Are stromatolites biofilms?

Are Stromatolites Biofilms?

Strictly speaking, stromatolites aren’t biofilms themselves—but they are built by them. Think of them as the fossilized cities of ancient microbial life, with biofilms as the architects.

At their core, stromatolites are layered rock structures formed over long periods in shallow water environments. The real builders are microbial mats—complex, living biofilms dominated by cyanobacteria. These tiny organisms form sticky, slimy layers that trap and bind sedimentary grains like sand and silt. Over time, minerals can precipitate and cement these layers together, creating a durable, laminated structure we recognize as a stromatolite.

The process is slow but powerful. As new layers form on top of older ones, a record of time is etched into the rock—one layer at a time. This layering is what makes stromatolites so valuable to scientists: they represent some of the oldest known signs of life on Earth, with fossils dating back over 3.5 billion years.

Modern examples still exist, though they’re rare. You can find living stromatolites in places like Shark Bay in Western Australia and the Bahamas, where extreme conditions limit competition from more complex lifeforms. Here, cyanobacteria thrive in salty, warm waters, continuing the same ancient process that shaped early Earth’s atmosphere—by producing oxygen through photosynthesis.

So while stromatolites themselves are mineral structures, they’re direct products of biofilms. They’re not just rocks—they’re monuments to life’s quiet persistence, built grain by grain, layer by layer, over eons. Without the invisible work of microbial communities, these geological wonders wouldn’t exist.