Astrobiology Revealed #28: Cassie Hopton

on ammonium sulfate limits to life on icy moons

by Aubrey Zerkle

For this Astrobiology Revealed, we asked Cassie Hopton about her recent paper, “Growth, physiology, and metabolism of Halomonas meridiana in aqueous ammonium sulfate with implications for icy moon astrobiology.” Cassie is a PhD student in the UK Centre for Astrobiology at the University of Edinburgh. Cassie explains how ammonium sulfate could act as a double-edged sword for life in the subsurface oceans of icy moons like Titan and Europa. (This interview has been edited for length and clarity.)

Q: How did you get involved in astrobiology research, and what aspects of this field excite you the most?

I initially encountered the work of my PhD supervisor, Charles S. Cockell, as a first-year undergraduate studying BSc Biochemistry at the University of Bristol. While watching the documentary television series Cosmos, I was inspired to find out how my developing knowledge in biochemistry could intersect with space studies. A quick search immediately led me to Charles’s research group, the UK Centre for Astrobiology, my first introduction to the field of astrobiology. I was enthralled by Charles's research and was surprised to learn that he, too, had studied BSc Biochemistry at the University of Bristol. It was by chance and luck that Charles not only had a PhD opening that coincided with the end of my master's degree, but it was also for a project that was directly linked with my skill set and interests.

Being part of the UK Centre for Astrobiology has been an invaluable experience. I love the sense of adventure that comes with this research area. Of course, I am mostly in the laboratory, but being able to envision and hypothesize about these extraterrestrial worlds really incites the imagination. It almost feels like you’re remotely exploring an alien world. I’m really interested in icy moons – I find the combination of hostile, icy surfaces with potentially habitable subsurface oceans quite fascinating.

Q: In your recent paper in Frontiers, you investigated how ammonium sulfate limited growth in an extremophile microbe. What's the significance of ammonium sulfate to life?

It is a bit paradoxical. Ammonium is a bioavailable form of nitrogen – it is easily captured and utilized by all kinds of life on Earth. Nitrogen is essential to make amino acids and DNA, among other molecules. Ammonium sulfate is a solid salt containing ammonium that is commonly utilized as a fertilizer for plants to promote growth.

But too much of anything can be a bad thing. Just like if you ate 400 bananas a day you might suffer from potassium poisoning, life can receive too much ammonium sulfate and suffer consequences. Some of these are external chemical effects. For example, ammonium sulfate is quite acidic, and lots of ammonium sulfate will increase the salinity of an environment. Any form of life that is not adapted to live in acidic or salty conditions may therefore cease to grow, or cease to exist, in the presence of substantial amounts of ammonium sulfate. Other effects are internal. Some ammonium ions and sulfate ions can permeate into the cell. If in excessive amounts, these ions may disrupt normal growth processes. 

Q: What about its significance with respect to potentially habitable extraterrestrial environments - where has it been found, and is it thought to be common in the universe?

When in water, ammonia can exist as two species: the more toxic ammonia (NH3) or the less toxic ammonium ion (NH4+). NH4+ dominates under more acidic conditions, whereas NH3 dominates under more alkaline conditions. An extraterrestrial environment may become less habitable for life as we know it if there were high concentrations of the toxic NH3. For NH4+, life on Earth has been found to survive in up to one molar and higher. So even at high concentrations of ammonium sulfate, life can survive quite well. But high concentrations will increase acidity and salinity, as I mentioned, and not all life is adapted to survive in such conditions.

Prior to the findings of NASA’s Cassini spacecraft, it was speculated that the icy moons of Jupiter, such as Europa, Ganymede, and Callisto, and Saturn’s icy moons Enceladus and Titan, contained liquid water oceans beneath their thick ice crusts. The preservation of liquid water, despite the extremely cold surfaces, was thought possible due to anti-freeze components such as NH3. In 2005, Cassini revealed plumes of water erupting from the surface of Enceladus. These plumes are speculated to originate from the ocean below. Later analysis of a plume revealed not only water, but NH3 as well, providing evidence for a long-held theory.

Certain confirmation of ammonia on other icy moons is outstanding, but the fact that NH3 has been detected on Enceladus gives a good indication that it might be an oceanic component of its neighbor, Titan, and possibly other icy moons. Models indicate Titan could consist of an ammonium sulfate ocean, rather than NH3. On Europa, current models place the ocean at a fairly neutral pH, so if ammonia were part of its formation, it would likely be in the form of NH4+. Notably, ammonium sulfate is one species that may have been detected on the surface of Europa – although this finding is not conclusive. But overall, ammonia is a primordial molecule. It is ubiquitous throughout the universe, so this research may have wider-reaching implications than just icy moons.

Q: You performed these experiments using the extremophile Halomonas meridiana Slthf1. Can you briefly explain why this organism in particular is relevant to potential life on icy moons?

