Fig. 2. Schematic diagram of key factors in the search for MTB-like biosignatures.

The environmental context ought to be suitable for the growth of MTB-like life, including an appropriate magnetic field >~ 6 µT [66, 67, 95], nutrient enrichment, oxygen content, pH range, redox stratification, irradiation condition, and potential phage interaction. Some common biosignatures of MTB-like life include magnetosome morphology, magnetic properties, and chemistry. Morphological biosignatures include distinct crystal morphology (e.g., elongated hexagonal prismatic magnetite with faceted ends), particle elongation, and particle chains. Magnetic biosignatures include single-domain (SD) sized particles, remanence, coercivity, and anisotropy. Chemical biosignatures include intact crystal lattice that is relatively free of defects (though it may occasionally have [111] twinned crystals), chemically pure composition from selective transport of iron, trace elements that are selectively incorporated into magnetosomes, and isotopes of relevant elements in magnetosomes. Note that a good biosignature is something difficult to produce through inorganic processes; the MTB-produced magnetosome chain structures display the effect of Natural Selection for magnetotaxis, with each of the magnetic, morphological, and chemical features being driven by selection to maximize the efficiency of the cellular magnetotactic response. The central intersection of these factors is what makes magnetofossils a superb biomarker [6].