The Tinto River is born in the Sierra de Padre Caro and runs 90 kilometers from Huelva to empty into the city’s estuary. Its marked red color -hence its name- tells us that it is a river, at least, different.
The formation of massive iron oxides in the present and geological past of the Rio Tinto makes it an ideal place to study the processes that lead to the sedimentation of iron compounds both on Earth and on Mars: the discovery in the Terra Meridiani region of extensive deposits of iron oxides -probably from the time when liquid water ran over its surface-, similar to those found in the Tinto river basin, is what makes it a small-scale model of what could have happened in Mars millions of years ago.
It was that intense red color that caught the attention of a group of Spanish researchers led by Ricardo Amils , from the Autonomous University of Madrid, many years ago. His studies were able to determine that the main component of its waters is iron in the ferric state , which is responsible for that marked red color . Obviously, under these conditions, no one could imagine that any living being could live there. In addition, everyone assumed that its color was due to the heavy mining that has taken place in the area since the time of the Iberians, 5,000 years ago. However, the river gave them two surprises: the first, that contrary to everything imagined, in the waters of the river there was a wide biodiversity ; the second, that those responsible for the ‘pollution’ of the river were the very microorganisms that lived in it. Its metabolism processes metal sulfides and originates sulfuric acid and solubilizes heavy metals. Or put another way: they are living beings that eat iron and both the acidity and the abundance of dissolved iron in the water are their ‘poops’.
Certainly, the water of the Tinto River is not to take a drink on a hot summer day -it is true that its color already takes away all the desire to do so-. For two reasons: its extreme acidity and the high content of iron and sulfates . In some cases, the measuring instruments have given values close to 20 grams per liter of iron concentration. To get an idea, the water we extract from the wells has an iron concentration between 0.5 and 10 milligrams per liter; that is, we are talking about values a thousand times higher than normal. On the other hand, the high concentrations of sulfates together with the high acidity are telling us that what runs through the channel is not water but a solution of sulfuric acid . That is why it is easy to think that life had no place in the river, which already from its source presents these peculiar properties. How to explain them? For that we have to look at certain very particular factors that only occur in this region of the Iberian Peninsula.
From a geological point of view, the iron and the sulfate it transports come from the subsoil through which the groundwater passes, which when it rises to the surface creates the Tinto River. In fact, the mining interest that has been held in this area since time immemorial is precisely the existence of exceptionally rich deposits of metallic sulphides , which are part of a large geological structure known as the Pyrite Belt , since it is mainly made up of pyrite, a mineral of sulfur and iron; exactly the main components of the acid solution of the Rio Tinto. And if we look from a biological point of view, we find that the microorganisms that live there do not obtain energy from sunlight or from eating other organisms, but from the oxidation of inorganic compounds formed by sulfur and iron.
Because of such a peculiar way of feeding, scientists call them chemilithotrophs , and they do so by oxidizing subsoil deposits. This produces two effects: by oxidizing the iron in the pyrite, they generate ferric ions which, in solution, give the water that reddish color. When the iron atom is released, the sulfide reacts with the oxygen in the air and the resulting sulfate acidifies the water, which also facilitates the solubility of the ferric ion: that is why it reaches such high concentrations of iron that have been found.
But the biggest surprise that the investigation of the Tinto River has provided is the biodiversity present in its waters . From a river considered dead, it went on to the discovery of extremophilic life forms and from there to the discovery of a vast biological diversity. In fact, it has been verified that a large part of the majority groups that make up the tree of life are represented: microorganisms belonging to the group of animals, plants, fungi…
What does such diversity mean in such an environment? If we compare these microorganisms with their non-extremophile congeners, we find that they are not very different. Coupled with this fact that the process of adaptation to that environment has been very fast evolutionarily, the Tinto River is telling us that life is not only robust but that it adapts to extreme conditions very quickly. But the most important thing is yet to come: thanks to this river, it is possible that it will help us define the minimum requirements for the development of life.
