Collaborative Research: Were Protists the Beginning of the End for Stromatolites?

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。微生物垫是许多底栖海洋和水生环境的显著组成部分。这些微生物垫的一个子集结合沉积物形成潜在的可石化结构，通常称为叠层石或微生物岩。虽然对微生物自养生物了解很多，但对它们的异养真核生物知之甚少。考虑到叠层石在地球大部分历史中都有广泛的地质记录，人们对叠层石缺乏了解是令人惊讶的。叠层石是由微生物活动、非生物碳酸盐沉淀和沉积过程共同作用形成的层状沉积结构。叠层石形成和保存的细节知之甚少，而前寒武纪晚期叠层石的出现和多样性的急剧下降一直是一个难题。一个流行的假说解释了大约10亿年前这种衰退，真核生物进化成了叠层石的捕食者。迄今为止，最普遍提出的掠食性罪魁祸首是一种身份不明的后生动物，尽管化石记录中缺乏这种生物的证据。原生生物，其中大多数预计不会留下明显的化石记录，是另一种可能的叠层石捕食者，但在这种情况下，它们在很大程度上被忽视了。本项目的假设是：(1)异养原生生物的活动导致叠层石（层状沉积物织物）向血栓（凝块沉积物织物）的结构变化；(2)异养原生生物导致新元古代叠层石的大量灭绝。由于不可能重建新元古代，对现代类似物的研究有助于间接检验这些假设。该项目的总体目标是描述与来自巴哈马和澳大利亚的现代叠层石和血栓石相关的真核生物群落，比较两个地点的群落，并将群落与叠层石/血栓沉积结构和生物标志物特征联系起来。总体目标将通过解决以下具体目标来实现：(1)通过形态学和分子方法确定现代叠层石和凝块的真核生物群落；(2)利用溶剂萃取、色谱和质谱方法分析现代和化石叠层石和凝块的真核脂质生物标志物；(3)利用荧光标记嵌入核（FLEC）方法，记录了居住在现代叠层石和凝块中的异养真核生物群落与细尺度沉积物结构的亚毫米分布；(4)采用溶剂萃取、色谱、质谱等方法，对异异体有孔虫培养物进行分析，寻找异异体有孔虫特有的脂质生物标志物；(5)现代叠层石与异养原生生物孵育后，利用FLEC方法确定它们的活动对沉积物结构的影响，并与叠层石化石的沉积物结构进行初步比较。知识价值：最古老的叠层石化石有34亿年的历史，是早期地球上普遍存在的微生物生命最明显的表现。叠层石丰度和形态的变化记录了生物和地质过程之间复杂的相互作用。该项目涉及叠层石成因和石化前蚀变的多个方面，但其核心是关注最大的地质谜团之一：叠层石的衰落与复杂生命的兴起之间的可能联系。更广泛的影响：因为任何本科历史地质学入门课程（以及一些初中和高中的地球科学课程）都将叠层石作为地球上生命的第一个高度可见的证据，所以这个项目的结果将会引起广大观众的兴趣。这个多学科项目涉及底栖生态学、分子生物学、沉积学和有机地球化学，包括从高中到研究生和教师的教育机会。该项目将支持一名WHOI-MIT联合项目学生一半的博士学位学习，以及一名MIT学生一半的博士学位学习。此外，来自麻省理工学院、康涅狄格大学和迈阿密大学的本科生将积极参与我们的一些实地收集和实验室分析。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Microbial mats are conspicuous components of many benthic marine and aquatic settings. A subset of these microbial mats binds sediments to form potentially fossilizable structures, often called stromatolites or microbialites. While much is known about microbialite autotrophs, little is known about their heterotrophic eukaryotes. The lack of understanding is surprising given that stromatolites have an extensive geologic record spanning most of Earth's history. Stromatolites are layered sedimentary structures formed by a combination of microbial activities, abiotic carbonate precipitation, and sedimentary processes. Details of stromatolite formation and preservation are poorly understood, and a drastic decline in stromatolite occurrence and diversity in the late Precambrian has long been a conundrum. A popular hypothesis to explain this decline at ~1 billion years ago is that eukaryotic organisms evolved to become predators on stromatolites. To date, the most commonly proposed predatory culprit is an unidentified metazoan, although evidence of such an organism is lacking from the fossil record. Protists, most of which are not expected to leave an obvious fossil record, are additional possible stromatolitic predators, but they have been largely ignored in this context. The hypotheses of this project are: (1) Heterotrophic protist activity caused the textural change from stromatolites (layered sediment fabric) to thrombolites (clotted sediment fabric) and (2) Heterotrophic protists caused the decimation of Neoproterozoic stromatolites. Since it is impossible to recreate the Neoproterozoic, studies of modern analogs serve to indirectly test these hypotheses. The overall goal of this project is to describe the eukaryotic communities associated with modern stromatolites and thrombolites from the Bahamas and Australia, compare the communities from the two sites, and to relate the communities to stromatolitic / thrombolitic sediment fabric and biomarker signatures. The overall goal will be achieved by addressing the following specific aims: (1) Identify, via morphologic and molecular approaches, the eukaryotic community of modern stromatolites and thrombolites; (2) Analyze modern and fossil stromatolites and thrombolites for their eukaryotic lipid biomarkers using solvent extraction, chromatographic and mass spectrometric methods; (3) Using the Fluorescently Labeled Embedded Core (FLEC) method, document the sub-millimeter distributions of the heterotrophic eukaryotic community inhabiting modern stromatolites and thrombolites in conjunction with fine-scale sediment fabric; (4) Using solvent extraction, chromatographic and mass spectrometric methods, analyze cultures of allogromiid foraminifers to survey for lipid biomarkers unique to them; (5) After incubation of modern stromatolites with heterotrophic protists, use FLEC methodology to determine how their activity affects sediment fabric and conduct preliminary comparisons of these modern fabrics to those of stromatolite fossils. Intellectual Merit: The oldest fossil stromatolites are 3.4 billion years old and are the most visible manifestations of pervasive microbial life on the early Earth. The changes in stromatolite abundance and morphology document complex interplays between biological and geological processes. This project addresses multiple aspects of stromatolite genesis and pre-fossilization alteration but at its core, focuses on one of the greatest geological enigmas: the possible connection between stromatolite decline and the rise of complex life. Broader Impacts: Because any undergraduate introductory historical geology class (and some middle and high school Earth Science classes) introduces stromatolites as the first highly visible evidence of life on Earth, the results of this project would be interesting to a wide audience. This multidisciplinary project involving benthic ecology, molecular biology, sedimentology, and organic geochemistry includes education opportunities from high school to graduate students and teachers. The project would support a WHOI-MIT Joint Program student for half of his/her PhD studies and an MIT student for half of his/her PhD studies. Additionally, undergraduate students from MIT, the University of Connecticut, and University of Miami will actively participate in some of our field collections and laboratory analyses.