Field notes on things that run themselves
The Slowest Handshake in Biology
The gray-green crust on a gravestone or a bare boulder looks like the least alive thing in the landscape — closer to a stain than a living thing. It’s actually two organisms from two different kingdoms of life, wearing one shape, in a partnership so old and so patient that some individual patches have been sitting on the same rock since before the pyramids were finished.
Call it a lichen, and you’re naming a relationship, not a species. The body is built by a fungus — the mycobiont — which supplies the structure, the grip on bare stone, and protection from drying out. Woven through that fungal scaffolding are photosynthetic cells, the photobiont: usually a single-celled green alga (about half of all lichens partner with one called Trebouxia), sometimes a cyanobacterium instead. The photobiont makes sugar from sunlight and air; the fungus takes a cut in exchange for housing and water. Neither one is “the lichen” alone. Pull them apart in a lab and you get an ordinary fungus and an ordinary alga, neither of which looks or behaves anything like the thing they make together.
For decades biologists thought that was the whole arrangement — two partners, one deal. Then in 2016 a team studying North America’s shaggy “beard” lichens found a mystery they couldn’t explain: two lichens made of what appeared to be the exact same fungus and the exact same alga were nonetheless different colors and made different chemicals, one bright yellow-green, one dull brown. Digging into the genetics turned up a third organism nobody had been looking for — a basidiomycete yeast, an entirely different branch of the fungal kingdom, living in the lichen’s outer skin. It isn’t a contaminant riding along; it appears to help build the very structure that makes a lichen a lichen. The partnership several biology textbooks still describe as a pair turns out, at least in this family, to be a trio.
Whatever the headcount, the deal is built to last, and that’s the real trick. Some lichens — flat, crusty ones that colonize bare rock — grow at less than a millimeter a year, so slowly that a single thallus can spend a decade barely widening a fingernail’s worth. That slowness is oddly useful: scientists use the known growth rate of these crusts to date the rock surface underneath them, a technique called lichenometry, applied to glacial moraines, rockfalls, and old gravestones alike. Individual thalli of one well-studied genus have been aged past a thousand years by this method, and the growth curves for a few exceptional patches imply ages running into the thousands more — a living thing, if you can even call the whole partnership one thing, older by far than any single cell inside it.
That last clause is not a throwaway. No cell in a lichen is a thousand years old; algal cells and fungal filaments live, divide, and die inside the arrangement the way flame consumes wax. What persists is the template — the relationship, re-formed continuously as its cells turn over. And the template gets handed down two very different ways. Most lichens can reproduce vegetatively, breaking off little dust-sized packets called soredia or coral-like nubs called isidia, each one a bundled starter kit already containing both fungus and alga together — plant one on a new rock and you get a complete lichen with no negotiation required. But when the fungal partner reproduces the ordinary fungal way, by releasing spores, those spores travel alone. A spore that lands on bare stone isn’t a lichen yet — it’s just a fungus, and it has to find a compatible loose alga or cyanobacterium already living on that rock and re-strike the deal from scratch. Every sexual generation of a lichen is a fresh handshake, not an inheritance.
A relationship built this way turns out to be almost absurdly hard to kill. In 2005, the European Space Agency sent samples of two lichen species into orbit strapped to the outside of a capsule, with no shielding at all, for two weeks — full vacuum, unfiltered solar ultraviolet, the sharp swings between blistering sun and deep cold that low orbit puts on anything exposed to it. Most of the cells came back alive. More strikingly, they came back working: exposed to essentially the entire solar UV spectrum with nothing to hide behind, the returned lichens still photosynthesized at close to normal rates within hours of rehydration. Almost nothing else multicellular and eukaryotic — nothing built the way you and a lichen’s algal cells are both built — has been shown to shrug off the vacuum of space and get straight back to work.
So the crust on the gravestone isn’t holding still because it’s dead weight. It’s two kingdoms of life that found, a very long time ago, that they persist better bonded than alone, and that keep re-signing that agreement in every generation, in cells that live only a fraction as long as the shape they’re renewing. The rock just holds still long enough to watch.
One loop I’m watching
Not everything that outlasts a human life on bare rock is alive at all. Some of it is built the way a lichen is patient — one imperceptible increment at a time — but by water and mineral alone, with no organism doing the choosing. Next time: a stone that grows one drop at a time.
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