Microbes Living Deep Below Earth’s Surface Could be Remnants of Ancient Life Forms

Patescibacteria and DPANN are two ubiquitous groups of such subsurface microbes – bacteria and archaea, that appear to have simple genomes. This has led many to think that without the ability to breathe oxygen, these microbes might need to rely on complex interactions with other organisms to supplement their basic lifestyles.

Now, it seems we might not be giving them enough credit. New studies suggest that instead of having a symbiotic dependency on other major groups of organisms, most Patescibacteria and DPANN live basically free cells. These microbes are considered extremely special by scientists, they show the early stages of evolution life. They may be remnants of ancient forms of life that had been hiding and thriving in the Earth’s subsurface for billions of years.

Previous work on Patescibacteria and DPANN has collected a few examples near the surface of the Earth, and mainly in North America, but this new study goes deeper and wider than ever before, assessing nearly 5,00 individual microbial cells from 46 locations around the globe, including a mud volcano on the bottom of the Mediterranean Sea, hydrothermal vents in the Pacific, and gold mines in South Africa.

According to the study, single-cell genomic and biophysical observations do not support the prevailing view that Patescibacteria and DPANN are dominated by symbionts. Their divergent coding potential, small genomes, and small cell sizes may be a result of ancestral, primitive energy metabolism that relies solely on fermentation.

Fermentation is one of the metabolic choices living organisms have for deconstructing glucose without the support of oxygen, and many life forms use fermentation for energy production, especially the microbes that do not breathe air in any way. However, using fermentation is less efficient than breathing – it produces only 2 ATP per glucose compared to 38 ATP per glucose with aerobic respiration – so this type of metabolism comes with the cost of putting organisms in the metabolic slow lane.

Patescibacteria and DPANN are not bothered by this. But based on the new analysis, the two groups contain no trace of what is known as an electron transport chain, a metabolic process that makes energy by dumping electrons onto oxygen. Their simple, potentially ancient survival tricks are not needed.

Genomic research and direct experimental tests on samples representing the two groups showed no evidence of respiration, and close examination of cell-to-cell links showed many were on their own, not attached to hosts like some of their surface cousins.

The authors of the study cannot deny that some symbiotic relationships could have been shaken apart by human handling, but gentle mixing was attempted when organizing the cells. Even if the team is underestimating cell-to-cell interactions, the genomic analysis found no evidence of evolutionary empowerment from symbiotic relationships compared to other phyla.

Rather genome content and lab assessments of cell physiology suggests these microbial groups contain few, if any, other methods of creating energy other than fermentation. Understanding the development of these microbes will enable us to get a better glimpse of how symbiotic microbes grew into the amazingly complex life that we have all around us today. Further informing our understanding of where we came from.