The
Middle Island sinkhole is open to Lake Huron, creating a gradient of
biological activity. A nine-metre boat is also visible in this aerial
photo for sense of scale. ((Scott Kendall/Bopi Biddanda/Grand Valley State University))Twenty
metres below the surface of Lake Huron, scientists have discovered
peculiar sinkholes where a bizarre ecosystem at odds with the rest of
the lake flourishes.
The huge lake's freshwater fish shun the dense, salty, oxygen-deprived waters of these sinkholes off northeastern Michigan.
Instead, brilliant purple mats of cyanobacteria — cousins of
microbes found at the bottom of permanently ice-covered lakes in Antarctica — and pallid, floating, ponytail-like microbes thrive.
Groundwater
from beneath the lake is dissolving minerals from the ancient seabed
and carrying them into the lake to form these exotic, extreme
environments, says aquatic ecologist Bopaiah Biddanda of Michigan's
Grand Valley State University, a leader of the team studying the
sinkhole ecosystems.
"These are almost primordial Earth
conditions, with high sulphur and low oxygen like in the ancient oceans
that covered the Earth three billion years ago," Biddanda told CBC News.
"It gives us a
window into the past and who knows what value it will hold."
The
researchers describe this little-known underwater habitat in this
week's issue of Eos, published by the American Geophysical Union.
Although above-ground sinkholes in the area were discovered decades ago, the submerged sinkholes were only recently uncovered.
Discovered 8 years ago
In
2001, researchers with the Connecticut-based Institute for Exploration
stumbled across them during an underwater archeological survey for
shipwrecks in Michigan's Thunder Bay National Marine Sanctuary.
Scientists
began to explore these sinkholes a couple of years later, finding some
just 20 metres below the surface and others extending 100 metres down,
where the sun never shines.
But their findings have trickled in over the last few years because of the logistical problems in exploring lakebed sinkholes.
"Finding these little spots in a huge lake — you can't even compare it to looking for a needle in a haystack," said Biddanda.
The
most recent findings show an ecosystem that has more in common with
Antarctic lakes and deep-sea, hydrothermal vents than it does with a
freshwater lake.
"We were amazed to find these brilliant
cyanobacteria mats," said Biddanda. DNA sequencing of the purple mats
show they are closely related to mats found in the ice-covered,
oxygen-poor Antarctic lakes.
Biddanda suspects similar ecosystems
once existed all around the Earth but largely disappeared as
the planet's atmosphere became increasingly oxygen-rich.
The team,
including researchers from the U.S. National Oceanic and Atmospheric
Administration, suspects similar sinkholes exist under the other Great
Lakes because, with the exception of Lake Superior, the lakebeds are all
composed of limestone, with ancient aquifers running beneath.
The researchers will
continue to
study the sinkholes this summer, keeping a sharp eye out for the
possible discovery of never-before-seen organisms and biochemical
processes.
