Astrobiology Revealed #19: Asena Kuzucan
on how gases expand the planetary habitable zone and limit E. coli growth
by Aubrey Zerkle
In this edition of Astrobiology Revealed, Asena Kuzucan discusses her paper “The Role of Atmospheric Composition in Defining the Habitable Zone Limits and Supporting E. coli Growth.” Asena is a postdoc in the Department of Astronomy at the Université de Genève in Switzerland, and part of the Centre pour la Vie dans l’univers (CVU). Asena describes the ins and outs of using 3D climate models to study planetary habitable zones, and clarifies the distinction between long-term habitability and short-term biological growth. (This interview has been edited for length and clarity.)
Your research spans everything from 3D climate models to microbial experiments! What piqued your interest in astrobiology, and how did you develop such a multi-disciplinary skillset?
My journey into astrobiology was rather unconventional. I originally pursued my PhD in particle physics at CERN, swept up in the need to understand fundamental questions about the universe. But as time went on, I came to understand that I needed something more interdisciplinary. I found studying particle physics in isolation to be frustrating because it prevented me from engaging with other sciences that had piqued my interest.
After departing from particle physics, I looked into positions outside of academia, but nothing really fit my curiosity. One day I was reading a book on astrobiology and suddenly had an epiphany: this was the field I had been searching for, a means of combining different fields of science. Astrobiology is a field that combines physics, biology, chemistry and geosciences to answer basic questions like the origin and limits of life, the potential habitability of other planets, and even the evolution of our own planet.
I then started the Master in Astrophysics program at the University of Geneva. There, I met Emeline Bolmont, the head of the CVU center, who has been my supervisor ever since. I told her about my ambition to specialize in astrobiology, and she introduced me to CVU which was also at its beginnings. I was privileged to be included in its inaugural experimental project, and then continued my research as a postdoc at CVU.
In the first part of your paper, you applied a 3D climate model to derive the habitability of H₂-dominated and CO₂-dominated planetary atmospheres, extending previous research that relied mainly on simpler 1D models. What are the advantages and disadvantages of using 3D versus 1D models for this type of study?
The primary advantage of a 3D climate model is that it can simulate the complexity of planetary climates realistically. While 1D models simplify the atmosphere to one column with vertically averaged conditions, 3D models better simulate planetary conditions by considering latitudinal and longitudinal temperature, pressure, and atmospheric process variations. This enables them to capture important phenomena like winds, cloud patterns, and heat distribution, each of which plays a vital role in the climate and habitability of a planet. Perhaps one of the strongest aspects of 3D models is their ability to represent regional climate variability and feedback mechanisms that are difficult or impossible to capture using 1D approaches. For instance, 3D models can represent the impacts of atmospheric dynamics on global cooling, particularly via variations in relative humidity.
Despite these advantages, 3D models also have some severe shortcomings. Their most significant drawback is the heavy computational demand. Running a fine-scale 3D simulation can be computationally costly and time-consuming even for high-performance computing clusters. This computational demand renders them less suitable for rapidly scanning many scenarios or running broad surveys over a wide set of planetary conditions. In addition, although 3D models are more realistic, they are based on assumptions and parameterizations that introduce uncertainties, particularly when used to simulate exoplanetary atmospheres with little observational constraints. Cloud formation, for example, is typically parameterized, hence precise predictions as to cloud behavior and therefore their climatic implications, are not straightforward.
In summary, while 3D models create a more realistic and more detailed look at planetary climates and habitable zones, this comes at the expense of more computational costs and complexity. These are best used for exact studies of single planetary environments, but 1D models can still be useful for surveying larger parameter spaces and initial habitability assessments.
What is the significance of H2 and CO2 from a planetary perspective?
We specifically targeted H2 and CO2 because these gases are realistic and potential atmospheric compositions for planetary habitability. From a planetary perspective, H2-rich atmospheres can occur through nebular gas capture or volcanic outgassing processes and contribute significantly to planetary environmental evolution. H2 is particularly important as it has a very strong greenhouse effect due to collision-induced absorption. This property enables planets to have liquid water at further distances from the parent star, potentially extending the habitable zone. Although small terrestrial planets may lose early H2 atmospheres over time by atmospheric loss, even a transient atmosphere rich in H2 would heavily influence conditions that favor life on a planet in its early days.
CO2 is already familiar to us as the dominant greenhouse gas that regulates climate on terrestrial worlds such as Mars, Venus, and Earth. It plays a crucial role in long-term climatic stability through processes like the carbonate-silicate cycle. Interestingly, in very high atmospheric pressures, the warming effect of CO2 can be counteracted by Rayleigh scattering, an effect that needs careful consideration when defining the limits of the habitable zone. By studying these two gases, we can better understand the inner and outer edges of habitability for planets with diverse atmospheric compositions.
What is the significance of these gases from a microbial perspective?
From a microbial perspective, H2 and CO2 are essential gases for anaerobic life, organisms that thrive without oxygen. Although E. coli is a facultative anaerobe and was expected to survive under anaerobic conditions, we wanted to compare its growth efficiency under anaerobic conditions versus Earth's current [aerobic] atmospheric composition.
Additionally, these gases are crucial for autotrophic microorganisms, which can synthesize their own organic compounds from inorganic substrates. For instance, methanogens, which are one of Earth's first life forms, use H2 as an energy source and electron donor, and CO2 as the primary carbon source. They combine the two gases through methanogenesis to produce methane, which is an important early biosignature gas. Similarly, acetogenic bacteria utilize CO2 and H2 in the Wood-Ljungdahl pathway, significantly influencing carbon cycling in environments lacking oxygen.
