的温度、干湿等环境条件和植物生理过程的变化[33,34]

2.1 长链正构烷烃

长链正构烷烃单体碳同位素组成(δ13Calk)已经得到了广泛的研究和应用。在大多数情况下,δ13Calk信号被解释为C3与C4植物相对比率的变化[35,36,37]。在光合作用过程中,C3与C4植物采用不同的固碳途径,并表现出有显著差异的碳同位素分馏系数。相对于C4植物,C3植物合成的有机物具有更偏负的碳同位素值(典型的C3植物总有机碳同位素值为-25‰~-27‰,而C4植物典型值为-11‰~-12‰)[38]。从光合作用产物到叶蜡正构烷烃,会发生进一步的碳同位素分馏,使得陆生C3植物叶蜡烷烃的δ13C均值为-33.1‰,而陆生C4植物叶蜡烷烃的δ13C均值为-21.7‰[39]

在纯C3植物或C4植物占比非常少的环境中,例如森林和淡水沼泽,δ13Calk变化通常被解释为相对湿度变化[40,41]。其中的机制是,大气湿度调控叶片气孔的开闭程度,进而影响到大气CO2向叶片内部的扩散速率。一般来讲,在湿润条件下,如热带雨林,光合作用过程中碳同位素的分馏系数受CO2扩散过程的影响较弱,因而Rubisco酶的分馏效应起到主导作用,合成的有机质表现出更偏负的碳同位素值。前期研究显示,叶片光合作用碳同位素分馏值(Δleaf)与降雨量之间存在显著的相关性[42]。在一项新近的美国亚热带Lake Tulane末次冰期古生态研究中,Arnold等[43]基于湖泊沉积物中长链烷烃δ13C值估算了Δleaf值。结果发现,在Heinrich事件2~4时期,Δleaf处于高值,而长链烷烃δD值处于低值,揭示出了末次冰期冷事件时的冷湿的气候特征。除了相对湿度,δ13Calk值还受CO2浓度及其碳同位素组成、温度、植物生活型等多种因素的影响[33,44]

在解译湖沼沉积物中δ13Calk信号时,还需要考虑从枯枝落叶堆积到被流水搬运等过程中的微生物降解及潜在的微生物输入[33,45,46,47]。由于微生物偏向于利用轻碳,有机质在经过微生物降解作用后,某些易被降解的脂类单体碳同位素组成可能会发生偏正。此外,在搬运过程中微生物自身产生的有机质输入也可能会对单体碳同位素的解译产生影响,特别是一些自养微生物产生的脂类可能具有比较偏负的碳同位素组成。气溶胶传输也可能是湖泊沉积物中保存的陆生叶蜡烷烃的一种潜在输入途径[48,49]

对于湖泊等沉积场所中长链正构脂类的研究,还需要注意原地生物贡献的影响。通过对青海湖水生植物和表层沉积物中长链脂类的研究,Liu等[50]提出水生的藻类和沉水植物也能贡献长链正构脂类,这些水生植物产生的长链正构烷烃的δ13Calk值非常接近陆地C4植物的特征,因而会对评估流域内C4植物的比率产生干扰。值得关注的是,湖泊内源贡献的长链脂类主要是C29及以下正构烷烃、C30及以下正构脂肪酸,而C31及以上碳链的正构烷烃主要是流域内陆生植物的贡献,因此,作者提出C31正构烷烃的δ13Calk值可以更准确地反映陆生植物对气候环境变化的响应[50]。此外,在一些湖泊中,水生生物还有可能利用来自甲烷的碳,使得合成的正构烷烃δ13Calk值显著偏负[40]。因此,对于湖沼沉积序列,在解释古气候和古环境变化时,要充分考虑沉积环境的多样性,具体问题具体分析。

