Astrobiology Revealed #1: Armando Azua-Bustos

Armando Azua-Bustos on Atacama’s “dark microbiome” and what it means for the search for life on Mars

By Aubrey Zerkle

In our inaugural Q&A for Astrobiology Revealed, we talk with astrobiologist Armando Azua-Bustos about his recent paper “Dark microbiome and extremely low organics in Atacama fossil delta unveil Mars life detection limits”. Armando discusses growing up in the Atacama Desert, why he thinks it's harboring 100 million-year-old DNA, and how we should (or shouldn’t!) be searching for life on Mars. (This interview has been edited for length and clarity.)

Could you please state your name and affiliation. 

My name is Armando Azua-Bustos. I’m a research scientist in the Department of Planetology and Habitability of the Center of Astrobiology, which is located near Madrid in Spain.

In your TED talk, you said the Atacama Desert was the “most Martian place on Earth”. Can you tell us what it is about the Atacama that makes it so similar to Mars?

First, it is a very dry place. The Atacama has been a hyper-arid desert for the last 15 million years and an arid place for the last 150 million years. Well before the demise of the dinosaurs, the Atacama Desert was a dry place that got progressively dryer over time.

Second, due to the clear sky of the Atacama, which among others explains why it is so dry, you have a huge level of UV radiation there. You might remember this UV scale that is used in public to tell you when to use a hat or sunscreen? That goes from 1 to 11. There are places in the Atacama that are measured at levels of 20 [on that scale]. That’s really high UV!

And third, most of the soils in the Atacama are quite saline, and part of the Atacama is covered by perchlorates, which are highly oxidized species that you find in the soil on Mars. So, the combination of extreme aridity, high UV, and soils with perchlorate, that’s why it’s a good analogue model for Mars. That was first proposed by Chris McKay in 2003, and in time many people have studied the Atacama as a Mars analogue model.

You also talked about how you have a unique perspective on this environment because you were born in the Atacama. How much influence did that have on you becoming an astrobiologist?

Yes, I was born and raised in the Atacama and lived almost 20 years there, until my family moved to the south of Chile. When I went to college, I went into wine making. But I quickly got bored in the wine making industry because, at least for myself, there wasn’t any intellectual challenge. We are making wines more or less the same way the Greeks did it.

I started getting bored, so I left that and I came back to study again. And I learned that, for the science, where I was living was more interesting than what I was actually studying. I had a distinct advantage in that it was a regular activity for me and my brothers and my family to go explore the Atacama for everything – the telescope at night, the stars, to look for lost gold, lost cities, looking for fossils, and whatever we could find around. As I was leaving there on the plane I remember saying “I will never come back here, because there is nothing to do in this wasteland!” But now life pulled me back. There is so much to do! At some point I said “Well, I have a huge advantage here, in which I know this place so well. So let’s apply the newly born research field of astrobiology in order to understand this environment as an analogue for Mars”.

I’m really interested in your recent paper about the Atacama “dark microbiome”. I’d like to hear about how this specific project developed and the science behind it. Let’s start with, why did you choose this particular site?

This site, Red Stone, is very near the coast, and the coast is rare for the Atacama. The surrounding desert has a huge mining industry, everything from copper, to nitrate, to lithium, etc. There you have the coastal city of Antofagasta, which is the biggest city in the area, where you have good hotels, restaurants, and a place to stay, a place to eat, a place to rent a truck that can go inland. And one of the ways to get into the desert and out of city is through [the Red Stone] area. For many years we noticed the red coloration of this site, which you don’t find in other places in the Atacama, and we wondered what was explaining this red coloration. So a few years ago when we were going in and out of the city we would stop at this site to sample there. And we found out the soils were leaching hematite, the same iron oxide that gives Mars its red color. This is an interesting environment here, because this is one aspect of [a Mars] analogy that has not been described any place in the Atacama. Now we have a site that is really arid, has high UV radiation, has a high percentage salt, AND it has hematite.

