University of Tübingen: Microbial life under extreme conditions
At the mouth of the Rio Tinto in south-west Spain, the acidic river water contaminated with heavy metals from ore mining and mineral weathering mixes with the salt water of the Atlantic Ocean. A unique community of microorganisms that love extremes forms there. They live in water as acidic as vinegar, are resistant to high levels of salt, and some are also fine with high levels of toxic metals. A research team led by Professor Andreas Kappler and junior professor Sara Kleindienst from the Center for Applied Geosciences at the University of Tübingen discovered this community. It examined where the microorganisms obtain energy for their metabolism under the extreme conditions and what influence they have on the deposition or leaching of heavy metals in the Rio Tinto estuary. The study was published in the journalApplied and Environmental Microbiology published.
The Rio Tinto – in English “Red River” – is rightly named: In places, the approximately 100-kilometer-long river in the Spanish province of Huelva glows magnificently orange to blood-red. Why its water is so acidic and why its estuary belongs to one of the most heavily polluted water systems in the world with toxic metals has not been clarified to this day.
Ore mining as early as the Copper Age
The magnificent colors of the water and the deposits are caused by the activity of various microorganisms.
“In any case, the pollution started very early, in the Copper Age, around 5,000 years ago,” reports Sara Kleindienst. Even then, people were mining ore in the upper reaches of the river above the so-called pyrite belt of the southern Iberian peninsula. Gold, silver, copper, tin, lead, and iron, as well as large iron sulfide deposits, are found in the rock belt. When the ore was mined, the iron sulfide came into contact with the oxygen in the air, allowing certain microorganisms to oxidize the iron and sulfur. “Blood-red, extremely acidic water is produced, which releases tons of other toxic metals such as manganese, cobalt, nickel and cadmium from the rocks and flushes them into the river every year,” says the researcher.
The new research showed that most microorganisms in river water obtain their energy from dissolved iron. “In doing so, they can form iron minerals and precipitate other toxic metals around their cell wall. These aggregates of cells and minerals are then transported downstream to the estuary,” says Andreas Kappler. “We were particularly interested in what happens when the acidic river water mixes with the sea water there.”
The high concentration of chloride in seawater is said to be toxic to acid-loving iron-oxidizing microbes. “Most of them disappear in the estuary. There, other iron oxidizers that can cope with the high salt content take over. In addition, the high levels of dissolved iron attract marine species of iron oxidizers,” explains Kleinschmidt. These are also the ones that form iron minerals in the estuary area and precipitate toxic metals such as arsenic and chromium, which are deposited in the sediment of the Rio Tinto. However, some of these minerals would be transported further to the ocean shores. “By gaining insight into this microbial community, we learn more about the influence of the microorganisms on the mobility of the toxic metals in the Rio Tinto,” says Kleinschmidt.
Mars-like conditions fascinate astrobiology
The iron-oxidizing bacteria in the Rio Tinto produce colorful minerals such as goethite, red hematite, schwertmannite and jarosite, which are deposited in the river’s sediment. “Intriguingly, the same minerals were discovered by the Mars rover Curiosity in the sediment of Mars crater Gale. The formation of such minerals could have been triggered there 4.1 to 3.7 billion years ago by similar acid-loving microorganisms in a large-scale river system as in the Rio Tinto,” says Sergey Abramov from the University of Tübingen, the first author of the study. Back then, Mars would have had wetter conditions and more moderate temperatures.
“In fact, the Rio Tinto attracts astrobiologists from all over the world to study hypothetical life on Mars,” adds Kappler. He also sees other similarities between the Rio Tinto estuary and Mars: In the former, the tides of the Atlantic Ocean periodically mix acidic river water and seawater; on Mars, similar processes may have occurred in an active sedimentary cycle at Gale Crater 3.7 to 3.6 billion years ago. During this period, the lake and river systems on Mars periodically dried up, and the climate at the mouth of the Rio Tinto caused a corresponding seasonal drying out of the floodplain.