New Research: Fog, symbiosis, and survival: the Atacama grit crust is a lichen, not a soil microbiome

There is a stretch of the coastal Atacama where it almost never rains. What keeps anything alive there is fog — camanchaca — that rolls in off the Pacific, beads onto the ground for a few hours, and burns off again. In that narrow window of moisture grows one of the strangest ground covers on the planet: the grit crust, a living skin of microbes wrapped around millimetre-sized granite pebbles.

We have been working on this crust for years now. Our newest paper, just out in Environmental Microbiology, asks a deceptively simple question: when you sequence everything living in the grit crust, what kind of community is it, really? The answer turned out to change how I think about biocrusts in general.

The assumption we tested

Biological soil crusts are usually described — and studied — as soil microbiomes: a mixed bag of bacteria, algae, fungi and the occasional lichen, all sharing the top few millimetres of ground. Lichens are treated as one component among many.

We were not convinced that framing fit the grit crust. So instead of looking at one group of organisms, we sequenced three marker genes at once across the community — 16S rRNA for bacteria, 18S rRNA for the eukaryotes and algae, and ITS2 for fungi — across dozens of samples spread along the fog gradient, from fog-soaked coastal sites to drier inland ones. Multi-marker metabarcoding like this lets you see the whole assemblage at the same time rather than guessing how the pieces relate.

What the sequences actually showed

The community is not organised like a soil microbiome at all. It is organised like a lichen.

At its centre sits Trebouxia, the green algal photobiont, dominating the eukaryotic side of the community. Around it we find lichen-forming fungi of the order Caliciales, and — this was the telling part — the bacteria are not generic soil bacteria but the Proteobacteria and Actinobacteria typically found associated with lichens. In other words, almost every major player in the crust has a place in a lichen symbiosis. The "soil community" is really a lichen and its extended cast of partners, microbial hangers-on included. That is what the word holobiome captures: not a single organism, but a symbiosis plus everything that lives with it as one functioning unit.

We also saw the crust change as it matures. The pale, white crusts are the earlier, more diverse stage. The black crusts are later-successional and pack far more biomass — the system gets less diverse but denser as it ages. And a reminder of how little we still know: a sizeable share of sequences matched nothing in the reference databases, a flag that real biodiversity is sitting in these samples that nobody has formally described yet.

Why this reframing matters

If the grit crust is a holobiome rather than a soil microbiome, it sits in an interesting gap. On one side you have lithic lichens that live on bare rock; on the other, the edaphic biocrusts that live in and on soil. The grit crust bridges the two — fog-driven chlorolichens that have effectively built a coherent soil layer out of pebbles and their own bodies. The lichen is not living in the crust. The lichen is the crust, and the soil follows from it.

That shifts the question for anyone studying drylands. If a biocrust is structured by symbiosis rather than by soil chemistry, then understanding it — and protecting it — means thinking about the symbiotic partnership and the fog that powers it, not just the dirt it grows on. And as a fog-fed, radiation-hardened, self-organising living system in one of Earth's most Mars-like places, it stays one of my favourite models for asking what life can do at the edge of what is possible.