Scientists dive into ice-covered lake to explore unique bacterial community
Posted October 16, 2009
There’s not much in the ice-covered lakes in the McMurdo Dry Valleys to interest anglers looking to land the big one. But for scientists who want to know more about some of Earth’s earliest organisms — and, by extension, to recognize what life may look like on other planets — those unique ecosystems represent a useful portal to the past.
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Indeed, the lack of fish or other animals high on the food chain has allowed the microorganisms that live within the lakes to flourish unmolested, developing into communities thick enough to accumulate in layers on the lake bottoms.
“The cool part is that you can see microbial ecosystems on a landscape scale. There aren’t too many places around the world where you can do that,” noted Dale Andersen , with the SETI Institute’s Carl Sagan Center for the Study of Life in the Universe and principal investigator on a project to learn more about the microorganisms that dwell in Lake Joyce.
Lake Joyce — one of about a dozen perennially ice-crusted lakes spread throughout the valleys — is all the more unique in that it is one of only two known lakes in the region where the microbes have produced microbialites. These carbonate structures, composed of the same minerals that make up a coral reef, grow right in the layers of cyanobacteria, called microbial mats.
Andersen explained that his team, funded by NASA’s Exobiology Program and supported in the field by the National Science Foundation (NSF) , is interested in learning more about the conditions that allow these organisms to grow and flourish in their dark and cold ecosystem. In turn, that information should shed light on the behavior of similar organisms billions of years ago.
“There are only a few places in the world where you can go to find living examples of those earlier ecosystems,” Andersen stressed. “The lakes in the Dry Valleys actually provide a very nice window back in time to compare notes, so to speak, with the fossil record.”
“They were abundant until just before [oxygen] accumulated in the atmosphere, so these structures might be able to answer some important questions about the evolution of cyanobacteria,” explained Sumner, making her first trip to the Ice this austral summer, via e-mail. Cyanobacteria, previously known as blue-green algae, get their energy from photosynthesis. Scientists believe they were responsible for the geochemical transformation that eventually pumped up the oxygen content in the oceans and atmosphere.
“That set the stage for the evolution of larger, more complex organisms that we see today,” Andersen said.
Added Sumner, “Lake Joyce plays a very important role in these studies. Most other examples of these structures are found in very shallow water hot springs and in temperate lakes. … We want to use this very different environment to help sort out biological influences on morphology from environmental influences.”
In other words, it’s a bit of a nature versus nurture experiment. Sumner explained that cyanobacteria with a thread-like cell shape that glide along surfaces dominate some modern bacterial mats. That particular structure may play more of a role in the development of the communities than photosynthesis.
Lake Joyce, with an ice cover up to six meters thick, allows only about one-tenth of one percent of the surface light to fall upon the benthic microbial mats, according to Andersen.
“These guys are right at the limits of photosynthetic ability,” Andersen said. “They are highly adapted to these low-light environments and are quite capable of harvesting energy from the sun via photosynthesis.”
Sumner said it is possible that a light-limited environment like Lake Joyce will provide new insights into microbial mat development, “but we have to understand the Lake Joyce microbial behaviors very well before we can extrapolate too much to the [fossil] rock record.”
Andersen, Sumner and the rest of their team will spend nearly two months camped out at Lake Joyce in the Pearse Valley to understand more about those behaviors. Several team members, including Andersen, will actually scuba dive in the lake, using fluormeters and microelectrodes while underwater to detect, measure and quantify photosynthetic activity of the benthic mats and obtain samples for lab work on the surface.
“It’s a very cold, tedious process. It’s hard because one must remain very still, working very gently, with the only real movement being your fingertips,” said Andersen, who was among the first people to dive in the lakes back in the 1970s. At that time, he said, everyone assumed the lake bottoms would be rocky and lifeless.
“[We] discovered at Lake Hoare, on that first dive, that there were these lush, luxurious microbial mat ecosystems,” Andersen said. “Those earlier observations set the stage for the research that has taken place for the last 31 years, looking at these ecosystems in the context of life adapted to an extreme polar setting, relevant to understanding life on early Earth, and helping to guide the search for life elsewhere, particularly on the planet Mars.”
Sumner said the filament structure of the bacteria cells possess evolutionary advantages in water — for example, they’re more stable in turbulent flow — and that “one might expect evolution to produce this simple shape even if the biochemistry of the organisms was entirely different.
“The morphological properties we’re looking at are centimeter-scale and easily preserved in rocks,” she added. “If present, they could be easily identified in images from a [Mars] lander or rover.”
Andersen has made two previous trips to Lake Joyce, where he first discovered the carbonate structures growing from the microbial mat communities at about 20 meters depth. This will be the first extended study of the carbonate structures in a Dry Valleys lake, he said.
“The initial observations that we have are that the structures are pretty cool and there’s lots going on, but we don’t know much about them,” Andersen said.
“Each lake is totally different,” he added. “The external factors seem to be pushing the communities in different directions. That’s part of what makes these very unique ecosystems interesting to study — they’re essentially right next to one another and they’re each so very distinctive.”