PACKING FOR OUR LONGEST JOURNEY — PAUL DAVIES

While the transport required to make an interstellar journey has been much debated, the biggest – and smallest – problem has been largely overlooked. PAUL DAVIES reports.

The microbe conundrum

Two overarching questions confront wouldbe human spacefarers: where to go and how to get there. Much attention has been given to the latter question. For interstellar travel to become a reality, major engineering advances are required, probably involving radically new propulsion systems. Many proposals are highly speculative, but we know of no fundamental physical principles that forbid interstellar travel; whether or not it becomes a reality boils down to technology, cost and motivation.

I wish to address the oft-neglected first question: the destination. Leaving aside fantastical speculations about faster-than-light travel, it is clear that journeying between the stars will take a very long time, even on the most optimistic estimates of technological advance. Therefore, interstellar tourism, or trade in physical substances (as opposed to information), is inconceivable.

Travel beyond the Solar System will be one-way only. Two possibilities then arise: that spacefarers will seek out and colonise other worlds, or that they will create permanent artificial habitats in space. Both scenarios have been popular in science fiction. I’m going to leave aside the vast engineering issues involved and dwell instead on a much trickier and more basic problem – the ecological requirements, especially those relating to microbiology.

Long-term human survival means more than growing enough food to eat and making enough oxygen to breathe. It demands creating a complete self-sustaining ecosystem. On Earth, complex multicellular organisms (e.g. animals, plants) form merely the conspicuous tip of a vast biological iceberg, the majority of which is microbial.

Almost all terrestrial species are microbes – bacteria, archaea and unicellular eukaryotes – and to date microbiologists have scratched only the surface of the microbial realm.

Microbes are everywhere – in the soil, in the air, in water, in the rocks beneath our feet, in the Earth’s crust to a depth of several kilometres. These busy little creatures are a vital part of the life-support system of our planet, both via their metabolic activity (such as recycling material) and through the exchange of genetic components. Even within your own body, microbes play a crucial role.

The microbial inhabitants of your gut, lungs, etc – known as your microbiome – outnumber your own cells. Without them you would die. So

It’s not just bacteria – a fraction of space worms came back with two heads

astronauts cannot be sent to the stars without, at the very least, their own microbiomes.

But it doesn’t stop there. Microbes do not live in isolation; they form a vast network of biological interactions that remains very ill-understood. The basic Darwinian process – replication with variation plus natural selection – is now recognised as an incomplete account of evolution. Darwinism can be regarded as the vertical transfer of information (from one generation to the next), but there is also much horizontal information flow, via gene transfer, cell-cell signalling, collective organisation of cells and much else.

Interwoven into this network are the activities of viruses, which infect microbes just as they do larger organisms. The subtle interplay of viruses, microbes and metazoa constitutes an ecological web of such staggering complexity that scientists have hardly begun to unveil it. The daunting nature of the problem may be glimpsed from the work of my Arizona State University (ASU) colleagues Hyunju Kim and Harrison Smith, who compiled data from over 28,000 genomes and 8658 biochemical reactions to create a map of information flow taking place, not just in localised ecosystems, but on a planetary scale. The biosphere, it seems, is the original World Wide Web.

Given that we can’t send the entire biosphere to another world, a fundamental problem arises: what is the minimum complexity of an ecosystem necessary for long-term sustainability? At what point, as more and more microbial species are dropped from the inventory of interstellar passengers, does the remaining ecosystem become unstable and collapse? Which microbes are crucial and which would be irrelevant passengers, as far as humans (and their animal and plant food supply chain) are concerned?

This is a Noah’s Ark conundrum with a vengeance: which species get chosen to go? Not only have we no clue as to the answer, we have little idea of the solution to a much simpler problem: identifying the smallest self-sustaining purely microbial ecosystem.

Can we pull the web of life to bits, extract a tiny subset of it, and expect such mini-webs to function forever in isolation? Any plan to terraform a planet ahead of human colonisation cannot proceed without a far deeper understanding of microbial ecology.