Fossilized Ancient Microbes

The images of putative fossilized nano-bacteria inside ALH84001 were perhaps more appealing to the public and the press, but turned into a major source of controversy within the scientific community. McKay et al. (1996) reported ovoid and elongated forms ranging in size from 20 to 100 nm in longest dimension (Fig. 1). These forms were similar to terrestrial nanobacteria and fossilized filamentous bacteria found in calcite concretions, travertine and limestone (Folk, 1992, 1993), a very suggestive argument in favor of their biotic origin. Using control samples the authors ruled out any possible artifacts associated to sample preparation, and the same analyses conducted in other meteorites recovered from Allan Hills did not show any of the structures, which ruled out a possible terrestrial origin. Bradley et al. (1997) found that similar bio-morphs were lamellar growth steps on pyroxene and carbonate crystals, and their segmented surface merely artifacts due to sample preparation. Accepting that argument, McKay et al. (1997) replied that it did not apply to the entire suit of bio-morphs in ALH84001, and some of them could still be remnants of Martian microorganisms.

But it was the size of these bio-morphs, 100 times smaller than the smallest known organism on Earth, what made the whole argument unconvincing for many. Soon the debate turned into the minimum size requirements for life, an issue that had not received much attention prior to 1996. The main critics of the biogenic hypothesis argued that something that small could not contain all the molecules necessary for the basic cellular activity. Before ALH84001, the concept of nanobac-teria was barely accepted in the scientific community. But the images presented by McKay et al. (1996) stirred a debate that lead into the meeting of a National Academy of Sciences (NAS) panel of experts in microbiology to discuss the size limits of life. Before the panel of experts released their conclusions, a number of publications related to nanobacteria had already seen the light. In 1998, Uwins et al. reported detection of living colonies of nano-organisms on Triassic and Jurassic sandstones and other substrates. These nanobes have cellular structures similar to Anctinomycetes and fungi, but their diameters range from 20 to 100 nm, and are composed of C, O and N. Ultrathin sections revealed membrane-like structures and different staining techniques indicated the presence of DNA. Kajander et al. (1998) claimed to having isolated nanobacteria from blood and blood products. The authors observed growth in culture plates seeded with samples that had been filtered through 0.07 ^m pores, and estimated a lower size limit for the nanobacteria of 80 nm in diameter. Kajander and Ciftgioglu (1998) also reported the culture of nanobacteria and the partial characterization of a nanobacterial ribosomal RNA. These results were later debated by Cisar et al. (2000) who argued that the observed bacterial growth, were in fact inorganic precipitates, and that the isolated RNA was likely a contaminant from the reagents used in the experiments.

Taking into account these precedents, and considering theoretical constrains about the minimum amount of biomolecules required for the basic living processes, the consensus of the panel of experts dictated that a sphere of 200 nm in diameter was the minimum volume required for a single-cell organism. With its 20-100 nm proportions, the putative bio-morphs in ALH84001, and some of the reported terrestrial nanobacteria, didn't seem to make the cut. However, new discoveries seem to call for a redefinition of the theoretical size limits of life.

Baker et al. (2006) extracted nanoarchea ranging in length from 193 to 299 nm, from naturally occurring biofilms in an acidic environment. These sizes fall dangerously close (and some below) the theoretical limit of 200 nm. The conservative estimates of the size limit of life may not be conservative anymore, and as new discoveries continue to lower this limit, the concept of Martian nanobacteria is likely to gain new thrust.

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