Astrobiology Top 10: Looking for Life on Mars — in a Canadian Lake
On the surface, Pavilion Lake, nestled among the peaks of Canada’s Marble Range, looks like a thousand other mountain lakes. It’s not unusually large or deep. It’s not especially acidic, or alkaline; it’s not overly salty; nor are there high concentrations of minerals dissolved in its water. Locals come here to fish, to boat, to swim, and to watch the summer clouds drift by.
But underwater lies an astonishing discovery that has drawn astrobiologists from around the world to this rural corner of British Columbia. Pavilion Lake is host to an underwater “forest” of microbialites, coral-like structures, in a variety of shapes and sizes, that may help guide the search for life on Mars.
“The size of these microbialites is unlike anything else that’s ever been documented,” said Darlene Lim of NASA Ames Research Center. Lim is the principal investigator for the Pavilion Lake Research Project (PLRP). “On a micro scale, there might be a lot of similarities to what you see in other lakes, other ponds, around the world, and also in marine environments. But on a macro scale, they look very different. And the fact that they’re in this very accessible, sort of regular, recreational lake is curious.”
Scientists have been studying the Pavilion Lake formations for nearly a decade. Scuba divers have retrieved samples from as deep as 100 feet below the surface for analysis. But the lake is too deep – more than 200 feet at some points – and the microbialites structures too varied for divers to survey it thoroughly. So this summer, a team of researchers went through a rigorous training program to become submarine pilots and then spent a week exploring the lake in DeepWorker submarines built by Nuytco Research of North Vancouver, British Columbia. Their goal was to map the distribution of microbialites in the lake, and to bring back a more-extensive set of samples, including samples from the deepest regions.
The streamlined, shiny black DeepWorkers – there were two of them – looked more like George Jetson’s flying car than a typical Hunt-for-Red-October-type submarine. There was just enough room for one person to fit inside and the pilot’s head stuck up into a Plexiglas bubble that hinged open for access. The unit was operated entirely by foot pedals.
At the start of each run, a custom-built barge, pushed by a small motorboat lashed to its back end, hauled the DeepWorkers into position. The pilots climbed inside, the hatches were sealed and the subs, assisted by a pair of divers, were lowered through a hole in the floor of the barge into the water.
The pilots then “flew” the subs along a course decided upon during lengthy scientific discussions earlier in the day. The runs typically lasted 2 to 3 hours. The pilots were guided by another pair of team members, who tracked the subs’ movements from CapComm, a rectangular flat-bottom watercraft with a metal roof decked out in Christmas lights. Although the pilots had excellent visibility, they literally didn’t know where they were going without continuous navigational updates from CapComm.
The microbialites, composed of calcium carbonate, range in size from small bumps just a few centimeters across to enormous structures as much as 12 feet high. They come in a variety of shapes; some have been described as looking like cauliflower, others like broccoli, still others like asparagus, or fingers. Some contain central columns that resemble chimneys.
But it’s not just their varied shapes that makes them so interesting. It’s the fact that no one knows how they formed.
“We are sure that the structures are here. We’re sure that they vary significantly with depth. And we’re sure that they’re not found in other analogous lakes. Those are the facts, the observational facts,” said Chris McKay of NASA Ames Research Center. McKay was one of the first scientists to scuba dive in Pavilion Lake. “When you look at structures like this, the standard hypothesis is that organisms are playing a role in creating them. But we haven’t proven that here.”
Present-day bacteria are playing some role in forming calcium carbonate structures in the lake, Lim said. “There’s trash down there. And the trash is fairly recent; at most it’s like 100 years old, but probably less than that. We know that there’s carbonate deposition happening on the trash. We know that there is microbial growth of some sort on the trash, there are microbial crusts that are developing on trash.”
Some cyanobacteria excrete calcium carbonate, so one possibility is that the crusts are composed of this bacterial waste. Or perhaps, the bacteria build up an electric charge along their cell walls, which attracts calcium carbonate in the lake water; or they secrete slime that the carbonates bind to.
But whether or not the process that is forming the modern-day carbonate crusts is the same process that formed the bulk of the large structures, “that’s what we’re trying to figure out,” Lim said.
The working hypothesis is that bacteria were involved in some way in creating the large structures. But it’s also possible that, although bacteria form crusts on the surfaces of the structures, the structures themselves were the product of a purely chemical, rather than a biological, process.
Cyanobacteria in other locations are famous for building a variety of structures, from thick rubbery mats to layered dome-like structures known as stromatolites, which are thought to have been the dominant form of life on early Earth. Today, though, they are rare, existing only in extreme environments.
The shallow waters of Shark’s Bay, in Western Australia, for example, are home to large fields of dome-shaped stromatolites. But Shark’s Bay is too salty for the tiny worms that like to snack on the bacteria. That’s why the stromatolites can thrive: there’s nothing around to eat them.
Pavilion Lake, however, is “normal.” It has all kinds of larger organisms living in it. It’s even stocked with fish. And therein lies the mystery. There are smaller, less diverse, carbonate structures in another nearby lake, Kelly Lake, but none have been found in any of the other lakes in the region. Something makes Pavilion Lake unique. It’s just that no one has figured out yet what that something is.
Researchers are approaching the problem from a number of different angles. Some are looking at the chemistry of the water and trying to understand the lake’s topography and underground water sources. Others are doing DNA analysis of the slime that coats the microbialites to learn what organisms are living there, and what they eat – and excrete.
Still others are comparing the carbon in calcium-carbonate samples from the slime layer to that in samples taken from the hard core of the structures, to determine whether there is a clear biological signature in the core. Living organisms prefer to use the lighter isotope of carbon, C-12, so they tend to leave the environment around them enriched in the heavier C-13.
Results on that front remain inconclusive, says Greg Slater of McMaster University in Ontario, Canada. Slater is a co-PI of PLRP. “The carbonate in the surface community has a signature of biological activity. But when you go in deeper and down into the structure, that signal doesn’t seem to be preserved.”
Which is odd, because if the structures were built by microorganisms, there should be some isotopic evidence of their biological origin. Slater plans to use powerful microscopes to compare the crystal structure of calcium carbonate from different depths within the microbialites. It’s possible that, over time, the structures have dissolved and recrystalized, in the process changing from one form of calcium carbonate to another – and losing their biological carbon-isotope signature in the process. If this avenue of research pans out, the results could provide new insight into how biosignatures are modified and preserved over time, and that understanding, in turn, could aid in future efforts to look for biosignatures on Mars.
Lim and her colleagues plan to return to Pavilion Lake in future years to continue their work – perhaps with an even more unusual submarine. Although the DeepWorkers enabled researchers to collect samples from the deepest parts of the lake, it’s difficult to maneuver them precisely enough to avoid damaging the microbialites. Lim is hopeful that a new combination-submarine-and-pressurized-underwater-suit, under development by Nuytco, will make it possible for divers to work in the deepest parts of the lake without having to resort to dangerous compression diving.
The Pavilion Lake Research Project 2008 field work was supported by the Canadian Space Agency, NASA, McMaster University, the University of British Columbia, British Columbia Parks, the Pavilion First Nations Band and Nuytco Research Ltd.
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