在泥炭地中,来自泥炭藓的脂类的碳同位素组成可以成为研究泥炭藓与微生物共生活动的有力助手。泥炭藓是主要的成炭植物,大多数泥炭藓种是以C23正构烷烃为主峰,其次是C25正构烷烃[51,52]。泥炭藓体内共生的嗜甲烷细菌能将泥炭与大气界面附近的甲烷氧化成CO2,可供泥炭藓进行光合作用[53]。由于泥炭地中生物成因的甲烷具有显著偏负的碳同位素组成(<-50‰),如果泥炭藓光合作用利用一定比率的来自甲烷的碳,将会在其脂类的碳同位素组成上留下印迹[54,55,56,57]。一些研究估计,泥炭藓光合作用固定的CO2约为10%,甚至高达30%,来自甲烷氧化[58]。这种认识也被用于重建全新世泥炭藓泥炭地中甲烷氧化活动的变化[54,55]

在大多数环境中,高等植物都利用大气中的CO2进行光合作用。在一些富碳环境中,例如,泥炭地,水位降低会造成大量的有机碳暴露在大气中,暴露的有机质会被分解为CO2,进而排放到大气中。这些分解产生的CO2在土—气界面传输过程中,有可能被地表植物利用来进行光合作用。这种老碳利用的方式并未得到重视[59,60]。在神农架大九湖开展的一项研究显示,该泥炭地在中全新世遭遇了显著的干旱事件[61]。对应于干旱事件,来自高等植物的长链正构烷烃的碳同位素组成并没有表现出相应的正偏,而是出现了显著的负偏。负偏的幅度随着干旱次数的增加而变大(图1)[62]。这种严重干旱时发生的δ13Calk值显著负偏的现象,有可能是因为植物利用了再循环的CO2,其δ13C值比大气CO2更偏负。这种作用机制在一定程度上减缓了因泥炭有机质分解向大气排放的CO2量。同时,这项研究透露出了叶蜡脂类单体碳同位素在揭示生物地球化学过程对气候变化响应方面的研究潜力。

图1

图1   神农架大九湖ZK-5泥炭柱记录的C29长链正构烷烃δ13C值对中全新世干旱的响应

Fig. 1   Response of the δ13C values of n-C29 alkane to the mid-Holocene drought in the ZK-5 peat core, Shennongjia

(a)大九湖ZK-5泥炭柱C29正构烷烃δ2H[62]; (b) 大九湖ZK-3泥炭柱藿类通量[61]; (c)和尚洞HS-4石笋磁学参数[63]; (d) 大九湖ZK-5泥炭柱C29正构烷烃δ13C [62]

(a) δ2H of n-C29 alkane in ZK-5 peat core from Dajiuhu peatland[62]; (b) Hopanoid flux in ZK-3 peat core from Dajiuhu peatland[61];

(c)Magnetic parameter in HS-4 stalagmite from Heshang cave[63]; (d) δ13C of n-C29 alkane in ZK-5 peat core from Dajiuhu peatland[62]


2.2 脂肪酸

相对于长链正构烷烃,长链正构脂肪酸(δ13Cfa)和脂肪醇的单体碳同位素组成研究很少,尤其是脂肪醇单体碳同位素组成。从叶蜡中脂类组成来看,叶片中往往含有更高含量的长链正构烷烃[64]。Chikaraishi[34]汇总了目前已经发表的陆生C3植物中各种叶蜡脂类的δ13C值分布范围。在沉积序列研究中,一些研究组专注于δ13Cfa,而另一些研究组侧重于δ13Calk。在同一套沉积样品中,δ13Cfa和δ13Calk的响应是否一致,这是值得关注的问题,目前这方面的对比研究还十分欠缺。这涉及到不同脂类碳同位素序列的对比问题,也可能从不同脂类碳同位素的差异上挖掘出更多的环境或生态信息。

