Latching onto Lichen

Categories: Extreme Life Interview
Rhizocarpon geographicum, species of lichen.
Credit: ESA

Bacteria can often survive the harsh conditions of space, and their toughness and adaptability have made them key candidates for the transfer of life between planets. But in a recent study by European scientists, lichen survived a trip in space even better than bacteria do.

The lichen flew on the European Space Agency’s Foton-M2 mission. A "Biopan" facility was located on the outer shell of the Foton return module and, once at the correct orbital altitude, it was opened to expose the samples of lichen inside. The lichen were subjected to the vacuum, wide temperature fluctuations, solar ultraviolet (UV) and cosmic radiation of space for over two weeks.

Two different species of lichen flew in the Biopan — Rhizocarpon geographicum and Xanthoria elegan — and they remained dormant while in space. When they returned to Earth, they exhibited no physical damage, and soon resumed their normal photosynthetic activity. Lichen are multi-cellular and eukaryotic, making them the most complex organisms to survive exposure to space conditions so far.

Field campaign in the Central Mountain system of Spain: Rosa de la Torre, INTA (left), Gerda Horneck, DLR (right).
Click to view larger image.
Credit: Torre

Astrobiology Magazine’s Leslie Mullen talked about the study with science team member Rosa de la Torre of the National Institute for Aerospace in Madrid and the European Astrobiology Network Association. They discussed how the lichen not only survived their journey, but seemed to shrug it off as a mere walk in the park.

Astrobiology Magazine (AM): Your study found that lichen are able to survive the harsh conditions of outer space. So let’s talk about what lichen are, and why they were able to survive.

Rosa de la Torre (RT): Lichen is an association between fungi and algae. There are two metabolic types on Earth: autotrophy, where the organism makes its own food, and heterotrophy, where nutrients are gathered from other organisms. Lichen is a combination of these two metabolic types. The algae make their own food through photosynthesis, and also supply nutrients to the fungi. The fungi, meanwhile, provide protection to the algae. The fungi form a shield, called a "cortex," and this protects the algae from extreme environmental conditions like UV radiation, extreme temperatures, or dryness.

We tested the lichen in low orbit, about 264 to 340 kilometers above the Earth. We discovered that lichen are very stable in these space conditions. When lichen become dry, they go into a dormant state. They sleep in this state until conditions become more favorable, and they can stay this way for a long time. We exposed our lichen to vacuum, extraterrestrial radiation, and extreme temperatures for two weeks.

The flight model which was integrated in Biopan, and a spare model that stayed as ground control during the flight.
Credit: ESA

All the lichen we have subjected to space or to space-simulated conditions have survived. After we put them in favorable conditions where they could recuperate, they exhibited high photosynthetic activity. We were surprised that the lichen reactivate so soon after exposure, and that there were no significant changes in the photosynthetic activity before and after exposure.

AM: If lichen is not dry and dormant, can it still survive radiation? I thought one reason radiation is so damaging is because it affects the water in our cells.

RT: Yes, they can survive. We see that happen on Earth. By about May, the snow starts to melt in the high mountain altitudes in Spain where lichen live. At this time, UV radiation is very intense because it’s nearing summer solstice. So you have the combination of the highest UV radiation of the year and available water for the lichen. This is their growing season, when they exhibit their highest photosynthetic activity.

AM: Could you extrapolate the lichen’s survival in space to mean they also could survive longer journeys, such as the trip to Mars?

Opening the flight model after landing.
Credit: ESA

RT: The tests that we have performed were about two weeks long, so we can’t say at this moment whether they could survive a long time in these conditions. We’d have to do a longer study, for six months minimum, to see if they could survive a trip to Mars. But I think that they have a potential capacity to be exposed to space conditions for a long time without dying.

AM: If they did survive, do you think lichen could grow on Mars? Could we use them to colonize the planet?

RT: After the Biopan was sent to space, a newspaper in Spain quoted me as saying that our lichen were ideal for colonizing Mars. (laughs) I didn’t say that. But I think that’s an interesting model to be tested, and could indicate the possibility for life to transfer between planets.

We have developed some tests to check the survivability of lichen on Mars, and their resistance to martian conditions. So now we are working on this step, to see if they can survive the radiation climate and the extreme temperatures on Mars.

The mean temperature on Mars is negative 60 degrees Celsius, with extreme daily fluctuations of 80 to 100 degrees C. These temperatures would not be a problem for lichen. You can find lichen in Antarctica at some gradients that are very cold and dry, like the Dry Valleys. One lichen that flew in the Biopan was Rhizocarpon geographicum, and you can find it growing in Antarctica. They can also go a long time without photosynthesizing and without growing. In less than ideal conditions, they go dormant for a long time but they’re still alive. Some of them have survived up to 3,000 years.

Electron microscopic image of lichen following post-flight analysis. The cells are complete and not broken.
Credit: ESA

Surviving on the surface of Mars could be difficult for lichen, though, because they are aerobic organisms and need oxygen to live. They also need water or humidity in order to grow, and Mars is very dry.

AM: Mars can get some low-lying fog. Would that be enough humidity to hydrate lichen and reactivate them, or would you need a more intensive water source?

RT: The lichen species we’ve tested don’t need a lot of water to photosynthesize. They could still be active with an indirect water source, like water droplets in vapor. But the amount of water vapor on Mars is about 25 percent of Earth’s, and that’s probably not sufficient for lichen to grow.

AM: What if they were sent to the poles of Mars, where there is water ice?

RT: It sounds like science fiction, but why not? Imre Friedman has said that the permafrost could be the most likely place to find life on Mars. Permafrost microorganisms such as bacteria, archaea, and fungi have survived a long time on Earth, even in the oldest Siberian permafrost, which is around 3 million years old. But life-supporting conditions at the surface of Mars disappeared around 3 billion years ago. And although there are lichen that survive in ice on Earth, we don’t know if lichen could survive in martian conditions. If microorganisms exist on Mars, they probably will be deep underground.

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