There is a tremendous variety of life beneath the surface of the earth. A new analysis of two large groups of subsurface microorganisms has now revealed that their evolutionary path to life in the dark has been more bizarre than we expected.
During the first 2 billion years of our planet’s existence, there was no oxygen in the atmosphere. Once the air on our blue planet changed, not all forms of life adapted, many microorganisms retreated to the less oxygenated parts of the planet.
Pseudobacteria and DPNN are two ubiquitous groups of such subsurface microorganisms – bacteria and archaea, respectively – which seem to have a very simple genome. This has led many to suspect that without the ability to breathe oxygen, these microorganisms may need to rely on complex interactions with other organisms to supplement their simple lifestyles.
Now, it looks like we might not have given them enough credit. New research suggests that instead of being symbiotically dependent on other large groups of organisms, most pathogens and DNA live as completely free cells.
“These microbes [..] There are truly special and truly exciting examples of the early evolution of life, “said Ramnus Stepnauskas, who studied microbial biology and evolution at the Biglow Laboratory for Oceanography.
“They may be the remnants of ancient forms of life that have been hidden and flourishing in the earth’s majesty for billions of years.”
Previous work on Pseudobacteria and DPNN has collected many examples near the Earth’s surface, and especially in North America, but this new study is much deeper and more extensive than before. Volcanoes, including hydrothermal vents in the Pacific and gold mines in South Africa.
The authors write, “Our single cell genomic and biophysical observations do not support the prevailing view that pathogens are dominated by bacteria and DPNN symbols,” the authors write.
“Their specific coding potential, small genomes and small cell sizes, could be the result of an ancestor, primitive energy metabolism that just depends on it. [fermentation]”
Fermentation is one of the metabolic alternatives available to living organisms to break down glucose without the help of oxygen, and many life forms use fermentation to produce energy, especially microorganisms that do not breathe air at all.
However, the use of fermentation is less efficient than inhalation – it produces only 2 ATP per glucose compared to 38 ATP per glucose with globic respiration – so this type of metabolism comes with the cost of placing the organism at a slower rate of metabolism.
Still, the pathogens and DPNN are just fine. Based on the new analysis, there is no trace of what is known as the electron transport chain in both groups, a metabolic process that generates energy by throwing electrons at oxygen. Their relatively simple, potentially ancient survival tricks just don’t need it.
Genomic research and direct experimental tests on samples representing the two groups found no evidence of respiration, and a close examination of cell-to-cell links revealed that most were not connected to hosts like their own surface cousins.
The authors cannot deny that some symbiotic relationships can be shaken by human care, but a humble attempt was made to mix the cells while sorting.
Even if the team is underestimating cell-to-cell interactions, genomic analysis has found no evidence of evolutionary breeding from symbiotic relationships compared to other files.
Rather, lab analysis of genome content and cell physiology suggests that there are several other ways to generate energy than to ferment these microbial groups.
“Our findings suggest that Pseudobacteria and DPNN are ancient forms of life that have never learned to breathe.”
“These two main branches of the evolutionary tree of life make up the bulk of the total microbial diversity on the planet – and yet they lack some of the capabilities that are especially expected in every form of life.”
The study was published in Frontiers in Microbiology.
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