在湖沼相沉积环境中,脂肪酸单体碳同位素组成可以在一定程度上反映有机质的来源。例如,在若尔盖沼泽的几项研究中,发现泥炭沉积物中C16-C28脂肪酸的δ13C平均值(-33.7‰)与草本和木本植物中脂肪酸的δ13C平均值十分接近,表明泥炭中的这些脂肪酸主要来源于高等植物[65,66];而泥炭沉积物中的C14和C15脂肪酸具有明显偏负的碳同位素组成,分别为-38.7‰和-37.2‰,表明这些短链脂肪酸可能来源于化能自养细菌[65,67]。同时,还发现若尔盖沼泽中草本植物(-33.4‰)的脂肪酸δ13C较木本植物(-32.5‰)略微偏负,与之相似的是草本植物的正构烷烃δ13C较木本植物也偏负,这可能是由于不同植物在生长过程中碳同位素分馏的差异所致,也可能是由于草本植物和木本植物的生长高度不同,草本植物紧贴地面生长导致吸收更多偏负的CO2[66]。与之相似,von Dongen等[68]发现荷兰Bargerveen泥炭地中的狭叶泥炭藓Sphagnum cuspidatum的C16脂肪酸具有较为偏负的碳同位素组成(-35.0‰),而同样生长在泥炭地中的杜鹃花科植物Erica tetralix根中的C16脂肪酸δ13C略微偏正(-28.8‰)。在一项采用2种不同纯化方法对比植物和泥炭脂肪酸δ13C的研究中,发现C4植物狗尾草(Setaira viridis)的C16脂肪酸δ13C比C3植物三叶草(Trifolium repens)明显偏正[69],这与C3和C4植物叶蜡正构烷烃δ13C的差异类似。总体上,在沼泽环境中不同植被来源的脂肪酸δ13C的大致变化规律为:泥炭藓<C3草本植物<C3木本植物<C4植物,而一些来源于自养细菌的脂肪酸δ13C可能更加偏负。

相对于沼泽环境,湖泊等沉积环境中脂肪酸δ13C在反映有机质来源上可能更加复杂。例如,在美国佐治亚州Altamaha河口沉积物中脂肪酸分布及其δ13C的研究中,发现长链脂肪酸δ13C比较偏负,被认为主要来源于陆源输入,而短链脂肪酸δ13C比较偏正,被认为主要来源于海洋浮游生物[70]。王丽芳等[71]对巢湖沉积钻孔中结合态脂肪酸的组成及其单体碳同位素进行了研究,发现湖泊内源的藻类和细菌是C16脂肪酸的主要贡献者,并且沉积柱从下到上C16脂肪酸δ13C值的逐渐偏正反映了湖泊的富营养化。陶舒琴等[72]分析了黄河悬浮颗粒物中脂肪酸的δ13C特征,发现长链脂肪酸(C28,C30)的δ13C变化范围为-31.1‰ ~ -32.2‰,表明其主要来源于以C3植被覆盖为主的黄土土壤有机质,而δ13C比较偏正的短链脂肪酸(C16,C18)可能主要来源于水生藻类和异养微生物。

除了反映有机质来源,脂肪酸单体碳同位素组成在反演古植被与古气候变化中也具有一定的潜力。例如,Hughen等[73]利用南美洲北部Cari​​aco盆地沉积物中的长链脂肪酸δ13C和平均碳链长度(Average Chain Length,ACL),重建了该地区末次冰消期的古植被变化,发现北大西洋热带地区和高纬度地区的气候变化是同步的,且热带植被变化响应气候变化具有几十年的滞后。在最近的一项关于日本富士山东北部Yamanaka湖泊沉积物叶蜡长链脂肪酸分布和单体碳同位素组成的研究中,Yamamoto等[74]发现15 000年以来C30脂肪酸δ13C的偏负对应于脂肪酸ACL的减小,表明植物在湿润条件下对可利用水增多时的生理反应,这与湿润条件下植物叶蜡正构烷烃δ13C的偏负类似;同时全新世的C30脂肪酸δ13C显示出与董哥洞石笋δ18O类似的波动趋势,指示了这一地区的水文气候的变化与季风活动有关。

