Spanish Rio Tinto

The water runs red at about pH 2, and a great deal is alive in it.

The Rio Tinto rises in the Sierra Morena of Andalusia and runs roughly south-southwest for about 100 kilometres to reach the Gulf of Cadiz at Huelva. Its red and orange colour comes from an extremely acidic chemistry carrying very high levels of iron and heavy metals. The name means coloured river, though tinto is also the Spanish word for red wine, so the pun is available in both directions.

The river sits inside the Iberian Pyrite Belt, an area holding enormous quantities of ore and sulphide deposits, and it has been mined for roughly 5,000 years, for copper, silver, gold and more, as far as 20 kilometres back from the banks. The acidity is very likely a consequence of that: acid mine drainage feeding into the water, with dissolved iron giving the deep red.

The chemistry does not last forever. The colour holds for about 50 kilometres, and past the town of Niebla the acid begins to be neutralised as other streams join it on the way to the Atlantic.

What lives in it is the reason scientists keep coming. A survey of the river's sediment published in Applied and Environmental Microbiology describes a constant mean pH of 2.3 and a high load of heavy metals, iron and sulfate, making the basin directly comparable to acid mine drainage systems, and then reports a high degree of biodiversity in it. The bacteria fell into five phyla, and every archaeon found belonged to a single order, the Thermoplasmatales. The researchers identified organisms running the iron, sulfur and carbon cycles, among them Acidithiobacillus ferrooxidans, Ferroplasma and Acidiphilium, and found ferric iron acting as the main electron acceptor.

That is why this river turns up in astrobiology papers. An iron-rich, strongly acidic, metal-loaded system that is nonetheless full of life is the closest thing on Earth to certain environments elsewhere, and the microbiology here is studied as a model for them.