Scientists have used powerful X-ray nano-imaging to investigate fossilised microorganisms preserved in one of Earth’s most extreme environments. The fossils from Spain’s Río Tinto could help researchers recognise ancient traces of life in rocks returned from Mars.

If life once existed on Mars, scientists are unlikely to find a perfectly preserved organism waiting inside a rock.

They may instead have to identify microscopic structures and chemical traces that are billions of years old.

The challenge is enormous.

Geological processes can alter biological material until it becomes extremely difficult to distinguish a genuine fossil from a structure produced without life.

An international research team supported by Université Grenoble Alpes and CNRS has now demonstrated a powerful way to tackle that problem.

Their laboratory is one of the strangest environments on Earth: Río Tinto in southern Spain.

A river that looks almost extraterrestrial

Río Tinto is famous for its extraordinary reddish colour.

Its water is highly acidic and exceptionally rich in iron and other heavy elements.

Despite these extreme conditions, the environment supports a surprising variety of microorganisms.

And microorganisms that lived there in the past have also been preserved as fossils in older deposits surrounding the river.

This combination makes Río Tinto particularly interesting to astrobiologists.

Conditions there have similarities to acidic, iron-rich environments identified on Mars. The researchers therefore regard these terrestrial deposits as useful analogues for investigating how potential Martian biosignatures might be preserved.

Looking inside a fossil without destroying it

The scientists wanted to see much more than the external shape of the fossil microorganisms.

They wanted to investigate their internal structure and chemical composition at extraordinarily small scales.

Until now, researchers lacked a method capable of imaging microbial fossils embedded in rock at nanometre resolution across areas hundreds of micrometres wide.

The international team therefore combined several advanced X-ray techniques.

One of the most important was ptychographic X-ray laminography, performed at the Swiss Light Source.

This was combined with X-ray nanofluorescence measurements at the European Synchrotron Radiation Facility in Grenoble and at the Sirius synchrotron facility in Brazil.

Together, these techniques allowed scientists to move between different scales and construct detailed three-dimensional views of the fossil microorganisms.

From micrometres to nanometres

This multi-scale approach is crucial.

A scientist investigating a potential fossil first needs to understand where it sits within the surrounding rock.

Then the analysis must move progressively closer.

At microscopic scale, researchers can investigate the organism’s general form.

At nanometre scale, they can examine extraordinarily fine structures and determine how different chemical elements are distributed.

The new imaging approach revealed details from numerous Río Tinto specimens, providing information about their nature, possible metabolisms, ecological interactions and the processes through which they became fossilised.

The problem of false fossils

Finding something shaped like a microorganism is not sufficient evidence that life once existed.

Minerals can produce structures that resemble cells.

Chemical reactions can generate patterns that look biological.

And millions or billions of years of geological alteration can destroy or modify the original biological material.

This is why scientists searching for ancient life need several independent forms of evidence.

Shape matters.

Chemistry matters.

Internal structure matters.

The relationship between the suspected fossil and the surrounding minerals matters.

By combining structural and chemical information at extremely high resolution, researchers can build a much stronger case for determining whether a microscopic structure truly has a biological origin.

Preparing for Martian rocks

That is where Río Tinto becomes particularly valuable.

Mars contains ancient sedimentary environments where liquid water once existed.

Future analysis of Martian material could therefore confront scientists with exactly the same question:

Is this tiny structure evidence of ancient life, or simply geology?

Studying difficult terrestrial fossils provides a way to practise answering that question before researchers encounter extraterrestrial samples.

The Río Tinto work could therefore help scientists anticipate some of the problems involved in analysing samples from Mars.

Two-million-year-old microorganisms

The microorganisms investigated in the study are approximately two million years old and were fossilised in Río Tinto’s iron-rich, acidic deposits.

That age is relatively young compared with the oldest evidence of life on Earth, which stretches back billions of years.

But that is precisely what makes these specimens useful.

Researchers can study how recognisable microorganisms become transformed during fossilisation in an extreme environment and determine which biological signatures survive.

The methods can then potentially be extended to much older and more altered material.

The researchers suggest that their approach could also be valuable for studying Precambrian rocks, which preserve some of Earth’s earliest evidence of life.

An international scientific effort

The project brought together scientists and facilities from several countries.

The research involved ISTerre at Université Grenoble Alpes, whose partners include CNRS, IRD and several French universities, as well as the Laboratoire de Géologie de Lyon, the Swiss Light Source, EPFL, Harvard University, the European Synchrotron Radiation Facility and the Brazilian Sirius synchrotron.

That international collaboration was necessary because identifying possible traces of ancient life requires expertise spanning geology, microbiology, chemistry, physics and advanced imaging.

Astrobiology is no longer simply about looking through a telescope.

It can involve using some of the world’s most sophisticated X-ray facilities to investigate a fossil smaller than the width of a human hair.

Learning to recognise life

The study illustrates a fundamental difficulty in the search for extraterrestrial life.

Before scientists can confidently announce that they have discovered evidence of life elsewhere, they need to understand exactly what ancient life looks like after geology has spent millions — or billions — of years altering it.

Earth provides the training ground.

Río Tinto is particularly valuable because its unusual chemistry creates an environment that helps scientists investigate how microorganisms interact with minerals and how their traces survive fossilisation.

The red river in Spain may therefore help answer a question about another red world.

If microscopic traces of ancient organisms are eventually discovered in Martian rock, recognising them could depend on techniques scientists first perfected by examining fossils here on Earth.

Source: Based on “Du microbiote fossile de Rio Tinto en Espagne à l’exploration de la vie martienne grâce à la nano-imagerie X multi-échelle,” published by CNRS on 25 July 2024 and Université Grenoble Alpes on 24 July 2024. The underlying study, “Unveiling Challenging Microbial Fossil Biosignatures from Rio Tinto with Micro-to-Nanoscale Chemical and Ultrastructural Imaging,” by Lara Maldanis and colleagues, appeared in the July 2024 issue of Astrobiology.