Where did life originate on Earth? Ancient hot springs under the seafloor could have the answer

The Barberton greenstone belt (BGB) in South Africa contains some of the oldest rocks on Earth. These volcanic rocks erupted as far back as 3.4 billion years ago on the ocean floor. At the time, cold seawater would enter into the hot oceanic crust, heat up and emanate in the form of hot springs on the seafloor, creating a hydrothermal system. At the same time, there would be marine sediments settling onto the seafloor, forming chert, a sedimentary rock that is largely made of silica.

Today the rocks of the BGB form a spectacular landscape and preserve traces of the life was present at the time they formed.

Researchers from all over the world have been looking for life in these cherts, and several studies have shown the presence of fossil microbes in these sedimentary rocks. These fossils correspond to organisms living in the sun-lit regions of the ocean, like modern-day plankton. However, it is possible that life also colonised hydrothermal springs, possibly much earlier, before spreading into the oceans.

Now a team of scientists, led by the University of Bologna, got a close-up look at these former hot springs using the ESRF beamlines. “Our findings provide the oldest microfossil evidence for subsurface methane-cycling microoganisms and expand the frontiers of early earth habitability”, says Barbara Cavalazzi, professor in paleontology at the University of Bologna (Italy) and associate researcher at the University of Johannesburg, who conceived and led the study. 

“Where did life evolve originally on Earth?” wonders Axel Hofmann, geologist at the University of Johannesburg and co-author of the study. “Some people think that these springs could be a suitable place where life formed. Life cannot be present in hydrothermal systems that are much hotter than 100 degrees, but the hot springs fossilized in Barberton were at temperatures of less than 100 degrees. That is the reason why I have been interested in looking for life in these systems”, he explains.

When looking for samples in the area, since it is impossible to see fossil microbes in the field, researchers guide themselves using certain structures in the rock and look for traces of carbon, as it is an indicator of life.

After searching for several years, Hofmann “got lucky” with a set of samples, as he puts it. In his samples, chert forming hydrothermal veins in volcanic rocks and corresponding to the pathways that the fluid would use to circulate through the rock contains carbon.

But where does this carbon come from?  Some carbonaceous matter may have been transported from the seafloor into the hydrothermal system from microbes living in the ocean. But some may have formed within the hydrothermal system and became preserved. Indeed, carbonaceous filaments with distinct biogenic morphologies were observed along the walls of the veins, suggesting their formation in the hydrothermal system. But do they really represent fossil microbes? And if so, what were their metabolic pathways? Cavalazzi says: “Having used a range of palaeontological and organo-geochemical techniques to demonstrate that the structures were true fossils, we came to the ESRF to investigate their metabolisms.”

Using the ID16B and ID21 beamlines, they looked for specific elements associated with the fossils, which may reflect their concentration of microbial life. Cavalazzi, who has been an ESRF user for more than a decade, explains: “at the ESRF, we were able to detect trace amounts of bioessential elements, including nickel. The incorporation of nickel as a part of the metabolic pathway is typical for some chemo-synthetic microbes”. “Hyperthermophilic chemosynthetic microbes thrive in hot waters and do not require sunlight (as energy source). Instead, they obtain energy from the hot hydrothermal fluid and synthesize organic matter from the ingredients present in the fluids.” she explains.

“Until now, there was no method that could pinpoint trace metals with the nanometre resolution needed to image the interior of a single-cell microfossil”, explains Alexandre Simionovici, from the Institut des Sciences de la Terre, Grenoble Alpes University, in France, and co-author of the paper. Together with Laurence Lemelle, a scientist at the Laboratoire de Géologie de Lyon: Terre, Planètes et Environnement, they have targeted transition metals as bio-markers and developed a non-destructive methodology to detect and quantify them in organic remains from rocks.

The scientists imaged the microfossils using nano-X-ray absorption near edge structure spectroscopy, performed for the first time at 50-nanometres lateral resolution. Cavalazzi explains: “The experiments were a success thanks to the very supportive and professional staff at ID16B and ID21, Remi Toucolou and Murielle Salomé, who gave us great technical advice”.

The experiment showed unique signatures of organo-nickel complexes, which allow to constrain the metabolic pathways of the microfossils. “Only with the ESRF could we show that the association of nickel with carbonaceous fossils is as high as in modern day bacteria that are chemosynthesizers and use this kind of metabolic pathway”, adds Hofmann.

The discovery of this evidence of life within a 3.42 Ga old hydrothermal system below the seafloor is truly remarkable for such ancient rocks. This setting could, arguably, have been conducive for life to have evolved much earlier in Earth's history.

The study of the underground habitats goes beyond finding out how life on Earth formed, says Cavalazzi: “the subsurface environments are one of the new frontiers in search for early life on Earth and for the astrobiological exploration of Mars”. She adds: “The metabolic pathways of microorganisms living in the subsurface environments evolved much earlier than phototrophs (organisms using sunlight for energy), so life could originated in protected subsurface habitats. Also, it has been suggested that hydrothermal and subsurface environments are among the most ancient habitats on Earth, so it is also possible that traces of primitive life are preserved in similar setting on Mars”.

Reference:

Cavalazzi B. et al, Science Advances  14 Jul 2021: Vol. 7, no. 29, eabf3963. DOI: 10.1126/sciadv.abf3963

Text by Montserrat Capellas Espuny.

Video by Montserrat Capellas Espuny and Mark McGee.