Stalactites hold tight to the ceiling, the saying goes, and stalagmites might grow high enough to reach it. But the simple mnemonic doesn't come close to covering the variety of weird, rocky shapes growing all over a cave. There are even, it turns out, rocks made from bacteria. They're not putting the "tight" in "stalactite" so much as the "ack!" Researchers found the microbe-made rocks in a cave in northern Sweden called Tjuv Antes. (It's named for a fugitive thief who allegedly hid there in the 1800s.) Therese Sallstedt and Magnus Ivarsson of the University of Southern Denmark's Nordic Center for Earth Evolution, along with colleagues from Sweden and Spain, saw many kinds of rocky formations in the cave. There were smooth crusts of stone. There were bubbly structures that looked like popcorn. There were structures with delicate fingers like coral (above). The walls of Tjuv Antes Cave are made of granite. But the researchers only found the stony growths on the cave's ceiling, which is made of dolerite. There were other things growing on the ceiling too. A whitish biofilm—a slick of live bacteria—clung to cracks in the rock. And a dark, brownish-black biofilm grew on many surfaces of the other structures in the cave. The researchers took samples of the various stones, and the biofilms covering them, back to the lab. Under the microscope, the biofilms looked to be mostly Actinobacteria, a common cave microbe. There are likely other types of bacteria inhabiting this slime too, the researchers say. They also found fungi, which may be living on the bacteria. Cross-sections of the lumpy stone structures revealed layers that were surprisingly distinct, says Magnus Ivarsson. Light and dark layers alternated in the stone. The light layers were made of the mineral calcite. The dark layers included silica, carbon, and what appeared to be the tiny spheres and strands of bacterial bodies. In other words, a fossilized biofilm
. Ivarsson says the stony formations in this cave formed in cycles, alternating between layers of inorganic calcite and films of bacteria. "The biofilm layers are formed seasonally, probably during spring and summer, when the microbial activity is high," he says. In this way the stones are similar to stromatolites, ancient rocks formed from layers of cyanobacteria. The live biofilm that researchers found in another part of the cave looks like an early stage in the formation of new stone, he adds, and minerals are already collecting there. It's not clear, Ivarsson says, whether the cave bacteria are the architects of the mineral layers of the stones, as well as the layers built from their own bodies. "Microbes are known to influence their close environment on a micro-level," he says. So it's possible that the calcite layers somehow result from the presence of microbes. Without bacteria involved, the cave's dripstones might be smaller, or might never have formed at all. How common is it for cave stones to be built out of bacteria? That's another mystery. "Microorganisms are usually found associated with speleothems," he says. ("Speleothem" is the general term for a stalactite, stalagmite, or other stony structure formed in a cave.) "I guess they are more involved in the formation of speleothems than we previously have thought." You don't have to travel to a Nordic cave to see stones built by bacteria. Just check your local subway station. "We have an ongoing study in the Stockholm metro," Ivarsson says, "where we study speleothem formation and microbial involvement." It's common for the stony structures to form in this kind of underground environment, anyplace water drips in. He's already found diatoms (a kind of hard-shelled algae) living on some subway walls. At a certain Stockholm station, it seems fungi are responsible for forming speleothems. "So, yes, I think microbes are strongly involved" in building cave stones in the subway, he says. Just like the human passengers, those speleothems are holding on tight. Image: Sallstedt et al.
Sallstedt, T., Ivarsson, M., Lundberg, J., Sjöberg, R., & Vidal Romaní, J. (2014). Speleothem and biofilm formation in a granite/dolerite cave, Northern Sweden International Journal of Speleology, 43 (3), 305-313 DOI: 10.5038/1827-806X.43.3.7