How do deep sea microbes survive with no sunlight? A world-first study from Monash University provides answers

What fuels the creatures that call the ocean’s deep depths home? It has long been a mystery, but a world-first study led by Monash University researchers finally gives us the answer.

And it might have provided added weight to the theory of where life first emerged on our planet.

It has long been thought that the bulk of ocean life was fuelled by photosynthesis via sunshine. This process is commonly associated with land plants. The organism uses sunlight to turn carbon dioxide and water into organic molecules. It is a process which occurs in almost all algae as well.

But the sun’s light can’t penetrate all the way down into the dark depths.

A five-year study led by Monash University’s Dr Rachael Lappan and Professor Chris Greening shows that fuel in the dark depths comes from a distinct process called chemosynthesis. Unlike photosynthesis, which uses light for organic growth, chemosynthesis uses inorganic compounds.

The ocean can be divided into three zones based on the amount of light that gets through at different depths.

On average, the ocean floor is roughly 3,688 metres below sea level. So a lot of ocean is shrouded in darkness.

Lappan and Greening’s research found that the chemicals responsible for fuelling chemosynthesis in the deep sea are two common gases: hydrogen and carbon monoxide.

Greening says that their analysis showed that chemosynthesis was the energetic process of choice for trillions of deep-sea microbes from the tropics to the poles.

“Hydrogen and carbon monoxide in fact “fed” microbes in all regions we’ve looked at: from urban bays to around tropical islands to hundreds of metres below the surface,” Greening says. “Some can even be found beneath Antarctica’s ice shelves.”

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The research involved chemical analysis from oceanic voyages as well as lab-based analysis of microbial cultures and the genes present.

“We found the genes that enable hydrogen consumption across eight distantly related types of microbes, known as phyla, and this survival strategy becomes more common the deeper they live,” explains Lappan.

The study followed the researchers’ previous work on soil bacteria which found that they too can live by consuming hydrogen and carbon monoxide from the atmosphere.

“The surface layers of the world’s oceans generally contain high levels of dissolved hydrogen and carbon monoxide gases due to various geological and biological processes. So it made sense that oceanic bacteria used the same gases as their terrestrial cousins,” Lappan adds.

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The researchers say that their findings also shed some light on how life may have evolved.

“The first life probably emerged in deep-sea vents using hydrogen, not sunlight, as the energy source,” Greening says. “It’s incredible that, 3.7 billion years later, so many microbes in the oceans are still using this high-energy gas and we’ve completely overlooked this until now.”

The study is published in Nature Microbiology.