Too much of a good thing — nutrients — can spur the growth of algal blooms in coastal waters, leading to low-oxygen conditions and the production of poisonous hydrogen sulphide. This can have drastic consequences. On the coast of Namibia, for example, residents are well acquainted with 'lobster walks', whereby the crustaceans literally march up onto the shore to escape the poisonous chemical.

In Namibia the blooms are a natural phenomenon, but researchers such as Marcel Kuypers, a biogeochemist at the Max Planck Institute for Marine Microbiology in Bremen, Germany, believe that similar sulphidic events elsewhere are triggered as a result of human activity. The New York-based Wildlife Conservation Society lists algal blooms as one of the 'deadly dozen wildlife diseases' likely to spread as Earth warms. But, it seems, nature may be able to soften the blow: Kuypers and his group have discovered an opportunistic bacteria that cleanses the toxic brew that the Namibian algal blooms produce.

Kuypers first travelled to Namibia in spring 2003 to study the loss of nitrogen from coastal ecosystems, in collaboration with the Namibia Ministry of Fisheries and Marine Resources. But that wasn't the only reason for the trip. “The sulphidic events were definitely on our agenda,” he says. “We had prepared to look for them, but we didn't hit on any hydrogen sulphide in the water column.” He returned in January 2004, timing the research cruise to improve their chances of observing a sulphidic event.

On the ship, Kuypers and his team received daily satellite images to look for areas of discolouration; these are created when hydrogen sulphide reacts with oxygen to form elemental sulphur in surface waters. “We were looking at these images and we didn't see anything; there was no discolouration whatsoever,” he says. “But we were measuring hydrogen sulphides in the water column.”

Because the depth at which they found the sulphides was 40 metres below the level at which any oxygen was present, it was clear that no chemical oxidation was occurring. But why? The researchers reasoned that biological processes must be involved, and incubated samples to look for microbes. On page 581, they describe finding large populations of γ- and ε-proteobacteria, which are known as chemolithotrophs because they live on inorganic matter. These bacteria, which had previously been found only near deep-sea hydrothermal vents, were consuming hydrogen sulphide in the bottom waters, preventing it from reaching the surface, where many fish and mammals live. This shielded the poisonous waters from human detection, so sulphidic events may be much more widespread than was thought.

On the plus side, the activity of chemolithotrophic bacteria protects animals in surface waters from hydrogen sulphide's toxic effects. But residents of the lower reaches, such as small fish hiding from predatory fish and birds, are still at risk. As sulphide poisons the lower layer, these fish are either killed directly or are preyed upon when they flee to the surface. “The areas where we found the sulphidic waters were teeming with life,” says Kuypers, who saw schools of whales and dolphins, and “massive amounts” of birds, which were presumably feeding on upwardly migrating small fish. If sulphidic events increase in frequency, he says, “The frequent occurrence of these events may actually decimate the fish populations off Namibia.”