A diver sets up a respiration chamber experiment near the purple microbial mats at Middle Island Sinkhole in Lake Huron (Image: Tane Casserley, NOAA)
We normally think of freshwater lakes and salty oceans as two distinct worlds. Not so in the North American Great Lakes. Just 20 metres beneath the surface of some of the largest freshwater reservoirs on Earth are deep brine-filled pockets.
Now, researchers are discovering that these unusual sinkholes are home to extraordinary communities of microscopic bacteria. The organisms are not new to science, but preliminary genetic analysis is showing that they are relatives of bacteria that live in the subglacial lakes of Antarctica. Others are functionally similar to the extremophile bacteria living on the black smokers of the deep ocean.
The sinkholes were discovered in 2001, when a sonar expedition searching for shipwrecks found deep pits, up to 100 metres across, in the lake floor. The underwater pits have formed in places where an ancient underlying seabed is dissolving.
As groundwater water rises through the carbonate bedrock, it dissolves minerals and carries them into the lake. "You have this pristine fresh water lake that has what amounts to materials from 400 million years ago being pushed out into [it]," says Steven Ruberg of the National Oceanic and Atmospheric Administration (NOAA).
Ruberg is part of a team of researchers who are studying the chemistry and biology of these odd pockets of life. So far, they have found that the water inside them is chemically very different to the overlying lake water. For starters, it is rich in sulphates and low in oxygen. It is also significantly more acidic, contains more chloride, less dissolved organic carbon, and supports entirely different life forms to the rest of the lake.
In these pockets, fish and plankton give way to dense communities of brilliant purple cyanobacteria that photosynthesise using sulphur instead of oxygen and give off hydrogen sulfide – the gas associated with rotting eggs. In deeper sinkholes, where light levels are too low for photosynthesis, bacterial communities form pale floating "ponytails" that can metabolise sulphur compounds without light.
Similar mats of cyanobacteria have been found in Antarctic lakes that are permanently locked under several thousand metres of ice. The light-free communities, on the other hand, resemble those found on deep-sea hydrothermal vents and cold seeps.
How these extremophiles came to colonise Antarctica, the deep oceans and aquatic pockets at the bottom of the North American Great Lakes is something of a mystery, say the researchers.
They have sent remotely operated submarines into the sinkholes and have seen how the mats of cyanobateria will occasionally peal off the bottom and rise up into the freshwater lake above, shuttled by microbial gases that come out of the sediment. This suggests one method of dispersal may be through global currents – but it still leaves much to be explained.