Collaborative Research: Establishing the Hydrogen Isotopic Window into Archaeal Lipid Biomarkers

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Life has three major groups - the eukaryotes, bacteria and archaea. This work focuses on the often-overlooked archaea. They are microbes critical to the cycling of matter and energy on Earth. They have played this important role throughout the history of the planet. Like all life on Earth, archaea are made up of elements such as carbon, oxygen and hydrogen. These elements form larger organic molecules that are required by all life such as DNA, proteins, and lipids. Geobiologists study these organic molecules to learn about life on Earth today and in the past. However, not all of these are preserved over long periods of time. Most DNA and proteins break down quickly and can only be used to study Earth today. Lipids are often much hardier and so are useful to geobiologists because they can be preserved in rocks for millennia. Lipids from archaea are particularly hardy and do not break down easily. This means that they can store information about the place in which the microbe lived. Long after they die, the lipids made by archaea can be extracted from muds, oils, and rocks. Geobiologists examine the structures and atoms of these lipids built by archaea millions of years ago to learn about Earth's past. However, it is not easy to decode the information in the lipids of long dead microbes correctly. To do this well, it is important to study these same lipids in modern systems and in the lab. This project will study several important archaea in the lab and shed light on how the different kinds of hydrogen atoms are used by archaea to build their lipids. The results will lead to advances in interpreting what lipids from archaea can tell us about the places they lived recently, and millions of years in the past. The proposed project emphasizes training of students at all levels and this includes advanced graduate (PhD and MS trainees) as well as undergraduate, high-school and middle school students. To engage high-school and middle school students in underserved districts, project members will use a graduate and undergraduate led peer-to-peer mentoring network called ManyMentors. This is an app-based mentoring platform that matches students in underserved classrooms who show interest in STEM with undergraduate and graduate students seeking STEM degrees. Specific teaching activities are all aimed at fostering transferrable skills. This includes critical thinking, written and verbal communication, and coding-literacy. All project members will participate in these activities. Archaea are a domain of life central to the cycling of matter and energy in low temperature (150 Celsius) environments today and throughout Earth's history. The primary objective of this project is to uncover how the hydrogen (H) isotopic composition of archaeal lipid biomarkers records geochemical and geomicrobial processes and may be used to reconstruct modern environmental systems and the recent geologic past. To read these records requires controlled experimental calibration of archaeal biomarker H isotope fractionation, which does not yet exist. This work will address this knowledge gap by calibrating the lipid H-isotopic signature of representative archaea across different redox regimes, carbon sources and energy fluxes reflecting the full suite of environments encountered in nature. Specifically, it will test the hypothesis that the structural and H-isotopic composition of archaeal lipids records the main hypothesized geochemical drivers of variability in archaeal biomarkers (energy and carbon availability, as well as nutrient/energy flux). To achieve this, the proposed work will employ a mixture of culturing methods including rate-controlled steady-state (chemostatic) experiments, combined with archaeal lipid characterization, compound-specific H isotope analysis, and a modeling effort. The output of this work will be a process- and mechanism-based interpretive framework for archaeal lipid H isotope fractionation in response to environmental forcings. That is, the proposed work will allow for (re)interpretations of past and present biogeochemical processes across wide temporal and spatial scales, from planetary (exosphere redox) to regional (basinal redox) to interfacial (pore-water redox). This project will train two PhD students in low temperature geochemistry and geomicrobiology and provide myriad opportunities to engage undergraduate students from diverse backgrounds in basic scientific research. Student engagement will be guided by an emphasis on critical thinking, lab and analytical skills building, and exposure to new ideas and questions in geomicrobiology. This will allow student to gain transferrable skills, such as rational experimental design, hypothesis construction and testing, and coding-literacy in the geosciences. These are requisite skills in the 21st century workforce, both in and out of the geosciences. Co-PI Kopf has a demonstrated commitment to open-source code, data collection and analysis software and has deployed such modules in the classroom as part of a larger effort to increase coding-literacy in geoscience courses. To expand the impact of coding modules developed at CU Boulder, Co-PI Leavitt will test and implement them in geomicrobiology and biogeochemistry courses at Dartmouth. Both PIs are committed to experimental pre-registration (via Open Science Framework) and the long-term archiving and preservation of all data products. All PIs and team members will participate in the ManyMentors network.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.