What did you originally set out to do at this site?

We first characterized the site from the geological and mineralogical point of view. It was already  described as a remnant of an ancient river delta, where you have a succession of sandstones and mudstones and different type of salts, including anhydrite and gypsum. That told us that well over 100 million years ago the Atacama was already a very dry place and that delta formed under arid conditions. So we have a very interesting site, the remnants of an ancient river delta that formed under arid conditions, that also had a lot of hematite. The question that followed was, what would be the habitability of such a place?

We started the same way with this site as I have done with other sites in the Atacama. I used culture dependent and culture independent methodology to describe the microbiology of the site. Through the culture dependent way, using different growth media, you can see what proportion of things we can grow and isolate from the site. Another way is the [culture] independent way in which, if you are lucky, you can extract DNA from whatever is in the soil there and you can do genome sequencing. I use [DNA sequencing] as a way to see how many microorganisms are at this site that I was not able to grow, because I don’t know what the heck they are eating, but at least I know they are there.

The surprise came when we had all the sequences from all the different types of samples at the site. When we collected all the information, almost 10% of the sequences were unidentifiable. Meaning when we compare these with the databases, which are quite populated nowadays with sequences from everything you can imagine on Earth, the databases could not find anything similar. We cannot tell the order, family, subfamily, or the genus, all we can say is that this DNA has come from a [domain] Bacteria. From time to time I have found such sequences in other sites of the Atacama, but not 10% of sequences! When we add the 10% of sequences that we cannot tell anything, with another 40% of the sequences that we can identify only from the higher ranks taxonomy, like order or domain, it’s almost half the sequences that we cannot tell where they are coming from!

This is interesting because we know there are a lot of microbial species out there that we haven’t found yet. We can’t sequence their DNA, but we can see their effects [in the environment], so many people have introduced the term “dark matter” for microbes that we have not described yet. Here we have sequenced the DNA, we know that it comes from microorganisms, but we don’t know the identity of such microorganisms. That’s why we proposed the term “dark microbiome” in parallel to the concept of “dark matter”. The dark microbiome is for microbes that we have DNA evidence they are there, but we have no idea what they are doing there.              

What would you say was the most exciting result you found?

First in this case of the dark microbiome I think there are two alternatives that could explain that. One is that there are some current, extant microorganisms there that are so different from anything we’ve discovered on Earth that you cannot tell [what they are]. I do not think that is the case. I think the most likely case is that the dark microbiome are the remnants of the DNA [from microbes] that used to live at that site when it turned into fossils 120 million years ago, the age of the site. It made sense to me that you have DNA sequences for some microorganisms not closely related to anything nowadays, because of the [proximity to the] ocean. And we have these clay minerals like on Mars that can maybe preserve this.

Second, now that we have a really nice model of Jezero Crater for example, the next question I had was - what are the instruments that are on Mars going to see when they look at the very same samples? So we tested this with 4 instruments [identical to those currently on martian rovers] to look for organics. I sent the same set of samples to each team, and the response was more or less the same – “Eh, Armando, we have been able to find very little if not nothing.” In some cases they had to alter the protocol, they had to change the conditions, to find anything. And each of them told me the true reality is that what there is on Mars is 10 times less than what we have here [in our samples]. So if we have issues detecting something with our instruments here, on Mars we will not see anything. That was very surprising also. And that’s what we reported, we said it’s not going to be easy to detect any evidence of life on Mars because we are sending instruments that are having real trouble with a site here on Earth. It might make sense that if you can bring pieces of Mars back to Earth you don’t have to go to all the trouble to transport all of this bigger and bigger equipment to find anything.

As you said, you did a really impressive amount of work on this site, characterizing the geology, the mineralogy, the microbiology, and the organics, which I think nicely encompasses the interdisciplinary nature of astrobiology. What was the most challenging aspect of the project?

