Astrobiology Revealed #23: Afonso Mota

on how fungi could survive on M dwarf exoplanets

by Aubrey Zerkle

In this Astrobiology Revealed, we chatted with Afonso Mota about his recent paper, “How Habitable Are M Dwarf Exoplanets? Modeling Surface Conditions and Exploring the Role of Melanins in the Survival of Aspergillus niger Spores Under Exoplanet-Like Radiation.” Afonso is currently a PhD student with the Institute of Astrophysics and Space Sciences at the University of Porto in Portugal. He also engages in science communication as a co-creator of the Portuguese astrobiology young career researcher group, AstroBioLusitanos. Afonso discusses why we need more info about exoplanet atmospheres, the importance of fundamental research, and how the fungus Aspergillus could be the perfect organism to take with us into space. (This interview has been edited for length and clarity.)

In your recent paper in Astrobiology, you listed your affiliation as the Aerospace Microbiology Research Group, which sounds fascinating. Can you tell us a bit more about that – what does an “aerospace microbiologist” study, and how did you end up in this research area?

That paper was from my Master’s thesis, which I developed there. My main affiliation was the German Aerospace Center, and the Institute of Aerospace Medicine was the associated group.

In terms of my history, I always knew I wanted to study space. I was always really interested in the universe when I was younger. But in high school, I didn't really enjoy physics - it was too much mechanics, like the ball rolling down the hill and stuff like that, which wasn’t as much my interest. I discovered astrobiology around that time, and the search for life outside of Earth, and I found it intriguing. I really enjoyed my biology courses, so I knew I wanted to do that from the beginning. So even though I did my Bachelor's in biology, I also took a minor in physics. When I started my Master’s degree in biology of extreme environments, I was looking for a Master’s thesis somewhere I could bridge these two fields of studying the universe, but from a biological perspective. And in the Aerospace Microbiology Group, that’s exactly what they did.

The PI of that group was Ralf Möller, who unfortunately passed away a few months ago, but he did a great job setting it up within the German Aerospace Center. A lot of the work being done there was related to the other side of aerospace, like commercial flights, the safety of passengers, microbiology related to the atmosphere, or to contaminating surfaces, or other things that are related. But at the time I arrived, they had also been developing the more astrobiology side for a few years. They were studying microbiology in space from a human space flight perspective and for the health and safety of the astronauts. But they were also studying space microbiology from a limits of life perspective, and trying to understand the extremes that life can endure and perhaps the ways that we could detect it on other worlds.

It's really impressive that you completed all of this work as a Master’s student! In the paper, you calculated the surface temperature and radiation environments of M dwarf exoplanets. For those of us who aren't astronomers, can you just tell us what those planets are and why they're of particular interest to astrobiologists?

M dwarfs are a type of star. Stars are classified according to their mass or how big they are. The more massive blue stars live very short lives, and there are very few of them in the universe. The less massive stars have lower temperatures, and they're much more common. The most common stars are these M dwarfs, also called red dwarfs, which are at the lower end of the spectrum. These stars produce light that is more shifted in the red, so this [points to the poster of Kepler 186f shown below] could be an M dwarf starlight. But they don’t produce a lot of energy, so planets that we could hypothesize to be habitable around these stars would likely have to be much closer to the star so they receive enough energy to form life and sustain biological processes. Of course, there are many ways of producing energy, but in general, the temperature has to be adequate for life as we know it. That's the big constraint. 

And why are these stars so important? Because they're so common. Red dwarf stars are about 75% of all stars in the universe. It's also slightly easier to detect planets passing in front of an M dwarf star because of the various constraints of astronomical techniques nowadays. All that has been increasing our interest in trying to understand whether these planets could be habitable or not. Because if they are, then we are greatly expanding the array of places in the universe where we could expect life to perhaps originate. If we’re considering only stars like the Sun, those are only 10-20% of stars, maximum. If we can include all 75% of planets that are red dwarfs, then we greatly expand our possibilities.

Kepler-186f - JPL Travel Poster, courtesy of the NASA/JPL-Caltech Visions of the Future Poster Series.

Which exoplanet travel poster did you point to when you said that it had red dwarf starlight?

This one, Kepler 186f, because of the red forest. But these are all red dwarf planets. This is TRAPPIST-1e [shown below], and this is Proxima Centauri-b. These are the planet systems that I studied, and in both cases, the planets are illuminated by red dwarf stars. TRAPPIST-1 actually has several planets and is one of the most promising exoplanet systems that we know of.

TRAPPIST-1e - JPL Travel Poster, courtesy of the NASA/JPL-Caltech Visions of the Future Poster Series.

In the discussion, you stated that models more complex than the one you used require information that is largely unknown for exoplanets – is there some key piece or pieces of information that would help make these models more accurate? 