2.3 三萜类

除了长链正构脂类,被子植物中通常含有较高含量的三萜类化合物,主要有奥利烷、乌散烷和羽扇烷等3个系列[75]。这些三萜类化合物被认为是植物体抵抗病虫害的一种方式[76]。在沉积物中,三萜类化合物可以被用来指示被子植物的输入,以区别来自裸子植物的二萜类化合物[31,77]。在成岩作用过程中,三萜类化合物会受到微生物改造,转变为各种三萜烯。沉积环境的相关信息,特别是与水文相关的氧化还原状况,会被三萜烯的组成记录下来,成为古环境重建的潜在指标[78,79,80]

三萜类和叶蜡脂类都来自高等植物,在响应气候变化方面,它们的碳同位素组成是表现出相似还是差异的响应是值得关注的科学问题。不过,相关的研究还非常少。一项在非洲东部Lake Challa开展的研究显示,从末次冰盛期以来,湖泊沉积柱中C31正构烷烃和脱A-环羽扇烷(羽扇烷的一种降解产物)的碳同位素表现出了非常不一样的变化趋势[81]。后者的变化非常平缓,而前者则表现出了明显的变化,在末次冰消期C31正构烷烃的碳同位素组成相对偏正,到了全新世,2种脂类的碳同位素组成的变化则非常相似。对于这种情况,作者解释为脱A-环羽扇烷主要来自C3植物,而C31正构烷烃同时来自C3和C4植物。基于此,作者提出可以将主要来自C3植物的脱A-环羽扇烷的δ13C值作为对应时段的C3植物δ13C值的端元值,由此可以更为准确地估算流域内C4植物的比率[81]。这其中需要考虑合成途径差异而造成的正构烷烃和三萜类碳同位素的差值[31,82]。由此可见,综合多种脂类的碳同位素组成能够更准确地揭示古生态和古环境的变化,还有可能发掘出更多的环境信息。

结合三萜类和长链正构烷烃的碳同位素组成,还被用来更好地认识古新世和始新世之间极热事件(Paleocene-Eocene Thermal Maximum,PETM)不同碳组分碳同位素负偏移程度差异的原因[83]。三萜类主要来自被子植物,而长链正构烷烃有被子植物和裸子植物的贡献。研究者还利用三萜类的碳同位素组成来探讨植物光合作用的碳同位素分馏系数是否在PETM时期表现出显著的特异性[84]

3 微生物脂类

微生物是陆地生态系统的重要组成部分。在改造植物有机质的同时,微生物输入自身合成的脂类。在微生物体中,脂类主要分布在细胞膜中,包括磷脂、GDGTs和藿类等。一方面,某些脂类只来源于特定的微生物类别或代谢途径,因此可以直接用来指示物源或代谢过程[2,6,85]。另一方面,得益于微生物活动对环境变化的灵敏响应,基于微生物的脂类指标已经成为湖沼沉积古环境和古生态重建的重要工具[5,86,87]

对于自养微生物,其脂类δ13C值主要受控于无机碳源的δ13C值与供应量、固碳途径和微生物的生长阶段[88]。异养微生物主要利用已有的有机物进行自身的生长。传统上认为,异养微生物生物量的δ13C值非常接近于其摄取的有机物的δ13C值[82]

3.1 磷脂脂肪酸

脂肪酸是构成生物体的重要脂类成分,在生物体内脂肪酸通常通过酯键与其他基团连接,例如脂肪酸与磷酸基团构成的磷脂脂肪酸(Phospholipid Fatty Acids,PLFAs)是生物细胞膜的主要成分。PLFAs种类十分丰富,除直链饱和脂肪酸外,还有支链饱和、单不饱和、多不饱和以及环丙基等各种结构,一些特定结构的PLFAs来源于特定的微生物。

