Interaction between Carbon and Sulfur Cycles in Antarctic Stratified Lakes and Fjords

---- Public Summary from Project ----The lakes and fjords of the Vestfold Hills region of Antarctica provide unique ecosystems for studying environmental changes in Antarctica over the past 8000 years. Studies of the changes in organic matter composition in sediment cores provide information how the microbial and plankton communities have changed over time in response to varying chemical and physical conditions. Our study will provide new information about how the cycles of the biologically-important elements carbon and sulfur are linked and why some sediments can preserve large amounts of organic carbon. This information will be useful for studies of palaeoclimate and will also provide valuable insights into the processes that produce petroleum source rocks.From the abstracts of the referenced papers:Preserved ribosomal DNA of planktonic phototrophic algae was recovered from Holocene anoxic sediments of Ace Lake (Antarctica), and the ancient community members were identified based on comparative sequence analysis. The similar concentration profiles of DNA of haptophytes and their traditional lipid biomarkers (alkenones and alkenoates) revealed that fossil rDNA also served as quantitative biomarkers in this environment. The DNA data clearly revealed the presence of six novel phylotypes related to known alkenone and alkenoate-biosynthesising haptophytes with Isochrysis galbana UIO 102 as their closest relative. The relative abundance of these phylotypes changed as the lake chemistry, particularly salinity, evolved over time. Changes in the alkenone distributions reflect these population changes rather than a physiological response to salinity by a single halophyte. Using this novel palaeo-ecological approach of combining data from lipid biomarkers and preserved DNA, we showed that the post-glacial development of Ace Lake from freshwater basin to marine inlet and the present-day lacustrine saline system caused major qualitative and quantitative changes in the biodiversity of the planktonic populations over time. Post-glacial Ace Lake (Vestfold Hills, Antarctica), which was initially a freshwater lake and then an open marine system, is currently a meromictic basin with anoxic, sulfidic and methane-saturated bottom waters. Lipid and 16S ribosomal RNA gene stratigraphy of up to 10,400-year-old sediment core samples from the lake revealed that these environmentally induced chemical and physical changes caused clear shifts in the species composition of archaea and aerobic methanotrophic bacteria. The combined presence of lipids specific for methanogenic archaea and molecular remains of aerobic methanotrophic bacteria (13C-depleted delta8(14)-sterols and 16S rRNA genes) revealed that an active methane cycle occurred in Ace Lake during the last 3000 calendar years and that the extant methanotrophs were most likely introduced when it became a marine inlet (9400 y BP); rDNA sequences showed 100% sequence similarity with Methanosarcinales species from freshwater environments and were the source of sn-2- and sn3-hydroxyarchaeols. Archaeal phylotypes related to uncultivated Archaea associated with various marine environments were recovered from the present-day anoxic water column and sediments deposited during the meromictic and marine period.

——项目公开摘要——
南极洲维斯特福尔德丘陵（Vestfold Hills）区域的湖泊与峡湾，为研究过去8000年间南极的环境变化提供了独特的生态系统研究载体。对沉积岩芯中有机质组成变化的分析，能够揭示微生物与浮游群落如何随时间推移，响应化学与物理条件的波动而发生演替。本研究将阐明具有重要生物学意义的碳、硫元素循环之间的关联机制，以及部分沉积物能够大量保存有机碳的原因。该研究成果不仅可服务于古气候（palaeoclimate）研究，还能为油气源岩（petroleum source rock）的形成过程提供极具价值的理论洞察。

参考文献摘要如下：
研究团队从艾斯湖（Ace Lake，南极洲）的全新世（Holocene）缺氧沉积物（anoxic sediment）中成功提取了浮游光合藻类（planktonic phototrophic algae）的核糖体DNA（ribosomal DNA, rDNA），并通过序列比对分析（comparative sequence analysis）鉴定出了该古老群落的组成成员。定鞭藻门（haptophytes）的DNA与传统脂类生物标志物（lipid biomarker）——烯酮（alkenone）和烯酸酯（alkenoate）——具有相似的浓度分布特征，表明化石核糖体DNA在该环境中同样可作为定量生物标志物（quantitative biomarker）使用。DNA数据明确显示存在6个与已知产烯酮和烯酸酯的定鞭藻相关的新系统型（phylotypes），其近缘物种为等鞭金藻UIO 102（Isochrysis galbana UIO 102）。随着湖泊化学环境（尤其是盐度salinity）随时间演化，这些系统型的相对丰度发生了变化。烯酮分布的变化反映的是种群结构的改变，而非单一盐生植物对盐度的生理响应。

通过结合脂类生物标志物与化石核糖体DNA的新型古生态研究方法（palaeo-ecological approach），研究团队证实：艾斯湖的冰后演化（post-glacial development）过程（从淡水盆地freshwater basin演变为海洋入口marine inlet，最终形成现今的湖泊咸水系统lacustrine saline system）使得浮游种群的生物多样性（biodiversity）在时间尺度上发生了显著的定性与定量变化。

位于南极洲维斯特福尔德丘陵的艾斯湖在冰后期最初为淡水湖泊，随后演变为开放海洋系统，现今则为分层滞水盆地（meromictic basin），其底层水体（bottom water）处于缺氧、含硫化物且甲烷饱和的状态。对该湖距今10400年的沉积岩芯（sediment core）样本开展脂类与16S核糖体RNA基因（16S ribosomal RNA gene, 16S rRNA基因）地层学（stratigraphy）研究后发现，环境诱导的化学与物理变化导致古菌（archaea）与好氧甲烷氧化菌（aerobic methanotrophic bacteria）的物种组成发生了明显偏移。

产甲烷古菌（methanogenic archaea）特异性脂类与好氧甲烷氧化菌的分子遗迹（molecular remains）——δ13C亏损的Δ8(14)-甾醇（13C-depleted δ8(14)-sterols）与16S rRNA基因——的共同存在表明，在过去3000个日历年中，艾斯湖存在活跃的甲烷循环（methane cycle）；而现存甲烷氧化菌（extant methanotroph）极有可能是在艾斯湖演变为海洋入口时（距今9400年）被引入的。核糖体DNA序列显示，其与淡水环境中的甲烷八叠球菌目（Methanosarcinales）物种具有100%的序列相似性，且该类群是sn-2羟基古菌醇（sn-2-hydroxyarchaeol）与sn-3羟基古菌醇（sn-3-hydroxyarchaeol）的来源。

研究团队从现今的缺氧水柱（anoxic water column）以及分层滞水阶段与海洋沉积阶段的沉积物中，分离得到了与多种海洋环境相关的未培养古菌（uncultivated Archaea）系统型。