Organisms beneath the seafloor barely need energy for life

A new study from Queen Mary University of London has revealed that life on Earth has a lower energy limit than expected. An international team of scientists has found microorganisms buried in sediment beneath the seafloor that can survive on about 50 billion times less energy than humans – a rate of energy consumption that was not previously known to support life.

The study has implications for understanding more about the origins of life on Earth, and about the potential for life on other planets.

The researchers used data on the abundance and distribution of carbon and microbial life in Earth’s deep biosphere and combined this information with the rate of biological and chemical reactions. The analysis enabled the researchers to determine the power consumption rate of individual microbial cells, or the rate at which they use energy. 

The study revealed that sub-seafloor microbes survive on far less energy than has ever been known to support any form of life. This expands the habitable boundaries of life to encompass lower energy environments, which could help to inform future studies of when and where life emerged on our planet.

“When we think about the nature of life on Earth, we tend to think about the plants, animals, microscopic algae, and bacteria that thrive on Earth’s surface and within its oceans – constantly active, growing and reproducing,” said study co-author Dr. James Bradley, who is a professor of Environmental Science at Queen Mary.

“Yet here we show that an entire biosphere of microorganisms – as many cells as are contained in all of Earth’s soils or oceans, have barely enough energy to survive. Many of them are simply existing in a mostly inactive state – not growing, not dividing, and not evolving. These microbes use less energy than we previously thought was possible to support life on Earth.”

“The average human uses around 100 watts of power – meaning they burn approximately 100 joules of energy every second. This is roughly equivalent to the power of a ceiling fan, a sewing machine, or two standard light bulbs. We calculate that the average microbe trapped in deep ocean sediments survives on fifty-billion-billion times less energy than a human.”

Study co-author Jan Amend is the director of the Center for Dark Energy Biosphere Investigations (C-DEBI) at the University of Southern California.

“Previous studies of life in the subseafloor – and there have been many good ones – focused predominantly on who’s there, and how much of it is there.”

“Now we’re digging deeper into ecological questions: what is it doing, and how fast is it doing it? Understanding the power limits of life establishes an essential baseline for microbial life on Earth and elsewhere.”

The research provides new insight into how microbes interact with chemical processes that take place deep below the seafloor. While oxygen provides the highest amount of energy to microbes, it is present in less than three percent of deep ocean sediments.

Anoxic sediments often contain microorganisms that produce methane as a result of their activity. Even though marine sediments are practically inactive, they are so abundant and persist for such extreme timescales that they make a significant contribution to Earth’s carbon and nutrient cycles.

“The findings of the research call into question not just the nature and limits of life on Earth, but elsewhere in the Universe,” said Dr. Bradley. “If life does exist on Mars or Europa for example, it would most likely take refuge in the subsurface of these energy-limited planetary bodies. If microbes only need a few zepto-watts of power to survive, there could be remnants of extant life, long dormant but still technically ‘alive’, beneath their icy surface.”

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer