Mysterious Red River

The Spanish river Río Tinto offers unique insights into the interplay of minerals and bacteria. Researchers from the University of Tübingen and even NASA are attracted to conditions in the river which are similar to those on the surface of Mars.

The Río Tinto in Spain has water red in color. — The Río Tinto in Spain has extremely acidic water and is red in color – due to microorganisms that dissolve iron and sulfur compounds from the riverbed.

The water of the Río Tinto shimmers red and orange. The “red river” in Spain, which winds through Andalusia for around hundred kilometers before flowing into the Atlantic at the town Huelva, looks almost unreal. The spectacular show of colors is not the result of man-made pollution, but of natural origin: Microorganisms release iron and sulfur compounds from the riverbed, which not only turn the water an extraordinary color, but also make it extremely acidic. Only a few microorganisms, algae and fungi survive here. For thousands of years, people have been mining raw materials such as iron and copper along the river.

Andreas Kappler has a photographic print covering the wall in his office in the Geo and Environmental Research Center (GUZ) at the University of Tübingen. It shows a close-up of the blood-red riverbed. “It’s fascinating when you drive through this extreme and hot landscape along the Río Tinto and see it with your own eyes. If you leave your finger in the river, it tingles, the water is acidic. I find it unbelievable that such a thing was set in motion by microorganisms.”

Portrait of Prof. Dr. Andreas Kappler. — Prof. Dr. Andreas Kappler.

Eldorado for science

The smallest of all living things fascinates the professor of geomicrobiology. He explores how minerals and microorganisms, especially bacteria, interact with each other. At Río Tinto, experts like him can access a unique natural laboratory that attracts researchers from around the world. Iron mineral deposits and extreme conditions in the region are similar to the surface of Mars and NASA has even tested equipment and analytical methods for finding traces of life on Mars here.

Metals such as iron, copper, lead, zinc and nickel remain dissolved in the water in their individual components due to the high acid concentration – normally they form solids and settle as sediments. It is estimated that up to 15 percent of the metal ions that are washed into the oceans worldwide come from the Río Tinto: Individual metal particles that carry an electrical charge and play an important role in chemical and ecological processes.

“The Río Tinto is one of the first areas where biomining has helped extract copper from ores,” says Kappler. “Even the Romans noticed that copper was released from the ores into the river water. Today we know that this is thanks to bacteria.” Many expeditions by plane and minibus have been made from Tübingen to the Río Tinto to gain insights for projects in basic geoscientific research. Researchers from the Autonomous University of Madrid also help with the work on site. The collected samples are then safely packaged and sent to the Tübingen laboratory.

“We are interested in what’s happening under the ground,” says Kappler. There is plenty of the mineral pyrite, also known as fool’s gold, in the region. Bacteria like Acidithiobacillus ferrooxidans, oxidize fool’s gold, dissolving it into its components of iron and sulfur. This results in a sulfuric acid solution, which in turn flushes iron and other metals (cobalt, manganese, nickel, cadmium) out of the rock and transports them into the river together with rainwater. “We are investigating which microorganisms play a role in this, how active they are under these extreme conditions and which of them can be found in the river and sediment.”

In 2020, Kappler, Sara Kleindienst and other researchers investigated which food the microorganisms in the Río Tinto metabolize and how they influence the transport of heavy metals to the estuary. While some minerals dissolve completely, the river transports others in the form of nanoparticles towards the sea. There, the metal ions of microorganisms, plankton, fish and higher organisms are used in enzymes or are deposited in sediments. “If we understand the transport processes, we can also better assess the impact on the environment and people.”

Biomining in waste slags

Kappler also has ideas on how the insights of the Romans could be used today: If they could dissolve metals from rock with bacteria – why couldn’t valuable metals be recovered from the waste slags from incinerators? This question has occupied geosciences researchers from Tübingen for several years. In a pilot study, funded by the Mannheim waste incineration plant, Kappler’s team was able to show that such biomining works in principle. In 2015, the project was awarded a prize by the Reutlingen Chamber of Industry and Commerce.

But we can’t expect Spanish bacteria to recover raw materials from German household waste on a large scale for the time being. “The difficulty lies in the subsequent separation of the metals,” says Kappler. “For effective metal recycling from slag, further investments would be needed to research the processes on a larger scale and to transfer them to the first pilot plants. Nevertheless, it is important to do basic research in the laboratory.”

Pyrite, also known as fool's gold and seen in the picture as sparkling particles, plays an important role in the chemical processes in the Río Tinto. — Pyrite, also known as fool's gold and seen here as sparkling particles, plays an important role in the chemical processes in the Río Tinto.

Looking toward Mars and the Tübingen subsoil

The latest project by the geoscientists could be a contribution to the question of life on Mars. In addition to iron minerals, nitrate was also discovered on Mars. “This is fascinating news for anyone who deals with the subject matter,” says Kappler. We know that nitrate is food for iron-oxidizing microorganisms and thus a potential basis for life. Kappler’s team also found bacteria at the Río Tinto that rust iron minerals and feed on nitrate. “We can study processes there that may take place on Mars in a similar way.”

A doctoral student is researching “nitrate-eating” microorganisms of Río Tinto that are involved in the dissolution of pyrite. These are exciting not only with regard to distant planets, but also with regard to an environmental problem directly under our feet: the nitrate pollution of groundwater. This problematic nitrate – for example from agriculture – can be removed by “denitrification”: Certain bacteria convert the nitrate into harmless nitrogen, typically with the help of carbon.

An interesting observation was made by a member of the project team: The Tübingen groundwater system contains bacteria that use the mineral pyrite as an energy source instead of carbon – and also remove the harmful nitrate from the water. “This is exactly what is happening in the Río Tinto riverbed,” says Kappler. “We are exploring the connections there in order to better understand what is happening underground. Our team is one of the world’s leading groups that works with exactly these organisms.”

The Río Tinto is an ideal “research playground”, says Kappler. “The region is well developed and safe and holds keys to a range of scientific questions, from the existence of early life to potential environmental technologies of the future.”

View on the river, that attracts research groups from all over the world. — As a unique natural laboratory, the river attracts research groups from all over the world.

The Geomicrobiology working group

Prof. Dr. Andreas Kappler heads the Geomicrobiology working group at the Geo and Environmental Research Center (GUZ) at the University of Tübingen. An interdisciplinary team of geochemists, mineralogists and microbiologists work here in well-equipped lab areas that were built in 2021.

In 2008, the University of Tübingen was one of the first universities in Germany to establish a professorship for the emerging research field of geomicrobiology.

Research focuses on the study of bacterial species; the analysis of minerals and their interactions; the importance of bacteria for the release of climate gases and pollutants; the emergence of life on Earth.

Text: Christoph Karcher