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

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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.