Considering the potential presence of similar atmospheric compositions on Mars and other exoplanets, exploring microbial responses to these gases provides critical insights into possible extraterrestrial habitability. Moreover, this work enriches our understanding of prebiotic chemistry, helps us identify potential biosignatures, and informs our expectations about life's resilience and adaptability beyond our planet.
Based on your model results, what was the biggest difference between H2- and CO2-dominated atmospheres?
According to our models, the primary distinction between atmospheres dominated by H2 and CO2 is their ability to sustain planetary warmth and, consequently, habitable conditions.
Because of the strong greenhouse effect brought on by collision-induced absorption, an atmosphere rich in H2 is particularly good at trapping heat. This characteristic greatly expands the outer bound of the habitable zone by allowing planets to maintain warm conditions and possibly liquid water, even at greater distances from their host star. Furthermore, these atmospheres tend to be extended due to the extremely light H2, which makes them simpler to detect using observational techniques. However, this low molecular weight also makes H2 atmospheres vulnerable to escape into space. As a result, continuous H2 replenishment such as through ongoing geological processes or volcanic activity is usually necessary for sustained habitability in an atmosphere dominated by H2.
However, CO2-dominated atmospheres have certain drawbacks despite also being significant greenhouse gas contributors. Although raising CO2 can initially accelerate global warming, increased Rayleigh scattering reduces its effectiveness above a certain pressure threshold. The warming potential of CO2 atmospheres is ultimately limited at high pressures because they reflect more incoming stellar radiation away from the planet. Therefore, planets farther from their star that have atmospheres made entirely of CO2 might have difficulties staying warm enough without the addition of other greenhouse gases (such as H2) to enhance their greenhouse effect.
Which type of atmosphere is more likely to be habitable?
Overall, our simulations indicate that, if there is a persistent mechanism to sustain atmospheric H2 over geological timescales, H2-rich atmospheres are more conducive to expanding the habitable zone away from the host star. Although CO2 atmospheres are also capable of supporting habitable conditions, their habitable zones are relatively smaller, and additional greenhouse gases are usually needed to maintain habitability at farther distances from the star.
In the second part of the paper, you grew E. coli in different gas compositions to mimic these atmospheres. How do those results compare with your model predictions that H2 should produce more hospitable conditions?
Our biological experiments with E. coli also suggest that H2 is particularly conducive to habitability, at least in some circumstances. In particular, we found that E. coli thrived (in the short term) in a pure H2 atmosphere but faced challenges in a pure CO2 atmosphere. But it's critical to make a clear distinction between long-term planetary habitability and short-term biological outcomes. As mentioned above, atmospheric escape makes it difficult to maintain a stable H2 atmosphere over geological timescales. Therefore, a planet's capacity to continuously replenish H2 through geological processes like volcanic activity determines whether or not it can support life in the long run.
On the other hand, a pure CO2 atmosphere had climatic and biological constraints. E. coli grew the least well in our microbial tests when exposed to pure CO2. Similarly, our climate modelling revealed that CO2 alone is unable to warm planets at farther stellar distances. However, our biological findings are based on studies using E. coli, a relatively simple organism that isn't suited for harsh environments. The fact that many extremophile microbes on Earth actually flourish in CO2-rich environments suggests that life is extremely adaptive and can evolve to survive even harsher conditions than those tested here.
Thinking closer to home, what does the lack of microbial growth in CO2-atmospheres mean for potential past life on Mars and Venus?
In the case of Mars, our results imply that a purely CO2 atmosphere on Mars would have presented difficulties for common microbial life, such as E. coli. However, Mars probably had additional factors that could have significantly enhanced its habitability. For example, if early Mars also possessed substantial amounts of H2, possibly produced by volcanic outgassing or serpentinization reactions, this could have created favorable conditions for some microbial life, like methanogens.
The situation is even harder for Venus. With surface temperatures of around 460°C, pressures of around 60 atmospheres, and extremely acidic conditions, Venus is now a very inhospitable world. But Venus may once have had oceans or more temperate weather. As early as 1967, Harold Morowitz and Carl Sagan suggested that life could have persisted on Venus by withdrawing up to more favorable atmospheric layers. But the runaway greenhouse conditions on Venus would have made long-term habitability enormously challenging. Our current understanding of Venusian habitability remains largely theoretical due to limited evidence, emphasizing the need for future exploration and investigation.
Do you have any plans for follow-up studies to test these ideas further?
Yes, my research continues to explore how CO2-dominated atmospheres might influence habitability, especially on early Mars. My recently completed project tested whether methanogens could survive and metabolize under simulated early Martian conditions. We specifically investigated an atmosphere composed of 2-bar CO2 with 15% H2, conditions shown by our 3D climate simulations to support stable surface liquid water, a critical requirement for life. Beyond microbial survival alone, we also explored how these microbes interacted with basaltic rocks, common on Mars. Basalt served as both a colonization substrate and a potential nutrient source, influencing microbial metabolism and possibly facilitating critical chemical reactions. The results of this study offer important insights into the kinds of biosignatures that might be preserved in Martian basalt, which has implications for future Mars missions seeking evidence of past life. I am currently preparing these findings for publication.
Building on these promising results, I am now extending this work to investigate clay-rich environments under similar atmospheric conditions. Clays are especially relevant because they effectively retain water and essential nutrients, potentially forming protective microhabitats for microbes. Additionally, clays have a high capacity for preserving biosignatures, making them priority targets for future Mars sample return missions. By studying how methanogens interact with clay minerals, I aim to better understand which environmental conditions would maximize microbial survival and biosignature preservation, guiding future exploration efforts.