How will we know if we’ve found Martian microbes?

By Reed Stubbendieck (@bactereedia)

curiosity
Figure 1. Curiosity Mars Rover taking a selfie. [Source]
Drew’s recent blog post spurred a conversation between the two of us about the first extraterrestrial life that humans will encounter. In the end, we both agreed that when/if humans discover aliens, they’ll most likely be microbial.

We are not alone in this assertion. At this very moment, the NASA Mars rover Curiosity (Fig. 1) is currently roaming the surface of the red planet using its suite of instruments to detect and characterize organic molecules that could be indicative of life from ancient aqueous environments. Intriguingly, data already collected by Curiosity has indicated that there are environments on Mars that may have once been habitable for microbial life!

Unfortunately, Curiosity is unable to directly detect living microbes, which begs the question: how will we really know that we’ve found genuine alien microbes?

To address this question, we first need to review the seven fundamental characteristics of life. All living organisms 1) are composed of cells, 2) are ordered, 3) grow, 4) reproduce, 5) pass down genetic information, 6) possess homeostasis, and 7) possess metabolism. In this post, we will consider how growth, reproduction, genetic information, and metabolism are currently used by scientists to detect life both on Earth and in the greater universe.

First and foremost, I am a microbiologist and prefer to follow an old proverb that states, “seeing is believing”. Thus, I would personally be most convinced of life on Mars if I saw an alien microbial colony emerge from a sample cultured on a Petri dish, which would demonstrate the necessary characteristics of growth and reproduction. However, unfortunately this level of evidence is most likely untenable for the foreseeable future.

As an example, on Earth if you directly count the number of bacterial cells from an environmental sample, such as soil or ocean water, using a microscope and then culture that sample on a Petri plate, only ~1% of those bacterial cells will form a visible colony. This phenomenon is known as “The Great Plate Count Anomaly” and has plagued microbiology since its inception. The anomaly is partially caused by how bacteriological medium is prepared, but is more majorly a result of our lack of understanding the nutritional requirements for different individual bacterial cells. Put another way, if we don’t know what Martian microbes like to eat, then we’ll be unable to coax them to reveal themselves.

iChip
Figure 2. The iChip after being removed from the ground. [Source]
On Earth, we’ve developed methods that circumvent and accommodate these picky eaters. For instance, the isolation chip (iChip, Fig. 2) is a relatively recent technology that allows microbes to be cultured in situ (at the place of their origin). This device works by trapping individual microbial cells into tiny wells that are sandwiched between semipermeable membranes. The membranes allow the passage of molecules between the trapped cells and their environment. Thus, the iChip allows microbes to access the nutrients they require without requiring scientists to determine specific requirements and formulate special media. This approach has been used to cultivate up to 50% of the microbes in a soil sample, which led to the discovery of a new antibiotic scaffold from a previously uncultivable bacteria!

Alternatively, as direct culture is a bottleneck for identifying living microbes, culture-independent approaches based on DNA sequencing have exploded in the field of microbial ecology. The primary approach that is used is called amplicon sequencing, which allow us to use specific DNA sequences as barcodes to identify different microbes. An alternative approach is to sequence all of the DNA present in a sample. This approach is called metagenomics and has been used to characterize the genes that are present in different environments on Earth. An advantage of metagenomics over amplicon sequencing is the ability to assemble entire intact genome sequences from environmental samples!

kate_rubins_nanopore
Figure 3. Astronaut Kate Rubins using the Oxford Nanopore MinION Sequencer in Space [Source]
Though once prohibitively expensive and technically challenging, advancements have rapidly decreased the cost of DNA sequencing and shrunken sequencers from the size of a refrigerator to a device that can fit in your hand (which has even been used in space, Fig. 3)! Thus, it may soon be possible to equip our future rovers with their own tiny sequencers. However, there are still important hurdles to overcome before implementing DNA sequence technology. First, direct DNA sequencing can’t distinguish between living and dead microbes. Further, contamination with microbes or microbial DNA from Earth may confound our analyses. Finally, a practical consideration is sampling throughput (the amount of samples that can be processed in a given period of time) and reagent usage.

