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Microbes beneath the seafloor found living on fifty-billion-billion times less energy than a human’

Tiny microbes living underneath the seafloor have been found surviving on a fraction of the energy a human needs to live – setting a new lower energy limit for life.

An international team of researchers led by Queen Mary University of London used data from the sub-seafloor to create a new global picture of the ocean biosphere.

They discovered that microorganisms buried in sediment beneath the seafloor can survive on less energy than was previously known to support life. 

The study has implications for understanding the limit of life on Earth and the potential for life elsewhere in the solar system, the team said.

Researchers combined data on the distribution and amounts of carbon and microbial life contained in Earth’s biosphere with the rate of chemical reactions.

Using this information they were able to determine the ‘power’ consumption of individual microbial cells – in other words – the rate at which they utilise energy. 

All life on Earth constantly uses energy in order to remain active, sustain metabolism, and carry out essential functions such as growth, and the repair of biomolecules.

The results show that sub-seafloor microbes survive using far less energy than has ever previously been shown to support any form of life on Earth. 

By stretching the ‘habitable boundaries of life’ to include lower energy environments – the team hope this could help them work out how early life started on Earth. 

Dr James Bradley, Lecturer in Environmental Science at Queen Mary said we tend to think about plants, animals, algae and bacteria when we think about life on Earth.

‘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 about 100 watts of power – or about the power of a ceiling fan or two lightbulbs, the researchers explained. 

‘We calculate that the average microbe trapped in deep ocean sediments survives on fifty-billion-billion times less energy than a human,’ said Bradley.

Jan Amend, Director of the Center for Dark Energy Biosphere Investigations (C-DEBI) at the University of Southern California, and co-author of the study, said previously studies of the sub-seabed focused on how much life 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,’ said Amend.

The findings raise fundamental questions about our definitions of what constitutes life, as well as the limits of life on Earth, and elsewhere. 

With such little energy available, it is unlikely that organisms are able to reproduce or divide, but instead use this miniscule amount of energy for ‘maintenance’ – replacing or repairing their damaged parts. 

It is likely, therefore, that many of the microbes found at great depths beneath the seafloor are remnants from populations that inhabited shallow coastal settings thousands to millions of years ago. 

Unlike organisms on the surface of Earth, which operate on daily and seasonal) timescales according to the Sun, these deeply microbes exist on much longer timescales, such as the movement of tectonic plates.

The research also sheds light on how the microbes interact with chemical processes occurring deep below the seafloor. 

Whilst oxygen provides the highest amount of energy to microbes, it is in overwhelmingly short supply – present in less than 3 per cent of sediments.

Anoxic sediments, however, are far more widespread, often containing microorganisms that obtain energy by generating methane – a greenhouse gas. 

Despite being practically inactive, the microbial cells contained in Earth’s marine sediments are so numerous, and survive over such extraordinarily long timescales, that they act as an important driver of earth’s carbon and nutrient cycles.

They even affect the concentration of CO2 in earth’s atmosphere over thousands to millions of years.

‘The findings of the research call into question not just the nature and limits of life on Earth, but elsewhere in the Universe,” added 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 zeptowatts of power to survive, there could be remnants of extant life, long dormant but still technically ‘alive’, under their surface.’

The results have been published in the journal Science Advances.

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