Lichens in Space28 May, 2020
So I think many of us have a basic understanding of what a lichen is: a symbiotic mutualism between a fungus and algae / cyanobacteria. The fungus provides a cosy home for the algae, along with water, nutrients and anchorage; while the algae photosynthesises and provides the fungus with carbohydrates. As Trevor Goward puts it: “Lichens are fungi that have discovered agriculture”. Put another way, lichen are fungi with solar panels. But I’ve only recently learned that the situation is far more complex than this simple 1:1 rendering of fungus and algae.
Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves. It is thought that lichens may be even more complex symbiotic systems that include non-photosynthetic bacterial communities performing other functions as partners in a holobiont.
Holobiont - now that’s a rad word.
A holobiont is an assemblage of a host and the many other species living in or around it, which together form a discrete ecological unit. The concept of the holobiont was defined by Dr. Lynn Margulis in her 1991 book Symbiosis as a Source of Evolutionary Innovation. Holobionts include the host, virome, microbiome, and other members, all of which contribute in some way to the function of the whole. Well-studied holobionts include reef-building corals and humans.
Another rather famous character in the lichen holobiont is the tardigrade! I wonder what it would be like to experience the lichen holobiont as a tardigrade - some sort of wondrous forested village perhaps? Maybe more like a landscape of networked villages, with all sorts of nanobiomes in between?
Anywho, I digress. While listening to an episode of the In Defense of Plants podcast yesterday, specifically Ep. 80 - Lichens and Their Conservation, I learned that one species of lichen has been shown to be capable of surviving exposure to space and a simulated Martian atmosphere!
The lichen Xanthoria elegans (which we can now think of as a holobiont and not a simple two-partner symbiosis) was exposed to space conditions for 18 months during a series of experiments on the International Space Station (ISS). This included exposure to vacuum, extreme cold, solar radiation and cosmic radiation. This was the first long-term exposure of eukaryotic organisms to space conditions in LEO, as well as a simulated Martian environment.
Upon returning to Earth, the samples were tested for viability: “the lichen photobiont showed an average viability rate of 71%, whereas the even more resistant lichen mycobiont showed a rate of 84%”. They were still alive and capable of growth! The Martian experiment showed even livelier results:
The X. elegans samples exposed on the ISS in Mars-analogue conditions showed slightly higher viability and clearly higher photosynthetic activity compared with the space vacuum-exposed samples.
Pretty astounding! The paper discusses these findings in light of the panspermian hypothesis:
The Lithopanspermia hypothesis is […] based on a proposal by Thomson (1871), suggesting that life could survive interplanetary travel. Though the 1.5 year mission duration is much shorter than the estimated length of a hypothetical interplanetary transfer e.g. 2.6 Myr for some Mars meteorites (Clark 2001), the results presented indicate that X. elegans might be able to survive a longer duration in space or might be a promising test subject for a Directed Lithopanspermia as proposed by Crick & Orgel (1973)
So cool. Humyns have long dreamed of space colonies and interstellar journeys. Perhaps these dreams are genetic memories as much as simulated futures. Perhaps we have already surfed solar winds in peer-to-peer assemblages, crossing galaxies in holobiontic parties; expectant, hopeful, that Life might grow yet further.
Here's a link to the full paper:
Viability of the lichen Xanthoria elegans and its symbionts after 18 months of space exposure and simulated Mars conditions on the ISS - Brandt et al. (2015)