There were two main reasons why this organism was the most appropriate to simulate ‘icy moon’ life. Firstly, H. meridiana was isolated from a location with geological and physicochemical similarities to those that could occur in the oceans of icy moons. Icy moon subsurface oceans, such as those of Enceladus and Europa, are hypothesized to contain hydrothermal vent systems. The waters of Enceladus, Titan, and Europa are also thought to be saline, and with the exception of Titan, could exhibit salinities comparable to Earth's ocean. The waters of these deep extraterrestrial oceans are also cold, likely below 0 °C, and under hydrostatic pressure. H. meridiana was originally isolated at a depth of 2000 meters from low-temperature hydrothermal fluid in the East Pacific Rise. Its environment of isolation is therefore highly relevant to icy moons. Because it came from this deep, low-temperature ocean environment, H. meridiana exhibited several adaptations to extremes of salinity, alkalinity, temperature, and pressure that were appropriate to icy moon environments.

Secondly, H. meridiana exhibited no known adaptation to ammonia. The purpose of my research was not to study ammonia adaptation, as is found in organisms like ammonia-oxidizing bacteria, but rather to study the limits of life in ammonia. Icy moons such as Enceladus and Europa are estimated to contain only small concentrations of ammonia, and the concentration of ammonia on Titan could be as low as 1.5%. Thus, it was not necessary to study an ammonia-adapted bacterium but rather to assess whether ammonia could be a chemical parameter that limits habitability for organisms without ammonia adaptations.

Q: You found that this organism can grow in quite high concentrations of ammonium sulfate, but it formed less glutamine in the process. What does that mean for its metabolism, and potentially the metabolism of other life on icy moons? 

Glutamine is an important molecule. It is part of the nitrogen assimilation process in many organisms. However, it is also possible for organisms to assimilate nitrogen without glutamine. My results suggested a switch from the nitrogen assimilation pathway that utilizes glutamine (the GS-GOGAT pathway) to one that does not (the GDH pathway). It was beyond the scope of my research to determine why this occurred, but I speculated that the NH4+ ion may have competed with potassium ions (K+) for transport through membrane protein channels. NH4+ has also been implicated in disrupting the balance of calcium ions (Ca2+) within cells. Both ions regulate the activity of the glutamine-producing enzyme glutamine synthetase. Thus, it is possible that NH4+ acted in a way that dysregulated this enzyme and reduced glutamine output.

What this means is that, if life were to persist in a highly concentrated ammonium sulfate environment, it is possible that this life would preferentially use the GDH pathway for nitrogen assimilation. Knowing what kind of metabolisms are likely to occur under extreme conditions is important, as one thing we look for when scanning environments for life is biomarkers. These include metabolic signatures such as compounds and proteins. If we know the GDH pathway is more likely, we can ensure instruments scanning for markers of life are attuned to pick up compounds and proteins associated with this pathway. 

Q: Would you say your results offer more hope or challenges for astrobiologists interested in life on icy moons?

For any research on potential extraterrestrial life, we need to remember that bacteria are adapted to Earth. Of course, we try our best to find suitable bacteria with metabolisms that could match the conditions in extraterrestrial environments. This gives us a good indication of whether life as we know it could survive. But we must remember that life on extraterrestrial planets may be just that – extraterrestrial. They may have entirely unique adaptations that allow them to survive in conditions we did not know were possible.

Equally, we could travel to icy moons and find that the oceans are habitable but not inhabited. The formation of life is a complex process. It depends on the right ingredients at the right time. So, it is also possible these oceans could be survivable for life as we know it, but that life has simply not formed.

That is not to say my results and the results of others are invalid. They form an essential part of the process to explore space. Researchers identify extraterrestrial environmental conditions that could support or limit life as we know it, then later these results can be used to determine appropriate targets for future space missions.

My results show that H. meridiana remained viable in high levels of ammonium sulfate. The levels of ammonium sulfate I utilized are probably beyond what is present on icy moons. Thus, ammonium sulfate is unlikely to be a barrier to habitability on icy moons. For all these reasons, I think my results offer hope and further indicate that moons such as Europa and Titan are worthy targets for life detection missions.

Q: What’s next - do you have any plans for follow-up studies to test these ideas further?

At this time, I have finished my PhD research, so I do not have any specific plans to follow up on these results. However, I have since published a paper in Microbial Ecology on how NH3 gas can migrate through the atmosphere and affect bacterial life from a distance. This has implications for how NH3 may travel from an icy moon ocean into the ice shell, affecting the habitability prospects of brine networks in this environment. I am also currently writing a review on how ammonia could impact the prospects for life on icy moons – for good and for bad – which will hopefully be published in 2026.

Q: Do you have any words of advice for others interested in pursuing astrobiology?

I’d like to encourage any students interested in astrobiology to undertake a degree in a ‘core’ science subject and attend any astrobiology summer schools or courses they can. For example, my undergraduate degree was in biochemistry. Many of my lab group members have undergraduate degrees in geology, physics, biology, etc. Understanding Earth and Earth life is essential to understanding extraterrestrial planets and the potential for life. Having a solid foundation like this really helps!

In between my first and second years as an undergraduate (after discovering Charles S. Cockell’s work), I enrolled in an astrobiology summer school at the Tuorla Observatory in Finland. It was very insightful. I was able to develop my interdisciplinary knowledge further, and I met so many astrobiologists from different countries and at different stages of their careers. There is funding out there for early-career students to attend events like this, and I’d really recommend it.