This cryo-electron tomography image reveals the internal structure of an ultra-small bacteria cell like never before. The cell has a very dense interior compartment and a complex cell wall. The darker spots at each end of the cell are most likely ribosomes. The image was obtained from a 3-D reconstruction. The scale bar is 100 nanometers. (Credit: Berkeley Lab)

Scientists don't just go looking for massive and distant galaxies when they want to explore the unknown. There's plenty to be learned from the other end of nature's size spectrum, too. In a recent Nature Communications paper, researchers present some of the first detailed microscopy images of what are probably the tiniest living things in the world.

These aptly-named "ultra-small bacteria" have an average volume of just .009 cubic microns. Since microns aren't exactly part of our standard measurement lexicon, here's a reminder: A micron is one millionth the length of a meter. That means that 150 of these bacteria could fit into the better-known E. coli, and 150,000 or more of them could sit on the tip of a human hair. Water is considered sterilized after it's passed through a filter with pores of .2 microns, so these bacteria easily made it into "sterile" lab samples.

The question of how many bacterial cells can dance on the head of a pin is actually pretty intriguing. There are size limits to life, because the structures that make life as we define it possible can't be made infinitely small. These ultra-small bacteria, which belong to three different phyla, are at the very lowest estimates for possible life size.

Cryo-transmission electron microscopy captured numerous hairlike appendages radiating from the surface of this ultra-small bacteria cell. The scientists theorize the pili-like structures enable the cell to connect with other microbes and obtain life-giving resources. The scale bar is 100 nanometers. (Credit: Berkeley Lab)

The single-celled organisms seem to manage their micro living by packing their DNA into tight little spirals and using limited metabolisms to run their systems. It's possible that they rely on other, larger bacteria to pick up the slack. They may use tiny hairs to connect to their microbial community and get support.

Found in groundwater from Colorado and studied in a lab at Berkeley, these microbes are probably pretty common, the researchers report. But even though they've been caught on camera, they remain fairly mysterious.

“We don’t know the function of half the genes we found in the organisms from these three phyla,” Berkeley professor and study collaborator  Jill Banfield said in a statement. “They’re enigmatic. These bacteria are detected in many environments and they probably play important roles in microbial communities and ecosystems. But we don’t yet fully understand what these ultra-small bacteria do."

Figuring out the confines of life on Earth is a vital part of looking for it elsewhere. Researchers are already focused on how extreme bacteria on our planet -- ones that live in steam vents and in oceans beneath miles of ice -- could be similar to ones living on others in the solar system.