There are several issues. It is hard to detect planets and even to get some of their basic characteristics. We can get some constraints on their mass, their radius, and somewhat their composition. But for smaller, potentially rocky planets that could harbor life, the bigger constraints are the atmospheres. We don’t know the atmospheres of any rocky exoplanets. Maybe we will be able to discern some more traits with the James Webb Telescope and eventually the ELT [Extremely Large Telescope], but right now we have very limited capabilities.

The atmosphere plays a crucial role in many factors, including the temperature and radiation protection. Those are the two factors I approached in my study, because they are some of the most influential in the ability of a planet to originate life and maintain it in the long term. If we don’t know the atmosphere, then we can’t know the greenhouse effect and how much heat it’s holding in. So, we could be assuming the planets are too cold or even too hot, since the atmosphere can also have a cooling effect. The atmosphere is completely essential to know what the surface environment will be like, so that’s really the crucial information we need.   

A lot of the current work uses a very simple model that is essentially a mathematical constraint of, for example, the equilibrium temperature. This assumes that the planet is just a black rock that has no atmosphere or anything else. That is relevant to compare between planets, but it doesn’t really give us a picture of what the actual temperature conditions could be like. Other, more complex modeling efforts could be more accurate, but only if you’re getting all your assumptions correct. Because you’re assuming what type of atmosphere it is, its density, its composition, and sometimes if the planet has a synchronous orbit or not. I approached it in a way to kind of bridge that gap, because a lot of these factors are unknown. So, I tried to find an intermediate modeling approach that offered better results than a very simple model but also had as few assumptions as possible.

Are there upcoming missions like the Habitable Worlds Observatory that will be able to provide more useful information about exoplanet atmospheres?

Yes and no! The Habitable Worlds Observatory is a very promising piece of technology and a candidate to drastically improve our knowledge of this, but it’s still in the initial design phases. We don’t know the budget constraints, building constraints, or even the technology included until it is built and launched. But something that I think is not so well understood, especially with the general public, is that even if we do have the capability to get the data, sometimes the problem is finding a way to translate it into an understandable piece of text or a plot. Actually extracting the information, including what molecules are there [in the atmospheres], also depends on assumptions. A lot of this work will depend on your analysis pipeline, and of course, these things are very complex.

For example, the James Webb Space Telescope was launched almost half a decade ago, but we are still in the early phases of trying to stretch how much data we can extract from the information it gives us. It’s not that the data isn’t there, but we need to be very careful in how we extract it to get the correct, or most likely, measurements. I think that’s also something the Habitable Worlds Observatory could certainly improve on. A lot of the knowledge we’re gaining now about treating large amounts of data from this new generation of telescopes will also help us in the future when we build larger and better observatories.

The point is, yes a mission like HWO should be able to actually look into terrestrial exoplanets and extract a lot of information about the atmospheres. Fingers crossed that it actually comes to be!    

The idea that there are different ways to interpret the same data, I think that’s such an important point! And we’ve seen this as the question of phosphine on Venus has evolved.

From your models, you estimated that Proxima b and TRAPPIST-1e could have habitable temperatures, which seems like good news for astrobiologists. Were these results pretty much what you expected, or were there any surprises?

I did expect similar temperature results, based on previous modeling that’s been done. For an Earth-like atmosphere, it lined up very well with what we expect from other, more complex models. But I was positively surprised by how flexible the model is. Even under various conditions, if you exclude some edge cases where you push the variables to their limits, the model is very consistent at predicting what the temperature would be. It agrees well with more complex models, and it’s not bound by the same assumptions. So, I could populate the expected temperature on, for example, Proxima b or Trappist-1e for a Mars-like atmosphere, and I believe that’s something that hasn’t been done before.

In addition to modeling exoplanets, you’re also a microbiologist! You subjected spores from the extremophilic fungus Aspergillus niger to the levels of radiation you calculated for these planets. Why did you choose A. niger as your model organism for exoplanet life?     

I worked with Aspergillus niger in particular, but the species is not so important. Here, I’ll discuss Aspergillus in general because the genus has a lot of common characteristics. First of all, it’s a filamentous fungus, so it is not just the simplest single-celled lifeform. It forms hyphae and more complex colonies. It also produces a pigment, which is melanin, and produces it in very high quantities. The spores are black because of that melanin. 

So, there were two main reasons why we decided to use this organism. The first one was precisely the presence of this pigment. But the organism itself is cosmopolitan, meaning it can exist anywhere. For example, the mold we have in our house is Aspergillus. It’s also been discovered in space stations. In both the Mir space station and the ISS [International Space Station], fungi like Aspergillus have been discovered growing due to the humidity and generally favorable conditions for them. But that’s also in microgravity, with increased radiation in the environment. So even though it’s cozy inside the space station, it’s still in an extreme environment. It was not grown on purpose there, so it wasn’t expected to grow so well and to essentially contaminate it. This cosmopolitan but still extremotolerant nature is what makes it so interesting for the possibility of not only being present on exoplanets, but colonizing them. Not necessarily to a global scale, but a very large scale.   

