A couple weeks ago, I gave a presentation on microbialites for our lesson on extremophiles. I chose to talk about microbialites because I’d never heard of them before, and because there’s been a lot of exciting research going on over the past few years regarding the potential for microbialites to serve as potential biosignatures for life on Mars.
Basically, microbialites are nothing more than organic sedimentary mineral deposits that are covered by a thin layer of microbes that become entombed in the mounds as they grow outwards. The difference between stromatolites and microbialites is that stromatolites are the Earth’s oldest known structure and show no direct evidence of life, whereas microbialites are relatively young in geological terms, as they are only about 12,000 years old. The reason that scientists are getting so excited about them is because they serve as macroscopic evidence of microscopic life. Chris McKay, a scientist with the NASA Astrobiology Institute, regarded microbialites as:
“They are helping us understand one of the big astrobiology questions how early life took hold and began to flourish on Earth, These fossils are like seeing a billion-year-old footprint in the sand and comparing it to a modern foot.”
NASA scientists realized the importance of microbialites over a decade ago, and began studying the ones that are growing in Pavillion Lake in British Columbia in 1998. The microbialites were formed layer after layer with the oldest lying on the bottom, thus the structure provides a record of growth and also yields important clues about the organisms that once lived there. In 2008, NASA researchers reported that the microbialites in Pavillion Lake were composed of calcium carbonate, ranging in size from small bumps just a few centimeters across to enormous structures as large as 12 feet high. Additionally, they came in a variety of vegetable-shapes: some were described as resembling cauliflower, other like broccoli, other like asparagus, and some like carrots.
How these "veggie"-microbialites came to be placed in this lake isn’t completely known. As of last year, scientists are sticking with the hypothesis that bacteria were involved in some way in creating the large structures. However, it's possible that the only role bacteria had was in the formation of the crusts on the surfaces of the structures, but the structures themselves are the products of a purely chemical, rather than biological, process.
Some cyanobacteria excrete calcium carbonate, so one possibility is that the crusts are composed of this bacterial waste. Another is that the bacteria built up an electric charge along their cell walls, which attracted the calcium carbonate in the lake water. Or maybe the cyanobacteria secreted slime that the carbonates bound to.
In other places around the world, Cyanobacteria are famous for building a variety of structures, from thick rubbery mats to the layered dome-like structures we know as stromatolites.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. This is what creates the mystery.
So, researchers are approaching this topic from a variety of different angles. Some of them are looking at the chemistry of the lake water to understand it's topography and underground water sources; and others are doing DNA analysis of the slime that coats the microbialites to figure out which organisms are living there, what they're eating, and what they're excreting. Still other researchers are comparing the carbon in calcium-carbonate samples from the slime layer to carbon from the core of the structures to see if there is a clear biological mark in the core. The basis behind performing this investigation is that living organisms generally use the lighter C-12 isotope, leaving behind the heavier C-13 isotope.
So far the research has been largely inconclusive. The carbonate in the surface communities have a signature of biological activity, however deeper down, the signature doesn't appear to be preserved. This is contradictory to what you would expect since if the structures were built by microorganisms, there should at least be some isotopic evidence of their biological origin. Though, there is a possibility that over time, the structures have dissolved and re-crystallized, and in the process of doing this, changed from one form of calcium carbonate to another, thus losing their biological carbon-isotope signature.
If this avenue of research turns up supportive findings, the results could provide new insight into how biosignatures are modified and preserved over time, which could aid in future efforts to for biosignatures on Mars.