The research was led by scientists from the Department of Energy's Lawrence Berkeley National Laboratory and the University of California, Berkeley. The existence of ultra-small bacteria has been debated for two decades, but there hasn't been a comprehensive electron microscopy and DNA-based description of the microbes until now.
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.
The scientists report their findings Friday, February 27, in the journal Nature Communications.
The diverse bacteria were found in groundwater and are thought to be quite common. They're also quite odd, which isn't a surprise given the cells are close to and in some cases smaller than several estimates for the lower size limit of life. This is the smallest a cell can be and still accommodate enough material to sustain life. The 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.
The bacteria are from three microbial phyla that are poorly understood. Learning more about the organisms from these phyla could shed light on the role of microbes in the planet's climate, our food and water supply, and other key processes.
"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 Jill Banfield, PhD, senior faculty scientist, Berkeley Lab's Earth Sciences Division and a UC Berkeley professor in the departments of Earth and Planetary Science and Environmental Science, Policy and Management.
"They're enigmatic," says Banfield, a co-corresponding author of the paper. "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."
Among the findings: Some of the bacteria have thread-like appendages, called pili, which could serve as "life support" connections to other microbes. The genomic data indicates the bacteria lack many basic functions, so they likely rely on a community of microbes for critical resources.
The scientists also discovered just how much there is yet to learn about ultra-small life.
"We don't know the function of half the genes we found in the organisms from these three phyla," Banfield says.
The research is a significant contribution to what's known about ultra-small organisms.
[Source: DOE/Lawrence Berkeley National Laboratory]