Whether we attempt to directly culture microbes or sequence their DNA from Martian samples, there’s another practical consideration to discuss: where do we sample? As mentioned above, our rovers will likely carry only a limited amount of reagents for bacterial culture or DNA sequencing. Thus, we need to determine a method to narrow our search space for microbes: we need to identify biosignatures of life.

Fortunately, Curiosity is already equipped with instruments that detect organic molecules. Remember, all living organisms possess metabolism, perhaps we can follow molecules like methane as biosignatures and locate microbes. Unfortunately, because Mars has no atmosphere, the planet surface is bombarded with ultraviolet light, which may destroy volatile biosignatures.

sagan_and_viking
Figure 4. Carl Sagan posing next to a model of a Viking Lander in Death Valley, California. [Source]
As an alternative, we can attempt to identify biosignatures under controlled conditions. One such method was employed in the 1970s during the Viking program. As part of this program, two landers (Fig. 4) were sent to Mars with the mission to search for evidence of life. One experiment performed by the landers was called “Labeled Release”. A Martian soil sample was combined with a mixture of seven radioactive 14C-labeled nutrients and monitored for production of labeled carbon dioxide (14CO2) gas, which would suggest that living organisms had consumed the nutrients and produced the gas as a waste product. The experiment gave mixed results: though both landers initially produced positive results, repeated injections of the labeled nutrients failed to yield additional 14CO2. Though controversial, it is now believed that the 14COobserved in these experiments was produced abiotically. Perhaps future experiments will combine radioactive labeling with culturing or sequencing to identify microbes.

The above list of approaches to identify life is by no means exhaustive. However, I hope this post has highlighted the difficulties that scientists face not only in our search for extraterrestrial life in the universe, but also in characterizing the vast diversity of terrestrial microbes that inhabit on our own planet!

Will you ever meet ET?

By Andrew Anderson (@AndersonEvolve)

One of my favorite shows is Firefly−if you haven’t heard of it, check it out. An aspect of the show I found enjoyable was, despite being science fiction, it explicitly stated there were no known aliens across the regions humans had expanded to. The thought of seeing the results of evolution on another planet would be amazing, and the mental games trying guess at what that life might look like are enjoyable. In reality though, I don’t think we will ever encounter alien life*, much less intelligent life, for a long while (>1000s of years) if at all. I am hardly the first to make such a claim, but I would like to bring up some of the reasons I don’t think we’ll encounter extraterrestrials within anyone’s lifetime.

The best place to start is with Drake’s Equation (see link for details), it’s basically a probabilistic statement made up of dependent chance events and time components. 

eq

Therefore, the chance of each successive event is multiplied by the previous then multiplied by the time that all chance events have occurred. As you can see the Drake Equation includes 6 conditions that have to be met, making the odds of an outcome lower with each condition (e.g. rolling a 1-5 on a die occurs ⅚ of the time, but doing it 6 successive times happens ~⅓ of the time). What’s more, we have no idea what the probabilities are for the later parts of the equation. The rate of star formation and the fraction of stars with planets are able to be reasonably estimated. Everything else is challenging to define. The big issue is we have a sample size of 1. We only know of one planet that formed life, evolved intelligent life, and sent signals−Earth. This is a two-fold problem. If I drew a number at random, what was the probability of drawing that number? Without knowing either how many draws it took or what range of numbers I had to choose from, you cannot know. Additionally, we now have what is known as a sample bias. While we are piecing together how life got started here, is that the ONLY way? Once life begins developing on a planet, what is the likelihood intelligent life appears? We might conclude it’s 100% given what happened on Earth, but remember the majority of Earth’s history is dominated by non-intelligent life*. Humans are a relative blip on the history of life and there’s no evidence that other groups evolved intelligent life. Were it not for a well-timed meteor, there still might not be intelligent life. All of this is to say, we’re not sure how to define all the portions of Drake’s Equation, so anyone claiming it supports their idea (even one saying it’s not possible) is on unstable ground.

