Astrobiology Revealed #14: Lígia Fonseca Coelho
on why purple is the new green when searching for extraterrestrial life
by Aubrey Zerkle
This week we chatted with microbiologist turned astrophysicist Lígia Fonseca Coelho about her paper “Purple is the new green: biopigments and spectra of Earth-like purple worlds.” Lígia is currently a 51 Pegasi b Fellow in the Department of Astronomy at Cornell University, but she started this project when she was on a Fullbright. In this Q&A, Lígia discusses her research path from cancer to exoplanets, the kaleidoscopic colors of “purple” bacteria, and why dryer planets make better targets for astronomers hoping to spot colorful life. (This interview has been edited for length and clarity.)
Lígia Fonseca Coelho, 51 Pegasi b fellow, working at Cornell University.
Photo: Ryan Young/Cornell University
How did you become involved in exoplanet research?
I started as a microbiologist working for the biomedical sector. My first research project as a grad student was on cancer drug therapy, I was developing a new peptide with anti-cancer properties for lung and breast cancer. But then I got this very generous PhD grant in the MIT Portugal Program, which is a consortia program between Portugal and MIT here in the USA. It was developed so that Portugal would have grad students embark on this PhD experience, and take some entrepreneurship classes given by MIT people, to broaden our perspective. It’s a very entrepreneurial and business-pro type of PhD program.
I always loved space, but I was doing cancer, and I was not really passionate about it. I believe in the mission very much, but I just didn’t have the curiosity. And that’s very important when you’re doing a PhD, because it’s what makes you stay after 6pm. You need to have the curiosity fueling you. So I was concerned about my future, because I didn’t have the passion for where my path was taking me, but it was kind of the safe choice. You didn’t have astrobiology implemented in Portugal back then. But then I got this great advice from one of the entrepreneurs, who said “Ligia, nobody is ever going to fund you for 4 years again, to study what you really love, with no agenda and no constraints. So pick what you really love, because this is your one-time opportunity to do so.” That changed my life, and made me think “Okay, so I’m gonna go with astrobiology!”
And that’s what happened! I had this big committee of PIs and supervisors. I had microbiologists, but they did ocean microbiology so were not really focused on astrobiology. I had polar scientists, because I was going to study icy worlds and I needed to do field work in the Arctic. I had analytical chemists, because I was going to do spectroscopy and other chemical analyses on the ice. And I had an external committee with Lisa Kaltenegger, because I was going to do the connections with astrophysics and exoplanets. It was a lot of people to please at the same time, and a lot of multidisciplinarity! It definitely taught me how the jargon, the expectations on the quality of the data, what is considered significant, are really different from each other [in the different fields].
All of that I had to navigate and learn. It gave me a very good skill set so that I could change to a deep astrophysics type of post-doc, where I wasn’t afraid to be the dumbest person in the room. Because yes I come from biology and I don’t know much about astrophysics, but I can learn <laughs>. And I don’t mind if you know that! So I was offered the job here by Lisa Kaltenegger, I was offered the opportunity to learn the astrophysics component, and time to do so. She’s an amazing mentor and an amazing PI that really focuses on making us grow as a scientist, and she is not afraid to give us the time for that. I started my post-doc and here I am!
In your recent paper you argued that “purple is the new green” when it comes to looking for life on exoplanets. Could you tell us briefly what purple bacteria are and how they’re different from the green life we usually think of?
Purple bacteria are phototrophic bacteria that do photosynthesis, but using different energy than green bacteria. The green photosynthesis that we know, when we’re talking about microalgae, cyanobacteria, and plants, is a chlorophyll a-based photosynthesis. That will need different parts of the spectra of visible light, blue and red, and will produce biomass. A plant will produce their own food, and they will be food for the remaining food chain here on Earth. It will also produce oxygen, and that is why it’s called “oxygenic photosynthesis.”
The purple bacteria do what we call “anoxygenic photosynthesis.” They also produce biomass, for themselves and whoever eats them, but they will not produce oxygen. Some of them are actually very sensitive to the presence of oxygen, which makes us believe they evolved in a time before the Great Oxidation Event had taken place, on the early Earth. At the same time, they use heat, or near-infrared [wavelengths of light] mainly. You can actually grow them in the dark in the lab with an infrared source of radiation. This means they’re always going to be outcompeted on present day Earth by the oxygenic photosynthesis because there’s just so much visible light flying around that [the oxygenic photosynthesizers] will just grow and grow and outcompete anything else. [Purple bacteria] do exist on Earth today, but in niche ecosystems.
We call them “purple” because the first ones that were found were purple, but they are this amazing diverse group of organisms. That purple pigment is a carotenoid, which is an accessory pigment that is visible to us. It’s not even the pigment that absorbs the infrared light and makes photosynthesis, that’s bacteriochlorophyll. But these carotenoids are so diverse that some of them are not even purple! They can have different colors! In figure 1 [of the paper] we showed all the different colors - it can be purple, it can be pink, it can be orangey, it can be yellow. So, you can have yellow purple bacteria! <laughs> That’s the problem with nomenclature and how taxonomy in microbiology evolved. We are calling things by one name that no longer makes sense after we start learning about their diversity. But through the lens of the telescope it doesn’t make any difference! What the telescope will mainly pick out is the bacteriochlorophylls, the diagnostic signature of these bacteria, which are very visible in a spectra.
Purple bacteria mat in Little Sippewissett Marsh. Lígia sampled some of the microorganisms measured in the last study in this location. Photo: Lígia Fonseca Coelho
They are really beautiful! I love the pictures in your paper of all the different colors of “purple” bacteria. What about these microbes makes them so likely to thrive on exoplanets?
