Chomping on Nano-Nuggets
Eight years ago, features resembling bacteria and measuring 20 to 100 nanometers across were discovered in the Martian meteorite ALH84001. NASA scientists interpreted these features to be the fossilized remnants of ancient life, but many scientists rejected that conclusion.
|Martian meteorite ALH84001
A nanometer is one millionth of a millimeter. The period at the end of this sentence is about one million nanometers long. The tiniest bacteria measure about 200 nanometers in size, and many believe that life can’t get much smaller than that.
A committee formed under the auspices of the US National Academy of Sciences determined that, due to the size requirements of such vital elements as enzymes and genetic material, organisms smaller than 200 to 300 nanometers in diameter could not be self-sustaining and therefore could not be considered to be "life."
Others contend that life can be that small, and as proof they claim to have grown nanobacteria in the laboratory. In addition to the nanobacteria in the Martian meteorite, spheroidal features measuring 50 to 200 nanometers have been found in sedimentary rocks on Earth. Some claim that these spheroids are the fossilized remains of once living nanobacteria.
The new study, conducted by Jürgen Schieber of Indiana University in Bloomington and Howard Arnott of the University of Texas at Arlington, suggests an alternative explanation for nanometer-sized features. The scientists report that protein balls measuring 40 to 120 nanometers across are produced when bacterial enzymes cause organic material to decay.
Schieber and Arnott dipped tiny pieces of bean, squid and beef into the muck from a pond, to ensure that the samples became coated with the full spectrum of naturally occurring decay bacteria. The samples were then buried in clay to simulate the burial of organic matter in sedimentary rock.
|Close-up of a Mars meteorite, showing what some have argued appears to be fossilized evidence of ancient microbial life.
Image Credit: NASA
Over the next two weeks, the researchers found the tissue samples experienced "explosive" bacterial growth, and balls measuring 40 to 120 nanometers in size were widespread. The scientists say that these "nannoballs" compare well with published examples of nanobacteria.
"Because gradual decay of tissues always led to formation of nannoballs, we surmised that the latter resulted when microbial enzymes interacted with the buried samples," the scientists write. The scientists also exposed tissues to various purified protein-degrading enzymes in separate experiments, and this confirmed that such enzymes were responsible for the nannoballs.
The enzymes snip the larger tissue elements like cell walls and muscle fibers into nanometer-sized units. Once snipped, the tissues contract into balls due to elastic forces. This enzymatic breakdown of organic matter may act as an aid to decomposition, the scientists suggest, reducing material to bite-sized nuggets for bacteria to ingest.
"Bacteria are osmotrophs and can only take in dissolved molecules liberated by exoenzymes utilized outside of the cell," write the scientists. "Seeing no subunits smaller than our nannoballs, we assume that in the subsequent degradation step, the nannoballs are broken down by further enzyme action into soluble molecules that can be ingested by bacteria."
Nannoballs are not always consumed by bacteria, say the scientists, because under certain conditions the tissues can become mineralized. This mineralization preserves the nannoballs, turning them into fossils in just a few weeks.
Although the nannoballs are not fossilized life forms, they can act as "biomarker" evidence for bacterial life.
Credit: University of Minnesota
"Most if not all alleged nannobacterial structures in sedimentary rocks are probably by-products of bacterial degradation of organic matter and not evidence for minute life forms called nannobacteria," the scientists conclude. "Nonetheless, mineralized nannoballs may indicate bacterial enzyme action on organic tissues and serve as a visual proxy for microbial activity."
Kathie Thomas-Keprta, an astrobiologist with Lockheed Martin at NASA’s Johnson Space Center, has studied the magnetite and carbonate mineralogy of the martian meteorite ALH84001. She says that if microbes on Earth produce nannoballs as they degrade certain minerals, as they do with the tissues in this new study, then the nannoball-like texture observed on the surface of carbonate globules in ALH84001 may be a product of such microbial etching.
However, she says it’s still possible that the features in ALH84001 are the fossilized remains of microbial life. Part of the problem with the debate over the size constraints of life, says Thomas-Keprta, is that microbes can shrink substantially after death.
"The size of a viable organism may be vastly different from the size of that organism when fossilized or mineralized," she states. "We do not understand how the size of organisms changes with fossilization or mineralization, nor do we know if particular categories of organisms can be better preserved than others."
While the physical shape and size, or morphology, of a structure is not enough to determine whether it was once a living microorganism, certain surface textures might be evidence of past biological activity. A granular surface texture composed of nannoballs, in conjunction with other biomarkers, may provide further evidence of past microbial life.
Schieber, J. & Arnott, H. J. Nanobacteria as a by-product of enzyme-driven tissue decay. Geology, 31, 717 – 720, (2003).