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How small can life be? Scientists attempted to answer this question last week in a paper in Nature Communications. A team comprised of researchers from UC Berkeley and Lawrence Berkeley National Laboratory looked at the diverse bacteria living in groundwater, capturing detailed microscopy images and analyzing the microbes’ genomes.

The bacteria were collected in filtered groundwater from Colorado and are thought to be quite common. They’re also quite odd, because the cells could be smaller than several estimates for the lower size limit of life. Researchers believe that this is the smallest a cell can be and still accommodate enough material to sustain life. The images indicate that these bacterial cells have densely packed spirals that are probably DNA, a very small number of ribosomes, hair-like appendages, and a stripped-down metabolism that likely requires them to rely on other bacteria for many of life’s necessities.

“These newly described ultra-small bacteria are an example of a subset of the microbial life on Earth that we know almost nothing about,” says co-author Jill Banfield. “They’re enigmatic. These bacteria are detected in many environments and they probably play important roles in microbial communities and ecosystems.”

How small are they? The cells have an average volume of 0.009 cubic microns (one micron is one millionth of a meter). About 150 of these bacteria could fit inside an Escherichia coli cell and more than 150,000 cells could fit onto the tip of a human hair. In addition, their genomes are about one million base pairs in length (the human genome is three billion).

“There isn’t a consensus over how small a free-living organism can be, and what the space optimization strategies may be for a cell at the lower size limit for life. Our research is a significant step in characterizing the size, shape, and internal structure of ultra-small cells,” says lead author Birgit Luef.

The genomic data indicate the bacteria lack many basic functions, so they likely rely on a community of microbes for critical resources. In fact, some of the bacteria have thread-like appendages, called pili, which could serve as “life support” connections to other microbes.

With these findings, the scientists realize there is still much to learn about ultra-small life forms, likely hiding right in front of us.

Image: Berkeley Lab

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