Survive on an extreme back burner

A deep-sea research submersible extracts sediment cores from the sea floor. (Image: Geoff Wheat, NSF OCE 1130146, and the National Deep Submergence Facility)

It doesn’t work without it – but how much energy is sufficient to survive? A study now shows that the amazingly frugal microbes in the sea floor require even less energy to survive than was previously known. The results shed light on the earthly existence limits and on the potential for life on other celestial bodies, say the scientists.

When we think about the nature of life on earth, we usually have in mind the plants, animals, algae and bacteria that exist on the earth’s surface and in the oceans. However, as has been known for some time, there is a huge biosphere that is largely hidden from us: there are enormous amounts of microbes in the subterranean oceans. An international team of researchers has once again dedicated a study to these organisms.

“Previous studies of marine life focused mainly on who existed and to what extent. Now we have gone deeper into the ecological issues of these creatures, ”says co-author Jan Amend from the University of Southern California in Los Angeles. The researchers focused on energy consumption, which enables all living things to maintain their metabolism and to ensure essential functions such as growth, repair and replacement of biomolecules.

Comprehensive view of a mysterious living space

The team used study results from cores extracted from the ocean floor worldwide. Using the data, they developed a global picture of the biosphere in the marine subsurface, which includes the most important life forms and biogeochemical processes. The scientists then created a model that illustrates the worldwide availability of energy in the seabed. To work out more details, they combined data on the distribution and amounts of carbon and microbial life with the rates of biochemical processes. This is how they came to assess the rates at which microbes convert energy in the marine subsurface.

The results show that some bacteria can survive in the marine subsurface with far less energy than was previously thought possible. The minimalism is hardly imaginable. “The average person uses about 100 watts of power. This corresponds approximately to that of a ceiling fan, a sewing machine or two commercially available light bulbs. Our calculations show that the average microbe that exists in deep sea sediments survives with about fifty trillion times less energy than a human, ”said James Bradley of the Queen Mary University of London. “Many of them live in a largely inactive state. They do not grow, do not divide and do not develop further. However, these microbes are not dead, but they consume far less energy than previously thought to survive, ”says the scientist.

In addition, as he and his colleagues report, their global inventory made it clear: only about 2.7 percent of ocean sediments are “oxic” – they contain oxygen, which enables the majority of life on our planet. The overwhelming majority is thus “anoxic” and the habitat of microbes that form methane (in 64.3 percent of sediments), followed by organisms that use sulfates as an energy source, the scientists report. Although they show little activity, the microbes contained in the earth’s marine sediments still make a significant contribution to the earth’s carbon and nutrient cycle due to their enormous total mass, the researchers emphasize.

Survival at the limit

As they explain, the organisms in the extreme areas probably only use the tiny amounts of energy to enable them to survive: the energy is used for “maintenance” – the replacement or repair of damaged parts. The scientists suspect that many of the microbes from the depths below the seabed are remains of populations that lived in shallow coastal areas thousands to millions of years ago. Then they were buried ever deeper and fell into a form of existence that allowed them to survive enormous periods of time.

The results now raise fundamental questions about our definitions of what constitutes life and the limits of life on earth and elsewhere, the researchers say. “The results question not only the nature and limits of life on Earth, but also elsewhere in the universe,” says Bradley. “If there was life on Mars or on the Jupiter moon Europe, for example, it would most likely find refuge in the underground of these energy-limited celestial bodies. And if microbes only need a few zeptowatts of power to survive, there could be remains of organisms beneath the icy surface of planets and moons that have slumbered for a long time, but technically still would be ‘alive’, ”says the scientist.

Source: Queen Mary University of London, Article: Science Advances, doi: 10.1126 / sciadv.aba0697

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