The Houses that Microbes Build
Of the various rock structures built by microbes, stromatolites may be the most famous. These gorgeous layered formations are found in shallow bodies of water. They typically grow upward in the shape of domes, columns, or cones that can reach meters in height and thickness. As the oldest evidence for life in the fossil record, they provide insights into the early evolution of life on Earth, and serve as potential "biosignatures" when looking for life elsewhere.
But stromatolites also have a lesser-known relative, a less flamboyant two-dimensional counterpart–which may be much more widespread. In many cases, the microbal communities don’t grow upward as columns or mounds, but instead build a flat deposit that can cover an area from a few millimeters to many kilometers.
"These planar structures occur everywhere–in lakes, rivers, and marine environments," says Nora Noffke, a biogeologist at Old Dominion University who has studied them for many years. "They’re very common, and have also been widely distributed throughout the earth’s entire history. But because they’re often buried in the sediments, you really have to know what to look for in order to see them–they’re not easy to detect."
Noffke recently co-authored a paper with Stan Awramik, a biogeologist at the University of California, Santa Barbara, who is a world-leading expert on stromatolites. The manuscript, published this month in a journal of the Geological Society of America, describes how each structure forms, highlighting their differences and similarities.
The name stromatolites comes from the Greek and means "layers of rock," or "beds of rock." The planar structures are called MISS, for "Microbially Induced Sedimentary Structures."
A Closer Look
Stromatolites and MISS are build-ups left behind by microbial mats–complex ecosystems of highly integrated microbial communities. As Noffke explains, these carpets of microbes–which someone could literally peel off, roll up, and carry away–actively rearrange and bind loose sediments, leaving these distinctive rock structures behind.
"They’re biological constructions that serve the purpose to stabilize the sediment, and to make it habitable for the microorganisms," says Noffke. "When they form a mat community, they work together to build these constructions and protect themselves against erosion by water currents and against the deposition of sediments."
The difference between stromatolites and MISS are likely due to the interaction of many factors, explain Noffke and Awramik, from the biological composition of the mat, to the gain size and microtexture of the sediments.
A striking feature of stromatolites, however, seems to be the precipitation of the mineral carbonate in the slimy substance that bind the microbal community together. With MISS, that carbonate production just doesn’t occur, or is very rare.
Stromatolites form mainly in environments that are rich in calcium and carbonate, whereas MISS mostly occurs in sandy coastal environments. But Noffke and Awramik speculate that the lack of carbonate precipitation in MISS may also involve genetic factors.
Noffke has studied MISS in many sites around the globe, ranging from modern day MISS to ones dating back to the Archean Eon, the time interval when life first appeared on Earth more than 3.5 billion years ago.
Implications for Mars
If these fossilized structures provide reliable evidence for the presence of life on early Earth, they can also be added to our catalogue of potential biosignatures when searching for life elsewhere, especially on Mars.
"Interpretation of relatively subtle textures (such as some of the MISS examples) is extremely important as we inch our way forward in understanding the complexities of the Mars environment in the past," says Penelope Boston, a geomicrobiologist at the New Mexico Institute of Mining and Technology. "While microorganisms individually are currently beyond our capabilities to see on missions like the Mars Science Laboratory Mission (MSL), macroscopically visible "field marks" like the MISS structures are very promising,"
"For now, we’re getting a first impression of whether there were habitable environments on early Mars, and if so, whether they were the kinds of environments conducive to biosignature preservation," explains Jack Farmer, a professor of geobiology at Arizona State University and a participating scientist on the Mars Exploration Rover mission.
"But we have to remain open to the fact that things could be different on Mars," he adds. "While a catalogue of biosignature is useful, that’s not the end of it. We’re using knowledge from orbit and on the ground observations to find our way to the right places to explore. But we’ll also need to use the right exploratory tool to bring samples back to Earth, where they can be scrutinized in terrestrial labs. That’s where we’re headed, and definitive answers about past Martian life might come within another decade."