Collaborative Research: Biosignatures of coupled iron and carbon cycling in ferruginous lakes

地球生物学的一个重大挑战是推断出在地球进化的早期阶段形成生物圈的微生物种类，并了解它们的活动对早期海洋和大气的影响。应对这一挑战的最直接方法是利用现代环境，可以被认为是早期地球环境的类似物。这样的模拟环境是罕见且有价值的。该项目将使用美国中西部的两个湖泊——明尼苏达州的布朗尼湖和密歇根州的峡谷湖——最近发现，这两个湖泊的水域含有大量溶解的亚铁，这使它们成为缺氧的“含铁”海洋的很好类比，这些海洋在地球历史上持续了20多亿年（在太古宙和元古代）。通过实地考察和分析工作，这些研究人员将研究一种早期形式的光合作用，这种光合作用依赖于铁的氧化还原循环（光养铁）和相应的甲烷循环，这可能是古代碳循环的重要组成部分。该项目将限制所涉及的微生物的活性和生物特征。这是理解地球上氧气的增加、代表大多数现代铁矿的大量富铁沉积物的沉积、以及主要的气候扰动和早期地球上神秘的碳同位素漂移的必要步骤。一个主要的新问题是，在含铁的海洋中，温室气体甲烷在多大程度上产生并排放到大气中，以及这对全球碳循环和气候有何影响。由于先前对早期地球上类似的含铁湖泊的调查利用了国际站点，该项目将通过建立这两个国家研究站点为美国科学家服务。该项目将支持两名早期职业研究人员，培养两名研究生，并让几名本科生参与研究。项目成果和已建立的研究地点将用于湖沼学研究生的实践培训。通过与这两个湖泊的土地管理者合作，该项目将提供详细的理化基线，为未来湖泊流域的水质监测工作提供信息，并将为明尼阿波利斯市的环境教育推广项目做出贡献。光嗜铁被认为是前寒武纪含铁海洋初级生产力的主要途径。然而，目前的类似物——分生含铁湖泊——要么受到光养铁的光照限制，要么主要通过基于硫的光合途径固定碳，要么位于不适合季节性监测的地区。布朗尼湖光照充足，无氧光养生物（包括光铁养生物）群落丰富，更深的峡谷湖有较宽的缺氧过渡带，营养物质比布朗尼湖低得多。总之，这些湖泊包含了一系列的条件来研究不同的营养水平、地形和季节性对光铁营养初级生产力和甲烷循环的控制。该团队将监测布朗尼湖和峡谷湖水柱的水和碳同位素地球化学、微生物群落组成和颗粒元素组成/矿物学，以确定：(1)调节光养铁生物存在和活动的物理化学条件；(2)常驻微生物在铁和碳循环中的作用；(3)类似微生物群落可能在前寒武纪富铁沉积物（如带状铁地层）中留下的无机和矿物生物特征；(4)光养铁和产甲烷/产甲烷对碳和铁循环的同位素印记。

One of the grand challenges of Geobiology is to infer the classes of microbes that shaped the biosphere at the early stages of Earth evolution, and to understand the effects of their activities on the early oceans and atmosphere. The most direct way of addressing this challenge is to use modern environments that can be considered as analogues for those of early Earth. Such analogue environments are rare and valuable. This project will use two lakes in the Midwestern U.S. - Brownie Lake, MN and Canyon Lake, MI - whose waters were recently discovered to harbor abundant dissolved ferrous iron, which makes them good analogues for the anoxic "ferruginous" oceans that persisted for more than 2 billion years of Earth's history (during Archean and Proterozoic eons). Using field and analytical work, these researchers will investigate an early form of photosynthesis that relies on the redox cycling of iron (photoferrotrophy) and the corresponding cycle of methane, which would have been an important part of the ancient carbon cycle. This project will constrain the activity and biosignatures of the involved microorganisms. This is a necessary step to understand the rise of oxygen on Earth, the deposition of extensive iron-rich sediments that represent the majority of modern-day iron ore deposits, as well as major climate perturbations, and enigmatic carbon isotope excursions on the early Earth. A major emerging question is to what extent the greenhouse gas methane was generated and lost to the atmosphere in ferruginous oceans, and how this contributed to the global carbon cycle and climate. As prior investigations of ferruginous lakes as early Earth analogues utilized international sites, this project will serve U.S. scientists by establishing these two national research sites. The project will support two early career researchers, educate two graduate students, and involve several undergraduates in research. Project results and the established study sites will be used in hands-on graduate training in limnology. In collaboration with land managers of the two lakes, the project will provide a detailed physicochemical baseline to inform future water quality monitoring efforts in the lake watersheds, as well as will contribute to the City of Minneapolis environmental education outreach programs.   Photoferrotrophy is thought to have been the major pathway for primary productivity in ferruginous Precambrian oceans. However, current analogues - meromictic ferruginous lakes - either suffer from light limitation for photoferrotrophy, fix carbon through predominantly sulfurbased photosynthetic pathways, or are located in regions unsuitable for seasonal monitoring. Brownie Lake has sufficient light and an abundant community of anoxygenic phototrophs (including photoferrotrophs), and deeper Canyon Lake has an extended oxic-anoxic transition zone and much lower nutrients than Brownie Lake. Together, these lakes comprise a range of conditions to investigate the controls that varying nutrient levels, physiography, and seasonality have on photoferrotrophic primary productivity and methane cycling. The team will monitor aqueous and carbon isotope geochemistry, microbial community composition, and elemental makeup/mineralogy of particulates from the Brownie and Canyon Lake water columns in order to determine: (1) the physicochemical conditions that regulate the presence and activity of photoferrotrophs, (2) the role of resident microbes in iron and carbon cycling, (3) the inorganic and mineral biosignatures that similar microbial communities might have left in Precambrian iron-rich sediments such as Banded Iron Formations (BIF), and (4) the isotopic imprint of photoferrotrophy and methanogenesis/methanotrophy to carbon and iron cycling.