There’s also this pigment, melanin. About 2 years before I started my work, some really interesting proposals had come out about how organic matter similar to melanin could have been delivered to the early Earth by meteorites and converted to melanin, for example, with UV radiation. If it could’ve been present in a considerable amount in the early Earth, it could’ve helped the origin of life, or at least been a protective factor for early cells.

We also know that melanin is present throughout the whole tree of life. We produce it, but bacteria do as well. Almost every branch of organism has the potential to produce some kind of melanin. This means that most likely the first instances of that molecule were incorporated very early in the web of life. Therefore, we thought it could be interesting to test that as a hypothesis for other planets. Even if it didn’t happen on Earth, it is feasible that it did happen on other planets. So, we wanted to test if the melanin in this fungus really gave it an advantage over organisms without protective pigments.

Am I right in thinking that melanin acts mainly like a natural sunscreen? Or does it also have other protective properties?

You’re absolutely right, it is kind of a natural sunscreen. That’s the main function we associate it with, radiation protection. Melanin is a varied group of molecules that have this brown-black color in the visible range. There are different types of melanins, but all of them offer some type of radiation protection, especially against UV radiation. They also have other cellular protective factors, for example, as antioxidants or in terms of cellular messaging or scavenging. The scavenging is related to being antioxidants, as it’s scavenging of reactive oxygen species, which are these nasty molecules that damage cells. Melanins are able to capture those molecules before there is damage and protect the cells from that. They also have roles in cellular messaging and other protective roles that we don’t understand as well yet, but we know that it’s a multifaceted molecule. It’s not just a sunscreen, there’s much more to it.

I could see how that would be really advantageous to have around for early life! I’ve read about tardigrades surviving radiation and various other harsh Mars-like conditions, and some researchers have talked about incorporating tardigrade genes into other animals to survive on Mars, or even incorporating them into human space technology. Do you think melanin would be useful for something like that? Should researchers be looking at incorporating melanins into space suits to make them more resistant to radiation? 

Yes, absolutely! It’s a more emerging field, but there is research being done on it. Tardigrades have a lot of complex cellular machinery that allows them to repair their DNA, for example. They can also go into a kind of stasis state, called cryptobiosis, where they go dormant to protect their DNA and proteins, and they kind of lay low without consuming a lot of energy for a long time. And they can resist very high radiation, and vacuum, and other conditions, then they wake back up when conditions are favorable again. But those are several factors, and not all of them are easy to incorporate when we talk about the human space flight perspective.

What has been studied a lot is using melanin as a physical shield. So, extracting melanin or using an organism that produces it intracellularly and having a barrier, either of the melanin or the organism. Of course there are several problems to fix, but at least for UV radiation, it is a very effective shield. There are hints that it can also protect against some X-ray radiation, although that is not so clear, or depends on conditions. The biggest problem there, from a human space flight perspective, is the cosmic rays. Heavy particles like protons, iron, argon, and other heavy ions can go through the melanin. For that, you need a lot of mass, like a water column that’s a few centimeters in depth. But water also adds a lot of weight to your space suit or spaceship.

For a lot of scenarios, melanin would be enough. It would also be feasible to produce in space when you talk about biotechnology, and the use of microorganisms for in situ resource utilization. Besides producing food, water, and oxygen, microbes could produce this melanin that you could use for radiation protection. We know that there are caveats, but it is a simple and potentially very important tool that we could have for radiation protection in space.

I love it! In 100 years, humans will be living on Mars, eating and wearing Aspergillus.                  

Probably, yes! I will be there for that.

So, what's next for you? Are you continuing to study how microbes survive in extreme exoplanet-like environments?

Yes, I started my PhD in September, and I’m continuing in the same field of investigation. I’m no longer working with Aspergillus, but I’m working on a broader project that involves the same key points. It’s bridging between the astronomy side of the planetary sciences, trying to understand and model how exoplanets could be. Then, translating that information into lab experiments with microorganisms, which then inform our models and predictions of habitability. I’m really trying to constrain and help update our definitions of habitability. I’m collaborating with some laboratories here in Porto in Portugal, but also with the Natural History Museum in France, where I’ll be using their planetary simulator. I’ll be working on more complex environmental simulations and coupling that with models of microbial communities on exoplanets. So that’s my roadmap for now!

Very cool! I’m excited to see how it turns out! Are there any final points you’d like to mention that I haven’t asked you about? 

I’d just like to encourage everyone to keep trusting science. I think nowadays in the world, we’ve been losing trust in science and the focus on the importance of fundamental science. We spoke a bit about applied science here, but most of my research is fundamental science. It’s cool, it’s great, and I love it, but it will also have applications. Maybe not in two or five years, but definitely in 20 or 30 years, we’ll gain important knowledge from this research. It is really important to keep funding this type of fundamental research and keep supporting it. That’s a bit outside of my actual research, but I think it’s equally important!