Don't drink the water. Despite its acidity, Rio Tinto is chock-full of eukaryotes.

Toxic River Teems With Life

The denizens of extreme environments such as hot springs and hydrothermal vents tend to be prokaryotes--simple organisms such as bacteria and archaea that lack a membrane-bound nucleus. But a new study finds that some extreme neighborhoods are more diverse. In the 9 May issue of Nature, researchers report the discovery of an unexpected variety of eukaryotes in the toxic, blood-red waters of the Rio Tinto in southwest Spain.

The red color and acidity of Rio Tinto are maintained by the metabolism of rock-eating prokaryotes that thrive on pyrite and other minerals in the river. With a pH of 2--about the acidity of lemon juice--researchers didn't expect anything but prokaryotic life to survive there. But when a team looked at water samples under the microscope and subjected them to genetic analysis, they got a surprise: eukaryotic algae make up at least 60% of the river biomass. Another 20% consisted of other types of eukaryotes, including fungi and amoeba, and the rest was made up of prokaryotes. That's the opposite of what's been found previously in extreme environments, says microbiologist and co-author Ricardo Amils at the Autonomous University of Madrid. The team also identified 33 previously unknown eukaryotes.

Eukaryotes may be tougher than researchers thought, Amils says. Because so few eukaryotes had been found in extreme environments, scientists assumed they weren't as rugged as prokaryotes. The findings also suggest that the eukaryotes living in the Rio Tinto may have toughened up in a hurry. Many of the Rio Tinto eukaryotes have close cousins that can't survive such acidic conditions, says first author Linda Amaral-Zettler of the Marine Biological Laboratory in Woods Hole, Massachusetts. This suggests that the hardy organisms "have figured out how to adapt to this extreme environment over relatively short evolutionary time scales."

To Roger Buick, a geologist at the University of Washington, Seattle, the findings suggest that "the ecological limits to complex life might not be as narrow as once thought." That could change the way people think about the potential for life elsewhere in the universe, he argues. "This implies that conditions amenable to complex life might not be confined to a very few planets in the universe."

Related sites
Linda Amaral-Zettler's site
Applied Microbiology site at the Autonomous University (in Spanish)
NASA Astrobiology Institute