An astrobiologist considers the implications of microbes' mining abilities.

Microbes have been boring ever since life began on Earth: boring into rocks, that is. But why? Perhaps to avoid competitors or predators, to escape from environmental extremes or simply to secure a site safe from turbulent waters. Or might they be mining minerals for essential nutrients? Although the reason may vary depending on environment and host rock, a recent paper shows that some microbes tunnel towards a particular mineral, suggesting that nutrient mining may be occurring.

Tony Walton of the University of Kansas in Lawrence describes (and illustrates, gloriously) microscopic tubes in submarine glassy basalts from Hawai'i that show all the complex features of microbial borings (A. W. Walton Geobiology 6, 351–364; 2008). The boreholes converge on olivine microcrystals but avoid plagioclase like the plague.

Olivine incorporates trace metals such as nickel, copper and chromium, essential nutrients for many microbes because they form the reactive centres in metalloenzymes and cofactors that catalyse key steps in vital metabolic pathways. These metals are sensitive to levels of oxygen and sulphides, so their bioavailability may have changed as Earth's surface environment has become more oxygenated and, periodically, more or less sulphidic. So the microbes may be mining olivine for metals that are now or were once rare in solution.

Two implications arise. First, although hominids have shown an ability to recognize different rocks for almost a million years, this geological aptitude may be more widespread and more ancient among other organisms. And second, as olivine occurs in martian meteorites and on Mars' surface, perhaps future astrobiological space missions should be alert to the possibility that fossils of microbial miners may occur in subaqueously deposited basaltic sands on that planet.

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