by Sharon Oosthoek

Answer: Brilliant purple mats of cyanobacteria and translucent ponytailshaped microbes. Question: What lives in the recently discovered sinkholes at the bottom of Lake Huron?

The sinkholes, probably the remains of what once held some of the earth’s ancient seas, appear to contain an ecosystem that is at odds with the rest of the lake, according to Bopaiah Biddanda, an aquatic ecologist at Michigan’s Grand Valley State University and a leader of its team of scientists studying the sinkholes.

“What we’re finding is all new, in a place where it was not supposed to occur,” he says. “There’s a chance for discovering new organisms.”

The freshwater fish that swim above do not venture into these oxygendeprived, sulphur-laden sinkholes in Michigan’s Thunder Bay National Marine Sanctuary, off the northeast coast of the state’s lower peninsula. It is an extreme environment resulting from water, containing dissolved minerals from the limestone bedrock, issuing up from aquifers that flow through an ancient seabed below the lake. The sinkholes happen to be an ideal spot for cyanobacteria, which can undergo photosynthesis using sulphurous compounds. Studies of the DNA sequencing of these purple organisms reveal that they are cousins of microbes found, at the bottom of permanently ice-covered lakes in Antarctica.

“Cyanobacteria were probably all over the world about three billion years ago and then they got segmented as the world became more oxygenated. But in pockets where there is still low oxygen, they continue to exist,” says Biddanda.

Lake Huron’s limestone sinkholes extend anywhere from 20 to 100 metres below the surface, depths to which the sun’s light does not penetrate. While local residents are familiar with above ground sinkholes and a small number of accessible shallow underwater ones, the sinkholes deep under the surface of the lake were discovered by accident in 2001. Scientists with the Connecticut based Institute for Exploration were searching for shipwrecks when their side-scanning sonar picked up deep sinkholes – ranging in size from a square metre in diameter to as big as a football field.

It is not surprising these deep sinkholes were discovered only recently, says Biddanda, whose team has been studying them since 2003. “You could be sailing over one and not know it because the water is dense and hugs the lake floor.”

Biddanda believes that similar sinkholes exist beneath the other Great Lakes where aquifers run beneath the limestone bedrock, except for Lake Superior where the bedrock is granite, which does not dissolve as easily as limestone.

He argues that these unique ecosystems are yet another reason to protect the lakes. “Here we have almost a coral reef – purple mats next to green and white mats. It’s spectacular.”