Magnetite Crystal Chains: Most Promising Evidence of Past Microbial Life on Mars, Also Useful on Earth

It is not easy to identify "biosignatures", suitable targets in the search for traces of microbial life on early Mars: They need to have withstood billions of years of extreme conditions and to provide unambiguous evidence. Organic molecules were probably destroyed by the highly oxidative environment, at least near the surface. Mineral structures offer hardly more than strong indication, not unambiguous evidence, and no direct information about the organisms that may have produced them. The reason is that soft-bodied bacteria do not leave behind good fossil traces. The only known exception seems to be magnetotactic bacteria, widespread on Earth in most aquatic environments, that produce inside their body chains of magnetite crystals. Single crystals showing properties observed in terrestrial bacteria (definite size range, morphology, chemical purity, rarity of crystallographic defects) may indicate biological origin, but provide no absolute evidence: It is impossible to prove that such crystals can not be produced by non-biological processes on Mars. However, magnetite crystal chains of bacteria have properties that are the direct consequence of being produced by organisms. In the living cell, they are surrounded by a biological membrane or organic matrix. Even in terrestrial fossils, crystals of a single chain show high level of uniformity in size and shape (because the matrix functions as a template), gaps between crystals (organic substance between crystals) and remarkable bends (due to the elasticity of the organic matter between crystals, a property absent in the mineral world). Magnetite chains of bacteria are complex structures whose formation requires the interaction of a series of chemical and physical processes and feedback loops, as well as genetically stored information, the ultimate proof of life. Non-biogenous chains produced in the laboratory do not show such characteristics. Bacterial magnetite chains in terrestrial magnetofossils can be observed in situ by backscattered scanning electron microscopy. While this method permits to locate such chains in sediments, it yields rather fuzzy images. Single chains excised by focused ion beam (FIB) milling instrument can be used to produce images by more advanced methods such as HAADF electron tomography. We suggest that fossil lakebeds on Mars (e.g. Gusev Crater) would be promising targets to search for fossil chains of magnetotactic bacteria.