The most challenging part was the pandemic! Because it was more or less in the middle of the pandemic that we started sending samples to different teams involved, and in many cases they were not able to go into the labs, the labs were closed for a long time. Also, those instruments are so interesting that they are always in high demand, so I had to wait for people to be able to analyze those samples among many other samples they had to analyze. We had finished the characterization of the site on our end by the middle of 2020, but we had to wait until more or less 2022 to get all the [data] from the different teams because of that. Covid was a very interesting time for anyone trying to do proper science.

You mentioned all the different teams of collaborators - how important are collaborators in doing this type of research?

Collaboration is fundamental. I learned early in my career that you can move faster and better when you collaborate on something. If I need to do DNA electrophoresis or whatever on some samples, I can start trying to do it in my lab. And maybe I can get it to work after a few months. But why reinvent the wheel if I can contact my colleague next door or in the next city that does it on a regular basis. And they would do it much better than I would do. This work would not have been possible without collaboration of all of those that were involved, because everybody used their expertise in order to find all the incredible things in these unusual samples.   

Going back to what you said earlier, what do you think these results mean for the search for life on Mars?

In the case of Mars I think we have to redefine again, first at a very basic level, what are we looking for in particular on Mars. We are looking for organics, this is typical, right? Because Perseverance has SHERLOC, which is able to detect organics by fluorescence. And we have detected already organics on Mars. But being very strict and rigorous, would you say that’s evidence of life? You can’t, unless another method [can] analyze the very same samples and you have something like a very long chain of carbohydrates or nucleic acids, maybe then we have possible [evidence].

You may also have read this paper that came out in the past week in which they detected a huge variety and amount of organics in the asteroid Ryugu. Does it tell you that the asteroid Ryugu has evidence of life? No! But it has a lot of organics. So, let’s imagine in a year we have found a lot of organics on Mars, does it count as evidence of life? Probably it’s a big chunk of a material like Ryugu that has fallen there. The discussion to be having now, I think, is what exactly are we looking for as evidence of life on Mars. And then after we say what are we looking for, okay what are we going to use to look for what we need to be looking for. That’s the way science moves forward.

Also keep in mind that if we are willing to have a human mission to Mars and maybe even a temporary base on Mars in the next 30 years, once you get humans to Mars, there’s no way to protect the planet anymore. Unless you could sterilize the astronauts on Earth so then they could be completely clean to send there. There would be no way to do that! So we are against time in order to analyze a pristine environment before they send a person to contaminate everything.

In our paper in Scientific Reports a few years ago we show that in an environment like the Atacama, wind is very efficient in transporting life across the desert in a matter of hours. So, if you have a human base on Mars, in this place where we know we have planetary dust storms all the way around Mars, how do you prevent that from contaminating your site with Earth bacteria? You can not.

Do you think sending a man to Mars is a bad idea?

I mean, it’s a good idea. But not yet, not until we figure out a way we can come in and out as clean as possible.

So what’s next? What are the next steps in this research?

In the case of Red Stone, there are people interested from different institutions to drill down at that site, in order to understand better what happened in time. With [drilling], we have to keep in mind that it just works for a minute part of a very big site. When you walk from the point of origin across the river delta, there probably will be new findings every step of the way.

In my case, I am interested in trying to see what these DNA sequences are actually telling me. Where are they coming from – is it the weird biosphere, or is it the very old biosphere? As I said I think it is a very old biosphere but I have to put my thinking cap on in order to see how we come up with which is the case.     

Is there anything I haven't asked about that you would like to add?

Also very important to me is to try to protect the site, because it is very close to the city and very easy to be affected by the humanity that are around. I’m trying to convince the local authority that this site needs to be protected, with controlled access so that not just anyone can get with their trucks into the site. Also, they must see this as a huge business opportunity for the locals, because they can sell the concept that right here we have the most Martian place on Earth. So let’s protect the site, let’s build a nice Mars café looking over the site, and then sell whatever you want to sell there. And make it a point that this is an attraction you don’t have anywhere else on Earth!