| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | August 12, 2015 |
| Latest Amendment Date: | August 12, 2015 |
| Award Number: | 1529963 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Enriqueta Barrera
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | August 15, 2015 |
| End Date: | July 31, 2017 (Estimated) |
| Total Intended Award Amount: | $235,000.00 |
| Total Awarded Amount to Date: | $235,000.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
660 S MILL AVENUE STE 204 TEMPE AZ US 85281-3670 (480)965-5479 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
School of Earth & Space Explor Tempe AZ US 85287-1401 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Geobiology & Low-Temp Geochem |
| Primary Program Source: |
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| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
The objective of this project is to identify the reasons why microorganisms can live where they live. The focus is on identifying livable conditions in the environment, with the goal of explaining how temperatures and geochemical compositions combine to allow and support microbial life. Two things have to be true for an environment to be habitable: there have to be sources of energy, and those sources of energy have to persist long enough for life to take advantage of them. Things that burst into flame are not good to eat. Habitability can be quantified by combining methods to calculate the amounts of chemical energy available to microbes with measurements of the rates that resulting reactions happen with and without microbes present. The importance of this approach is that it can be used in diverse environments from soils to deep in the Earth's crust, allowing an expansion of scientific understanding of how our planet supports life, and even in biological systems including the human gut where there could be surprising applications to improve human health. In this study, environments that support microbes that use iron reactions as their source of energy will be studied including hot springs, acid mine drainage, and cold springs fed by snowmelt. By examining the same processes across diverse environments, this case study of the microbial iron cycle will serve as a template for future studies of other chemical energy sources. Ultimately these efforts will allow researchers to explain underlying reasons for the immense microbial diversity found on Earth.
Two things have to be true for microbes to gain chemical energy from the environment. First, there must be a source of energy. This requires the presence of compounds in differing oxidation states that are out of thermodynamic equilibrium with one another. Second, there must be mechanistic difficulties that are keeping those compounds from reacting, which means that the chemical energy cannot dissipate by itself. Using this energetic reference frame, geochemical habitability can be defined and quantified by the combined presence of thermodynamic and kinetic limitations at diverse environments on and in the Earth. As an example, microorganisms across the phylogenetic tree of life gain energy by reacting dissolved reduced iron with oxygen in environments ranging in temperature from freezing to boiling and pH values between 2 and 7. However, not all combinations of pH and temperature are habitable. In high-pH environments this reaction occurs rapidly on its own, which prevents microorganisms from using it, and the pH where this kinetic barrier occurs decreases with increasing temperature. In acidic environments, however, the abiotic oxidation reaction rate is significantly slowed, allowing microorganisms to catalyze iron oxidation and conserve some of the energy released. However, increasing acidity lowers the energy yield, ultimately creating an energy boundary to habitability at the lowest values of pH. Combining such energetic and kinetic boundaries permits habitability to be mapped for individual reactions using geochemical variables that include pH, temperature, and concentrations of reactants and products of the reaction. It is a goal of this research to generate habitability maps for the case study of iron oxidation and reduction reactions. Geochemical data from fieldwork at hot springs, acid mine drainage, and cold springs fed by snowmelt will be used to calculate energy supplies. Field experiments of biotic and abiotic rates of iron oxidation and reduction will determine kinetic limitations. Complementary lab experiments will provide abiotic rates. Molecular analyses will reveal the microbes likely to be responsible for driving the biological iron redox cycle in these environments. The resulting multi-dimensional habitability maps for several iron oxidation and reduction reactions will provide a framework for future studies of many other chemolithotrophic metabolic process throughout surface and subsurface environments on Earth, which will quantitatively constrain the discussion of habitability on other planets.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Things that burst into flame are not good to eat! That seems obvious enough, as our food represents sources of energy that we can use because they do not react quickly on their own. But it has special meaning for microbes that make their living from chemical sources of energy. Whether a source of energy can support them depends on how quickly the reaction can happen. If fast on its own, with no involvement of the microbe, then the energy can escape before being captured. On the other hand, if the reaction happens slowly on its own, it represents a source of food.
That food can take forms that can seem unusual to us. The focus of this study was iron, which exists in two oxidation states where many microbes live: the reduced form, Fe(II), and the oxidized form, Fe(III). If Fe(II) is present at conditions where it could be oxidized to Fe(III), but that reaction has not yet happened, an iron oxidizing microbe can catalyze the reaction and live on some of the released energy. The product can be a rusty iron oxide staining of water, sediments, and rocks. The same idea, but opposite type of reaction, applies where Fe(III) is present at conditions where it could be reduced, leading to the disappearance of the rusty coloration. Many environmental variables affect the rate at which oxidation and reduction reactions happen on their own. As those variables change an individual reaction can go from extremely slow, and therefore represent a source of energy, to extremely rapid, and effectively ‘burst into flame’. As an example, the oxidation of Fe(II) speeds up as pH increases such that at low pH, acidic conditions, iron oxidizing microbes can thrive, but at higher pH, basic conditions, the reaction happens too fast on its own for the microbes to compete. Field experiments conducted in this project in cold springs in the Swiss Alps, streams impacted by acid mine drainage in southern Arizona, and hot springs in Yellowstone National Park allowed us to determine rates of nonbiological and biological iron oxidation and reduction reactions.
By taking advantage of highly variable natural systems we could determine conditions where the nonbiological reactions of iron were so fast that microbes could not use the energy sources, as well as biological rates of reactions where microbes are responsible for iron transformations. It was also possible to determine the percentages of biological iron oxidation or reduction as conditions approached the limits of where life could use individual reactions as energy sources. The synthesis of these field experiments was the generation of figures showing conditions of pH, temperature, and dissolved oxygen concentration where individual iron reactions can and can not be used by microbes. We call these figures ‘habitability maps’ because they reveal territory in these dimensions where conditions are habitable for microbes that catalyze specific reactions. One outcome of this habitability mapping project was to show that just above freezing temperatures an iron oxidation reaction can support life below a pH of 8, but that at near-boiling temperatures in hot springs the upper pH is 5 for biological use of the same reaction. Experiments at temperatures in-between these extremes showed a shift between these pH limits. Another result for the same iron oxidation reaction is that, despite the pH being in the habitable range, there is not sufficient dissolved oxygen for iron oxidizing microbes to thrive in many of the highest temperature hot springs we studied. This means that what looks like a temperature limit to microbial iron oxidation may actually be an oxygen limit.
Reddish-brown iron staining is easy to observe in natural settings where iron oxidation occurs. Now, a few simple measurements allow predictions of whether that iron staining is caused by microbes or not. Yellowstone hot springs that show vivid evidence for iron oxidation provide opportunities for the public to understand whether microbes are responsible. We have explained our research results to Yellowstone rangers who are experts at relaying science to the public. The story of how Yellowstone hot springs helped researchers map the habitability of iron reactions is a rich opportunity for outreach as part of the broader impacts of this research.
Last Modified: 01/24/2018
Modified by: Everett L Shock
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