Now we can get into some fun probabilistic concepts. Drake’s Equation doesn’t offer much help, but it comes down to a fairly simple question: is Earth, and the process of life on it, rare or common? Some would invoke the mediocrity principle, that is if you only have one observation, it is more likely you observed a common event than a rare one. Imagine it’s your first time snorkeling on a reef; the fish you see are probably the more common fish on the reef. The problem with this is, since we are alive, we had to come from a planet that met the criterion for life, so there is no way our limited observations would NOT include a planet with life on it. I actually think life on Earth could be more along the lines of the Wyatt Earp Effect; given the amount of attempts (planets in this case) it is an almost certainty that 1 will hit on the rare event. Wyatt Earp was notorious for winning gun fights without getting hurt, but given the number of gunfighters and gunfights, someone had to survive multiple gun fights through sheer luck. So life forming could be rare, but it happened at some point and, since we’re here, we see that outcome.

My point so far is we have no idea what the odds of life are, and I acknowledge that my suspicion it’s rare is just that, a suspicion. I think life likely does exist somewhere else out there, but we won’t see it anytime soon because: physics. Let’s assume there is a planet we want to check out for life. There’s a candidate at our nearest star ~4 light years away. Let’s assume that we have a spacecraft right now capable of traveling ~2% the speed of light, the current record is ~1.5%. That means it would take 200 years to reach the planet, plus 4 more just to hear if they found anything. Even if we hit on our first pass, it still wouldn’t happen in our lifetime. Our ability to detect non-sentient life is severely limited and there are no ways to be certain other than direct exploration.

Instead we would have to hope that lifeforms on another planet are sending some signal we can detect, which means they are likely intelligent, or they are likewise looking for life*. Radio waves are electromagnetic waves and travel at the speed of light but there is a delay. Try to imagine a conversation on Messenger where you only see what was written 4 years ago, that’s what it would be like to converse with our nearest star (Alpha Centari could only now tell us how they felt about Lost). This also makes Sci-Fi movies amusing with how communication and observation of events unfold (think about every intense radio conversation in space and how far apart they were–likely they were getting that message minutes or hours after it was sent). Humans have only been sending out signals for ~100 years which means only things 50 light years away could be responding at this point (travel to and from) which is 1/2000 of the distance of the Milky Way. We’ve only been listening for 50 years which means some civilization has to be sending signals at the right time to have them reach Earth in this exact range of listening time*. Consider a planet looking at Earth but is on the other side of the Milky Way, they would not find anything and have to wait 100,000 years just to hear it. All of this assumes we know what to look for or what to broadcast.

In order for us to find life it would have to be close, sentient, and signaling. While the universe is incredibly large, we’ve now severely narrowed our search window, which means life has to form easily and evolve intelligence frequently if we are to see it while you and I are here. I just don’t think that probable. Got something to add or something I missed?* Give me tweet!

Post-Script:

Scott Mattison (@FoolsPizza) added some thoughts (* in text, his thoughts in italics) that should be shared and responded to (my thoughts in bold).

  • I imagine we will find microbial life on colonized planets. I mean, we think we might have bacterial life on other planets in our local system This seems most likely, although we don’t have every step down for the formation. What we do know doesn’t seem like something any other planet would have gone through in its formation. 
  • Non-intelligent or non-sentient or just not up to our levels. It could be reasonably argued multiple forms of sentient life developed on Earth. Humans just won the competition. (Neanderthals vs. Homo sapiens). Also there is the question of intelligence level of apes and dolphins which clearly can learn communicative behaviors In this context I equate intelligence with the ability to send and detect signals to space. This is not at all the correct definition, but for a short piece it’s the easiest term I can think of. Yes, this means humans didn’t become intelligent until quite recently.
  • Fermi paradox –It starts with supposition that life is common and we should see them. I don’t agree with the premise.
  • This gets even worse. EM waves decrease intensity as they spread out through space through the inverse square law (not that inverse square law, the other one).Essentially intensity decreases on the order of distance squared. So even if someone is screaming out into space. They have to scream really loud to appear over the background noise and be screaming in our direction (although we can get pretty sensitive detection especially if they are sending out patterned signals). −Neat.
  • We could also see visible signs from ancient, more advanced races. Concepts of super structures that utilize the energy of their local star would be observable. (we have had some pretty well publicized incorrect interpretations of these things). −It’s still a matter of timing, the ancient civilizations had to have formed at just the right time for us to see, or the structures are durable enough to keep going.  I guess the fun question is does intelligent life persist once it is formed?