PLFAs技术已经被广泛应用于各种环境中活体微生物生物量的定量测量,反映微生物群落结构的变化等[89,90,91,92]。在此基础上,PLFAs单体碳同位素(δ13CPLFA)可以反映在原位生长的微生物碳同位素组成的细节,提供微生物利用的碳和能源的信息[93],深入揭示微生物参与的有机质代谢过程,将特定的微生物种群与土壤过程联系起来[94,95,96]。Cowie等[97]对加拿大安大略省北部酸性矿坑水中的微生物进行培养,利用PLFA单体碳同位素揭示了自养和异养微生物在碳源利用过程中的同位素分馏差异。Mills等[93]研究了日本一处矿井沉积物和地下水中PLFAs的单体碳同位素特征,发现指示II型甲烷氧化菌的PLFA181ω8c的δ13C值在沉积物和地下水中明显不同,表明甲烷氧化菌在2种环境中可能采用不同的碳同化途径,并且地下水中δ13CPLFA值更加偏负,指示了可能由化能自养作用主导的微生物群落。Brady等[98]研究了加拿大西部Pavilion湖泊中淡水微生物的PLFA丰度和单体碳同位素组成,发现不同种类的PLFA的δ13C与总有机碳δ13Corg之间的差值Δδ13CPLFA-org有明显差别,可以指示自养或者异养微生物过程,并且利用PLFA单体碳同位素揭示了湖泊中不同位置微生物自养和异养过程的差异。

除了探究天然环境中PLFAs的单体碳同位素特征,许多研究还采用13C标记配合微生物培养的手段研究各种碳源底物在微生物中的代谢方式。Huguet等[99]对泥炭进行有氧与厌氧条件下的培养,对比了泥炭中细菌来源的脂肪酸和支链甘油二烷基甘油四醚(Branched Glycerol Dialkyl Glycerol Tetraether,bGDGT)脂类的生产率,并通过13C和氘(2H)同位素标记的方法,识别出2种支链PLFA与bGDGT具有相似的生产模式,为解决沉积环境中bGDGT的生物来源提供了重要信息。Veuger等[100]向潮汐沉积物中加入13C标记的葡萄糖和15N标记的铵,探究有机质降解过程中的可水解氨基酸,单糖和脂肪酸的变化,发现磷脂脂肪酸和总脂肪酸具有相似的变化:在前期标记阶段没有13C的损失,而在后期阶段13C损失比较快,表明这2种来源的脂肪酸的不稳定性非常相似。同时根据13C的富集程度识别出181ω7c,a150和i150 3种相对抗降解的脂肪酸,而在沉积物中藻类物质的降解过程中也观察到这3种脂肪酸的产生。

3.2 藿类

藿类主要分布在细菌的细胞膜中,充当着类似于真核生物细胞膜中甾醇的功能[101]。藿类化合物种类众多,前驱物主要有C35的细菌藿四醇及衍生物、C30的里白醇。多种类型的细菌可以合成藿类化合物,包括甲烷氧化菌、蓝细菌、固氮菌和紫色非硫细菌等[102,103,104,105,106,107]。Pearson等[108]汇总了藿烷环化酶(控制藿类合成的一种关键酶)的基因数据,发现只有10%的细菌包含藿烷环化酶基因。另外还有少量的真核生物中发现了藿类化合物,如高等植物、蕨类植物、苔藓、地衣和真菌,这些真核生物中的藿类化合物大多是二级代谢产物,由共生或者寄生的微生物所产生[109]

藿类广泛地分布于泥炭和湖泊沉积物中。在酸性泥炭地中,C31 αβ构型藿烷是含量最高的藿烷化合物,在一些样品中,其含量超过了正构烷烃主峰碳的含量[110]。基于对大九湖泥炭地藿烷环化酶基因[111]以及现代环境中藿类总量与水位关系的认识,Xie等[61]提出泥炭沉积中藿类主要由好氧细菌合成,藿类化合物的沉积通量可以用来反演泥炭地水位的变化。即在高水位条件下,泥炭表层处于淹水状态,好氧细菌活动受到抑制,产生的藿类总量非常低;相反,在低水位条件下,泥炭表层处于通氧状态,适合好氧细菌的繁殖,能够产生较高含量的藿类。

