Reading Archaean Biosignatures
Using a new instrument that can locate elements on the nanometer scale, NASA scientists are exploring tiny bits of organic matter that could be the oldest traces of terrestrial life. Possible “biosignatures” have been found in rocks dating back 3.3 to 3.5 billion years, long after deformation by heat and pressure would have obliterated any whole-cell fossils these rocks may once have contained. These biosignatures would be embodied in suggestive concentrations of elements, like carbon and nitrogen, that are associated with life, and in the ratios of specific isotopes.
In an effort to recognize early biosignatures with greater precision, Dorothy Oehler and Everett Gibson of Johnson Space Center have begun using a new, high-precision instrument called NanoSIMS (for nanoscale secondary ionization mass spectrometry) that can locate particular elements associated with life to within about 50 nanometers. NanoSIMS is a fine-scale elaboration on SIMS, also called an “ion probe,” which spews a beam of cesium ions that releases ions from the sample surface. Individual detectors on a NanoSIMS instrument are tuned to pick up particular ions, including carbon, nitrogen, oxygen and sulfur.
While an electron microscope makes images that show the location of structures, NanoSIMS makes “maps” showing the location and number of particular chemical elements that may comprise these structures. By analogy, while an electron microscope might be able to see a brick wall, NanoSIMS could see some of the brick in the wall.
“What NanoSIMS brings us is a new tool in our arsenal of weapons,” says Gibson, who has been involved in numerous studies of ancient biosignatures, including the Allan Hills martian meteorite, a 4.5-billion-year-old rock containing 3.9-billion-year-old carbonates that, he thinks, may carry a credible biosignature. “For the first time, we have the ability to get in-situ carbon and nitrogen compositions, and potentially in-situ isotopic compositions of fossil cell walls.” Isotope measurements on the nano scale are possible, but remain problematic, adds Oehler, who brings to the task long experience analyzing organic compounds for the oil industry.
The earliest putative biosignatures are difficult to interpret and often controversial, due to the generally poor level of preservation of organic material in some of Earth’s oldest sediments, but studies of younger rocks have confirmed that NanoSIMS can confirm biogenic structures by pinpointing the location of biologically important atoms like carbon and nitrogen. Oehler’s first NanoSIMS studies focused on chert, a fine-grained sedimentary rock, because, as the rock forms from solution, microbial cells in the solution fill with dissolved silica. Eventually this silica crystallizes, but when it does, it preserves morphological and chemical indicators of those cells.
Oehler says cherts from the Gunflint Formation in Ontario, Canada, contain “ cells, beautifully preserved” after 2 billion years, which can be seen with optical microscopy and have been analyzed by NanoSIMS performed by Francois Robert, at the National Museum of Natural History in Paris.
Matters are murkier in rocks dated to 3 billion years or older that contain organic compounds (molecules containing carbon and hydrogen), due to long deformation and heating. “As we go back to older and older rocks, the chance of good preservation gets more and more remote,” says Oehler.
Cell walls are key to these analyses, Oehler says. “One very simple, basic thing that characterizes all living things is packaging — a cell wall or membrane that keeps things in and out. Cells have to interact with the environment, have to metabolize, change the chemistry, control the osmotic balance, and packaging allows this. If we see a remnant of a little bit of structure, even a fragment of a wall, that would give us some confidence that we are looking at biological material.”
While electron microscopes produce an image, the output of NanoSIMS is more akin to a map that shows the presence of particular elements. In biosignature work, a NanoSIMS map shows elements that are important for life, including nitrogen and carbon. “Amino acids and proteins have a lot of nitrogen, and cell walls are made of amino acids,” says Oehler. “If the nitrogen map looks virtually identical to the carbon map, that likely indicates a cell wall or membrane. We don’t see as much nitrogen in organic carbon that has an abiogenic origin.”
To confirm that NanoSIMS can detect cell fossils, Oehler began by analyzing chert, a rock that faithfully records the cell structure of ancient microbes that lived in the water when the chert solidified. Oehler performed her first examination on an approximately one billion year old Precambrian chert from Bitter Springs Formation, Australia, that she had collected decades earlier for her Ph.D. thesis. “The Bitter Springs is famous for a microbiota containing between 20 and 50 species and it contains all sorts of things with clearly biological morphology, such as dividing cells, colonies, and long filamentous chains of cells.” she says. “I had a thin section from the Bitter Springs Formation hanging around, and I used it to show what real microfossils would look like” with NanoSIMS.
The cells are visible using both optical and electron microscopy and in NanoSIMS element maps, Oehler says. “We can see the cell walls and sheaths and their carbon-to-nitrogen element ratios.”
At the 2008 Lunar and Planetary Science conference, Oehler and Gibson reported that chert recovered from Australia, and dated to about 3 billion years, showed carbon and nitrogen distributions similar to what they saw in younger non-controversial microfossils, and thus was probably biogenic. In contrast, NanoSIMS analysis of non-biological organic matter revealed a much lower ratio of nitrogen to carbon, without the wall structure seen in the younger Australian microfossils.
Supported by a new NASA Exobiology grant, Oehler and Gibson are expanding this line of research to further comparisons of samples whose origin is known to be biogenic and those whose origin is unknown. “We are going to start with what we know, and step back in time,” says Oehler.
NanoSIMS “is a particularly promising instrument for assessing the biological origin of cell fragments in ancient rocks,” says Sherry Cady, an associate professor of geology at Portland State University, who studies the interactions of microorganisms and their environment. “The potential to locate organic remains typical of cellular remnants, characterize within them ratios of biochemically important elements, and relate such features to the biology and sedimentology of the host rock could provide the type of compelling evidence for early life we’ve been seeking in the ancient rock record."
If NanoSIMS continues to prove its mettle in locating fragments of cell structures, it may be applied to organic samples from meteorites, and eventually to samples recovered during space missions, says Gibson. “If we can demonstrate this works on well-known, well-characterized material that is recognized as biogenic by the geological community, we may have another analytical tool to use in assessing the origin of organic matter extraterrestrial samples when we get them.”