Lake Huron sinkholes harbor unexpected life forms

Compared to the dark depths of the ocean or the frigid ends of the earth, Lake Huron hardly seems like an extreme environment. But sinkholes discovered at the lake’s bottom have conditions just as harsh-and harbor life forms just as unusual-as those of deep-sea hydrothermal vents or Antarctica’s permanently frozen lakes.

And like their marine and polar cousins, the sinkholes’ peculiar occupants-mostly microbes-are providing scientists a glimpse of what early life on Earth might have looked like.

Located in the Thunder Bay National Marine Sanctuary area just off the index finger of Michigan’s “mitten,” the sinkholes are thought to have formed thousands of years ago, before the water now known as Lake Huron covered the area. The sinkholes formed in spots where flowing groundwater dissolved layers of gypsum underground, creating cavities into which the overlying rock collapsed. Today, groundwater still moves through the system, dissolving minerals out of the rock and flushing them up into Lake Huron through the sinkholes.

An Extreme Environment

Researchers first stumbled upon some of the sinkholes in 2001 while surveying the Thunder Bay area for shipwrecks. Subsequent explorations of Isolated Sinkhole, located about 10 miles off shore in more than 300 feet of water, revealed that the groundwater seeping out the lake-bottom fissure was very different from the lake water. The groundwater lacked oxygen and had a chloride concentration 10 times higher and a sulfate concentration 100 times higher than that of the lake.

That got Bopaiah Biddanda, an ecologist at Grand Valley State University in Muskegon, Mich., thinking. “If water with different chemistry is coming out,” he said, reflecting back on his thoughts at the time, “maybe there are different kinds of life there.”

So in 2003, he and his colleagues set out with divers and a remotely operated vehicle, or ROV, outfitted with a video camera and other equipment to take a look. “We had no clue what we would find,” he said.

Biddanda ranks what they did find-vibrant ecosystems of microbes that thrive in conditions too extreme for most modern life forms-as one of the “most exciting” discoveries of his career. Previously, such “extremophiles” had been documented only in places like deep-sea hydrothermal vents and frozen Antarctic lakes. “We found something that wasn’t thought to exist in the Great Lakes,” he said.

The microbes flourish in the sinkholes’ harsh chemical conditions-the water is sulfur-rich and oxygen-free-because they use hydrogen sulfide (the “rotten egg” gas) rather than hydrogen oxide (water) for their metabolic processes. That makes them similar to primitive life forms thought to inhabit Earth some two to three billion years ago, before oxygen was abundant.

A Spectrum of Microbes

The types of microbes the researchers found in Lake Huron varied from sinkhole to sinkhole, depending on the availability of light. Situated in just three feet of water, the well-lit El Cajon Sinkhole is home to organisms such as green algae and cyanobacteria, which are photosynthetic and therefore need light. Located 100 times deeper, Isolated Sinkhole is almost completely dark, and populated by white mats of bacteria that carry out chemosynthesis instead of photosynthesis.

Both photosynthetic and chemosynthetic microbes live at Middle Island Sinkhole, located at an in-between depth of 75 feet. Among Middle Island’s most fascinating occupants are thread-like cyanobacteria that cohere to each other to form brilliant purple mats. The mats are punctuated by fingerlike projections filled with methane gas released from the sediment below.

In their constant quest for light, the photosynthetic microbes making up the mats glide past one another or over obstructions, such as the particulate matter that settles out on them from the water above. “One day, I put pebbles on [a mat sample in the lab],” said Biddanda, “and by the next morning the mat had ‘climbed’ over them.”

According to a DNA analysis conducted by University of Wisconsin-Stout biologist Stephen Nold, the purple cyanobacteria’s closest known relative is a photosynthetic bacterium found on the bottom of an Antarctic lake.

Along with the assortment of microbes, researchers have occasionally-and surprisingly-found more complex life forms, like microscopic worms, living in the harsh sinkhole environments. “We don’t know how they are managing against the odds,” said Biddanda.

That’s just one of the many intriguing aspects of the sinkholes scientists are eager to explore. Researchers are currently working to answer a myriad of biological, chemical, and geological questions-from how old the sinkhole groundwater is, to how chemicals and nutrients cycle through the system, to whether compounds produced by the newly found microbes have medicinal value.

The work of unraveling the mysteries of Lake Huron’s sinkholes has just begun.

Jennifer Yauck is a science writer at the UWM Great Lakes WATER Institute. GLWI (glwi.uwm.edu) is the largest academic freshwater research facility on the Great Lakes.

CORRECTION (MAY 6, 2010): The original version of this article misstated the type of rock that is thought to have been dissolved by groundwater, thereby creating Lake Huron’s sinkholes. The rock primarily affected is gypsum, not limestone. Limestone, however, is one of the main rock types found in this area.

Explore some more!

Cruise along with an ROV as it explores Lake Huron’s sinkholes-visit miearth.org/play.php?vid=268.

Meet sinkhole researchers, read expedition logs, check out expedition videos and more by visiting NOAA’s Ocean Explorer page at oceanexplorer.noaa.gov/explorations/08thunderbay. Teachers, be sure to check out the page’s “Education” link (left side of page)-you’ll find sinkhole lesson plans for grades 5-12 and other useful resources.