研究者对泥炭沉积中藿烷的碳同位素进行了研究。结果发现,处于显著地理隔离的不同泥炭地,C31 αβ构型藿烷的δ13C值在表层泥炭中非常相似(约-26‰),相对总有机碳同位素组成显著偏正,被认为是利用了相对富集13C的糖类进行异养生长(图2[20,62,112]。在大九湖的ZK-5泥炭柱中,C31 αβ藿烷的δ13C值在末次冰消期相对偏负(约2‰)[62]。不同于C31 αβ藿烷,C29 ββ藿烷的δ13C值变化幅度更大,最负时接近-40‰,指示出可能有来自嗜甲烷细菌的贡献(图3[62]。在PETM时期的褐煤中,研究者观察到C29 ββ藿烷的δ13C值发生了显著的负偏,最低值接近-80‰,负偏的幅度大于C31 ββ藿烷的δ13C值[113]。泥炭藓提取的脂类中,裂解产生的C32 ββ藿酸的δ13C值约为-39.8‰,比泥炭藓自身的有机质的碳同位素组成偏负,很可能来自内共生的嗜甲烷细菌[53]。从泥炭藓中提取的藿类的δ13C值表现出随变干而增大的趋势[19]。在红原泥炭地全新世泥炭沉积中,里白烯δ13C值在中全新世干旱阶段显著负偏(<-50‰)[114]。已有的研究充分体现出了利用藿烷单体碳同位素来研究湿地生态响应的优越性。

湖泊沉积中的藿类单体碳同位素组成也得到了一定程度的关注,不过主要是集中于表层样品中,缺乏连续的时间序列研究。在湖泊环境中,尤其是青藏高原湖泊和高纬湖泊中,里白烯δ13C值普遍显著偏负[115,116,117,118,119,120,121]。例如在青藏高原寇查湖末次冰消期以来的沉积物中,里白烯δ13C值最负可达-62.7‰[122],被认为与湖泊环境中的嗜甲烷过程有关联。

图2

图2   泥炭沉积中生物体、糖类和脂类δ13C分布示意图(据参考文献[88]修改)

Fig. 2   Schematic diagram showing the relationship between organic carbon constituents of plants and heterotrophic bacteria in peat deposits (modified after reference[88])


图3

图3   神农架大九湖ZK-5泥炭柱中藿烷和C29正构烷烃δ13C序列对比[62]

Fig. 3   Comparisons of δ13C records in the ZK-5 peat core retrieved from the Dajiuhu peatland, Shennongjia[62]

(a) C29 ββ藿烷; (b)C31 ββ藿烷; (c)C31 αβ藿烷; (d) C29正构烷烃

(a) C29 ββ hopane; (b)C31 ββ hopane; (c) C31 αβ hopane;

(d) n-C29 alkane


3.3 GDGTs

GDGTs是近些年来得到广泛关注的生物标志物,基于GDGTs建立了多种古气候指标[83],可以用来探讨陆地生态系统与地球环境的协同演化[123]。根据结构的差异,可分为类异戊二烯类(isoprenoid Glycerol Dialkyl Glycerol Tetraether,iGDGT)和支链GDGT(branched Glycerol Dialkyl Glycerol Tetraether,bGDGT)。前者来自古菌,而后者由生源尚未确定的一类细菌产生。单体碳同位素的研究为探索bGDGT的生物来源提供了有用的线索[21,99,124]。根据相对偏正的δ13C值以及在土壤剖面中的分布,bGDGT的生物源被推测为生活在厌氧条件下的异养细菌[21]。在一些湖泊环境中,研究者观察到了非常负偏的bGDGT δ13C值,被认为摄取了来自甲烷氧化细菌的有机质[24,124]

