The Treaty

A lichen on a stone looks like the simplest life there is — a grey-green crust, flat against the rock, one patch of one thing. It has a shape, an ecology, a Latin name; it sits there for centuries being, to all appearances, a single modest organism — the least a living thing can be. For most of the history of biology that is exactly what we filed it as: a lowly sort of plant. It is not a single organism. It is at least two, from different kingdoms, and the thing you are looking at is not a creature but the arrangement between them.

A lichen is a fungus that has taken a photosynthesizer indoors. The body — the crust, the leaf, the little branching shrub of it — is built almost entirely by the fungus: it makes the structure, anchors to the rock, holds water, shields against light and drought. Sealed inside that fungal architecture is the other partner, an alga or a cyanobacterium, doing the one thing the fungus cannot do for itself: turning sunlight into sugar, and feeding it outward to the fungus that houses it. The visible individual is, in effect, a fungus wrapped around the garden that feeds it. And our naming quietly admits which partner is in charge — a lichen's scientific name is the name of its fungus. The alga inside, the one actually running the photosynthesis the whole arrangement lives on, does not get its name on the door.

That a named species was secretly two organisms was, when first proposed, an outrage. In 1867 the Swiss botanist Simon Schwendener put forward what came to be called the dual hypothesis: that a lichen is a fungus living on an alga. The leading lichenologists of the day were appalled and dug in against it — the notion that one of their well-catalogued species was really two kingdoms cohabiting struck them as not just wrong but faintly obscene, and the period's own descriptions reached for the language of captivity, the fungus cast as a tyrant and the imprisoned alga as a captive maiden it held and farmed. The argument lasted a generation. And it pressed biology hard enough that it needed a new word for the thing it was being forced to admit: out of the study of lichens and a few cases like them came symbiosis — Frank's coinage in 1877, de Bary's address of 1878, "the living together of unlike organisms." We had to invent a word for togetherness because a crust on a rock would not resolve into one creature.

And here is the part the gentler tellings leave out: it was never settled that the togetherness is friendly. The fungus does not merely shelter the alga; it pushes feeding structures into the algal cells, drawing the sugar out. The alga can be found living free and untroubled all over the world; the fungus is almost never found living alone anywhere — it needs what it has enclosed. Whether the right word is partnership or farming or captivity is a fight lichenologists are still having; one of the field's own, Vernon Ahmadjian, spent a career studying the relationship and ended by calling its supposed mutual benefit a myth, the alga a victim. So the lichen is not a tidy fable about cooperation. It is a relationship whose terms are disputed, between an extractor and a producer, and the partner that built the walls is the one whose name we use.

It does not even stop at two. For a century and a half the rule was one fungus, one alga, and yet some lichens stubbornly could not be rebuilt from just those two in a laboratory, as though a part were missing. In 2016 the part was found: a second fungus, a yeast, woven into the outer skin of many lichens, unnoticed for a hundred and fifty years. The two-partner rule broke, and it has kept breaking — there are bacteria in there too, and the real count of who lives in a lichen is simply open. Every time the instrument gets sharper, the single individual resolves further into a crowd, and the crowd keeps gaining members.

What is strange is how well this disputed, crowded, internally-extractive arrangement holds. Lichens are among the toughest things alive — two species once rode the outside of a space capsule through open vacuum and full solar ultraviolet for two weeks and came back photosynthesizing — and among the slowest and oldest, crusts on Arctic rock estimated at thousands of years. And the arrangement is, in part, portable: many lichens disperse as soredia, tiny packets of fungal thread already wrapped around a few algal cells, so the whole partnership blows away and resettles together rather than each member having to find the other again. Though others scatter as bare fungal spores that must locate a new alga wherever they land — so that even reproduction cannot decide whether the unit is the partnership or the fungus alone.

Whether the crust on the stone is a marriage or a captivity, we still do not know.