We take advantage of what we learn about life here on Earth. For the example of green photosynthesis, we know that if you have no limitation of water and you have no limitation of [light] on the right wavelength, then our landscape will mainly be dominated by green colors. If you take that as an example, and just shift the wavelength far to the infrared like it exists around M stars, then it’s plausible to believe those landscapes would be dominated by purple, or the other colors we show. With bacteriochlorophyll, you actually have a pigment that prefers [the infrared light], and that does photosynthesis too. So those planets are going to have a strong [spectral] feature of bacteriochlorophyll instead of chlorophyll.
You brought up how interdisciplinary this project is, which is fairly representative of the field of astrobiology - you’ve got microbiology, spectroscopy, even modeling. What would you say was the most challenging aspect of this project?
That’s a great question! I think the most challenging aspect was not any of those components. This project is including the Carl Sagan Institute here at Cornell, which was started by Lisa Kaltenegger, and Lisa has also done a lot of work on this color catalog. I want to ensure that the credit [for this work] is also given to Lisa, because this is a project envisioned by both of us. We wanted to put in a database all these diversity of colors that can be dominating the surface of exoplanets. With this said, it means that we are creating this amazing and big library of cultures that I’m taking care of and growing here right now. It’s becoming difficult to select where my time goes in the growing of them. Where do I stop measuring? Which biota will stay out of our exoplanet research? The more I get, the more I grow, the more I have to tend to them. So the main difficulty has been to try to tame our curiosity and focus on a group of biota. They’re all so beautiful and interesting!
The most interesting part, and what kept me very motivated was the modeling. Because it was a new thing for me, and it came from all the mentoring I have here in the astronomy department by my colleagues and my mentor. And I’m still learning with them. That part was very stimulating, because I was feeling I was growing as a scientist as I was doing it.
I did some microbiology earlier in my career, so I definitely commend you for that work. It is not easy keeping those little guys alive! You get so attached to them, and sometimes they just die, and you don’t even know why!
We have a pet one, that I can show you very quickly (shown in the photo below)! We’ve been seeing how long we can keep it without dying. It’s been going since February 2023. It’s seasonal - we have times where it goes not purple at all because everything’s dying, then the one that survives eats everything and reproduces again and becomes very purple. It became this self-sustaining purple planet! There’s some input of CO2 for photosynthesis, because we keep [the lid] open, and there are probably some other volatile products it’s producing. So it’s not a closed system like a planet is, but it’s the closest we get to a self-sustaining system.
Wow, that’s so fun! Is it a specific species, or is it from a specific environment?
It is a specific species of Gloeobacter, which is a purple cyanobacteria. We also use it in the study, to show that even on a cyanobacteria world with oxygenic photosynthesis, purple is more interesting than green in terms of the spectra. The spectra [of the purple one], instead of one step function it has several, so you maximize your ability to see things and to model things. So we kind of adopted it as our pet!
Winogradsky column of green biota (left) and Lígia's "pet" purple bacteria (right). Photo: Ryan Young/Cornell University.
In your paper, you modeled the spectra astronomers would see if purple bacteria dominated different types of Earth-like planets, everything from ocean worlds to ice worlds to full Earth analogs. Do your results suggest there are some types of planets where they might be easier to spot?
Yes on the “dry Earths”, those were the frozen Earths and the snowball planets, we would see more distinct features because we are using [spectra from] the dryer version of our biota. In 2022 we published the other color catalog of life, and we proved that in drier environments the biopigments are much more reflective. We show the same here. This would mean an Earth with not a lot of liquid water around, not a lot of water activity - in those cases the biosignatures are stronger in our simulations, as we already hinted in the first paper. Even with clouds, you could see the biosignatures. Even with overlap with atmospheric features, you could see the biosignatures. We used a slightly colder version of the atmosphere, because we are modeling a lot of frozen and snowball Earths, and still you could see the biosignatures.
That’s amazing! You also produced a pigment database with spectra for different types of purple life. How do you expect astronomers will use your database to look for life on exoplanets, are there current missions or future missions planned where they can already start doing this?
Yes, and thank you for that question! First, the data is already available. It’s on a separate website, but it is in an archive for everybody to use. We also did supplemental material with simulations for Habitable World Observatory and for the Extremely Large Telescope, and we envision that these data can be used for simulations and modeling in preparation for those missions. The missions are being designed to look for planets around M stars mostly, for a lot of reasons. Because of the size and proximity to the star, the habitable zone is kind of shifted so you don’t need as many [telescope] hours to look at these planets.
There are a lot of advantages in looking for planets around M stars, so we want people to be able to use realistic biota on the surface when they’re doing these models [of what to look for in the planets’ spectra]. We hope this is one of the stepping stones [for scientists] to have enough data to create high-fidelity models and simulations. Right now people are mainly using rocks, basalt, simple surfaces. Now they have the tools to increase the complexity and fidelity of their [model] surfaces, if they want to use biota. This is one of the immediate uses the data can have, and that we hope people will start to implement. The missions are being designed now, so now we need to have the right data to ensure that in 20 years we are not saying “Shoot, wrong wavelength!” <laughs>
Hypothetically speaking, if we found evidence of purple bacteria on an exoplanet, do you think people would be as excited as they would be if we found evidence of something more familiar, like green life or technosignatures?
I don’t think it would make a difference in how excited people would be. I think any evidence [for extraterrestrial life] that has high significance and that is widely accepted and rigorously tested would make everybody very excited. It definitely will create a reaction, and it doesn’t matter if it’s purple or if it’s green in this case. I do believe people are very excited about the purple version. In previous papers people talked about the purple early Earth, and people are very passionate about the idea that Earth was purple before. There’s this connection with purple that people have!