Day 36: Bizarre life lurks in Great Lakes sinkholes

The transition zone between the oxygenated Lake Huron water, and the hypoxic groundwater below, lies just avove the lower ledge. The green algae near the top shelf perform oxygenic photosynthesis, while purple cyanobacteria down below perform anoxygenic photosynthesis. White sulfur- oxidizing bacteria (the stringy mats) found in the transition zone perform chemosynthesis. Diver photo by Tane Casserley, National Oceanic and Atmospheric Administration

DO YOU KNOW WHAT THAT IS? Seriously, this is one of the coolest studies I’ve read all week, and not just because it documents biology at temperatures as low as 4 degrees C (39 degrees F) at the bottom of Lake Huron. 

This is a study that says some forms of life will still thrive after global warming chokes out the oceans, and that stinky little microbes undeniably exist on Titan, Saturn’s largest moon.

Well … no. Not really. But it certainly ignites my imagination in those ways. 

Lead study author Bopaiah Biddanda, of Grand Valley State University in Muskegon, Michigan and a boatload of colleagues have been studying sinkholes in Lake Huron that were originally discovered in 2001 and 2002. They’ve found that the bizarre ecosystems are supported by regular flow from groundwater vents, and that they host microbes that don’t need oxygen. 

“Here,” the study authors write in Eos, “typical lake inhabitants such as fish and phytoplankton are replaced by communities dominated by microorganisms: bacteria and archaea that perform unique ecosystem functions.” Eos is a weekly newspaper put out by the American Geophysical Union. And archea are just single-celled life forms.

In the deepest, most oxygen-deprived parts of the sinkholes, the authors describe “brilliant purple mats formed by filamentous cyanobacteria” that cover carbon-rich sediments. “Carbon-rich sediments” is a nice, scientific way of saying these sinkholes collect decayed materials that form a thick, black sludge at the bottom. In the oxygen-depleted water, cyanobacteria carry out photosynthesis using sulfur compounds rather than water and give off hydrogen sulfide, the gas associated with rotting eggs. Where the sinkholes are deeper still and light fails, microorganisms use chemical means rather than photosynthesis to digest the sulfurous nutrients.

The newly characterized underwater ecosystems bear striking similarities to deep-sea marine vents, subsurface sulfur springs, the sulfate-rich Lake Cadagno in Switzerland, and Antarctic lakes that are permanently entombed in ice. Besides offering an easily accessible habitat for scientific exploration, the sinkholes are veritable launch pads for the imagination.

For one thing, the authors wonder how life forms that don’t require oxygen have come to exist in such far-flung reaches of the globe. They point out that many microbial organisms are transported through the air, and that the bacterial mats they’re seeing under Lake Huron can break free, floating to new locations on the the currents.

“Dutch microbiologist noted, ‘Everything is everywhere, but the environment selects,’” they wrote. “Clearly, the environments in Antarctic lakes, Lake Huron, and Lake Cadagno are selecting for similar microbial communities.”

Alternatively, the oxygen-deprived communities could be holdovers from our world as it was 400 million years ago, before photosynthetic plants started using the carbon dioxide and producing oxygen.

And that’s where my imagination starts to go a little wild with this study. One of the consequences of pollution, particularly agricultural pollution from fertilizers, is that we create huge oxygen-deprived zones where the rivers dump into the ocean. Sure, it’s nice to think of addressing that, so fish can continue to live in these places. But if we don’t, I suppose we can take heart in the idea that SOMETHING will live in the changed environs — like, perhaps, brilliant purple mats and floating white ponytails of bacteria with other-worldly chemistry.

This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan. Credit: JPL/NASA

Now, what of the Titan connection? Well, that is even farther afield than my previous speculation. 

“In the lower sediment depths,” the authors wrote, “methanogenic archaea reduce carbon dioxide and produce methane. Sediment profiles display high methane concentrations at depth and lower concentrations near the sediment surface. This suggests that methane is being consumed at the sediment-water interface by methanotrophic [methane-loving] bacteria.”

Titan, you see, is a truly weird moon. Scientists are pretty sure now that methane and ethane fill the air with a dirty haze that rains down as ash. Rivers run with a gasoline-like liquid that collects in black, glassy lakes. Sure, it probably smells rank. But besides that, I think the place would be ideal for finding life forms that have evolved in entirely different circumstances. And that is very, very exciting. 

Naturally, the study authors don’t get quite as carried away with their work as I have. But Biddanda and his co-authors did say the findings “might also lead to the discovery of novel organisms and previously unknown biochemical processes, furthering our exploration of life on Earth.”

Indeed.

The new study was funded by the National Science Foundation, the National Oceanic and Atmospheric Administration and a Michigan Space Grants Consortium Graduate Fellowship.

My source was the study itself, available through the AGU’s website.