3.4 其他

除了上述类别的微生物脂类,来自颗石藻的长链烯酮化合物的碳同位素也得到了广泛研究。主要集中在海相沉积物中,被用来重建新生代大气CO2浓度的变化[125]。在少数的几项陆地湖泊烯酮单体碳研究中,烯酮δ13C值表现出较为偏负的特征。例如在南极Ace Lake中,烯酮δ13C值范围为-31.7‰~-36.1‰[126];而在格陵兰湖泊中,烯酮δ13C值约为-40‰[127]。这种较负的碳同位素特征,被解释为较低的湖水温度下碳同位素分馏系数增大,或者是利用了来自有机质分解产生的CO2

一项在非洲湖泊开展的研究,探讨了末次间冰期湖泊沉积物中来自葡萄球藻的葡萄球藻烯的δ13C变化[128]。在湖泊沉积物中,短链异构烷烃(主要是C17和C18)主要来自蓝细菌[129]。来自蓝细菌的这些短链烷烃的δ13C值通常表现出较大的变化范围[120,130,131],例如,在CO2供应受限的碱性淡水水体中,来自蓝细菌的n-C17烷的δ13C值会显著偏正[130]

4 研究展望

伴随着单体碳分析仪器的日益推广,近30年来的研究已经充分揭示出,湖沼沉积物中脂类单体碳同位素组成,尤其是微生物脂类的单体碳同位素组成,具有非常重要的古环境和古生态应用潜力。在今后的发展中,以下几个方面值得重视:

(1)重视实验技术。目前主流的单体碳测试仪器要求化合物的量约数十微克,这接近常规色谱柱的容量上限,容易造成色谱峰不规则,影响峰分离,并降低测试精度。另一方面,较高的浓度要求使得很多样品量较小的样品无法进行准确地单体碳分析。因此,需要建立小样品量的单体碳测试方法。对于化合物组成复杂的样品,需要建立目标化合物分离纯化的方法,除了常规的分子筛、尿素络合方法,可以考虑制备气相色谱和制备液相色谱法。对于分子量相对较大的脂类(如GDGTs,BHPs),需要开发并推广能够直接测试其碳同位素组成的新技术[24,25]。单体放射性碳同位素也在得到日益广泛地重视[132,133],在年代学、陆地有机质搬运等领域取得了重要的进展[134,135,136]

(2)在关注单个化合物的碳同位素组成的基础上,可以考虑联合多种脂类的碳同位素值,利用它们指示的不同生态系统成员的优势,从更多的侧面去认识生态系统对气候变化的响应过程和机制。这方面已有一些探索性的研究,例如非洲湖泊中长链正构烷烃和脱A-环羽扇烷单体碳的对比研究。在泥炭沉积中,此方面尤其值得重视。泥炭有机质含量非常高,包含有来自高等植物的各种脂类和来自微生物的多种脂类,可以从碳同位素角度深入地刻画泥炭地生态系统对气候变化的生态响应[88]

(3)同一脂类碳同位素和氢同位素的联合也是值得考虑的方向。叶蜡脂类的单体碳和氢同位素组成会受到类似环境条件的控制,因此联合2种同位素比值可以更好地识别环境控制条件。Feakins等[137]在南美洲亚马逊流域开展的一项研究中,利用正构烷烃和正构脂肪酸碳同位素和氢同位素的联合,能够更好地识别河流颗粒物中有机质的来源。

(4)微生物脂类的碳同位素组成通常和代谢途径有关,由此赋予了微生物脂类单体碳同位素被用来揭示生物地球化学过程对过去气候变化响应研究的优势。在当今全球变暖背景下,陆地生态系统如何响应是非常值得关注的重要科学问题。利用湖泊和泥炭沉积中保存的脂类的碳同位素特征来研究过去生态响应,可以为预测未来气候条件下的生态系统响应提供借鉴。

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