THE RELEVANCE OF BACTERIAL BIOMARKERS IN ASTROBIOLOGICAL RESEARCH

摘要

"The need to be able to recognize the signature of life in rocks from Earth's fossil record as well as in extraterrestrial materials" is stated as goal 7 in the NASA Astrobiology Road Map. The difficulties associated with this effort has lately become prominent due to scientific controversies over the earliest evidence of life on Earth and the continuing discussions over possible evidence of biogenic activities in Martian meteorite ALH84001. The common denominator these debates ground on is the underlying difficulty or inability to demonstrate conclusively the biogenicity of the respective evidence, which in either of the above cases would have to be seen in evidence of fossil microbial life. A consensus that has emerged from these discussions, and is now seen as a critical requirement, is the demand for molecular evidence in addition to the morphological evidence to support those extraordinary claims. Efforts are being made in order to fulfill the requirement to combine morphological microbial fossils with proof of their molecular remains. Carbon isotopes have successfully been correlated with individual Proterozoic microfossils and FT-RAMAN spectra were obtained on supposed early Archaean microfossils, although this evidence is far from being conclusive. Notably, Toporski et al. [12] demonstrated the possibility to correlate more comprehensive molecular information with fossil bacterial biofilms using Time of Flight -Secondary Ion Mass Spectroscopy (ToF-SIMS), a technique that has been shown to be capable of detecting unambiguous biomarkers of the hopane class, in complex organic mixtures. These examples show that by means of a multi-disciplinary scientific approach, the questions raised above find themselves on the verge of being answered conclusively, although clearly further work is needed. However, they also emphasize the relevance of molecular bacterial biomarkers in the search for traces of bacterial activity in rocks, in order to fulfill goal 7 of the NASA Astrobiology Road Map. Biomarkers are defined as organic molecules found in the fossil record with their precursor compounds being produced by living systems; bacterial biomarkers thus are organic molecules specifically derived from biomolecules produced by prokaryotic organisms, including bacteria and archaea. Both groups represents groups of candidates possibly involved in the origin of life on Earth or elsewhere. If life once existed on an extraterrestrial planetary body and it indeed followed principles employed by terrestrial life, then one must assume that it is most likely represented by single-celled prokaryotic microorganisms. Evidence of life on Mars or Jupiter's moon Europa may still be present in the form of molecular biomarkers and could be targeted for in solar system exploration. Steele et al. (this volume) proposed new strategies for solar system exploration involving biotechnology and specific immune reactions to detect the presence of organic compounds in extraterrestrial deposits, including bacterial biomarkers as well as prebiotic molecules. Comprehensive .knowledge on bacterial biomarkers thus has great relevance in astrobiological research, particularly with respect to the search for evidence of life in rocks of terrestrial and extraterrestrial origin. A review on bacterial biomarkers can be found in Toporski and Steele (in review GCA).