Finally, after years of research, Boston and other scientists revealed that the microbes in Lechuguilla do much more than spit out a bit of soil. Lechuguilla is embedded in thick layers of limestone, the fossilized remains of a 250 million year old reef. The distribution chambers in such caves are usually formed by rainwater that seeps into the ground and gradually dissolves the limestone. In Lechuguilla, however, microbes are also the sculptors: Bacteria that eat buried oil reserves release hydrogen sulfide gas, which reacts with oxygen in the groundwater, creating sulfuric acid that cuts away limestone. At the same time, various microbes consume hydrogen sulfide and generate sulfuric acid as a byproduct. Similar processes occur in 5 to 10 percent of limestone caves worldwide.
Since Boston’s first descent into Lechuguilla, scientists around the world have discovered that microorganisms are transforming the Earth’s crust wherever they live. Alexis Templeton, a geomicrobiologist at the University of Colorado, Boulder, regularly visits a barren mountain valley in Oman where tectonic activity has pushed parts of the Earth’s mantle — the layer beneath the Earth’s crust — much closer to the surface. She and her colleagues drill boreholes up to a quarter mile into the uplifted mantle and extract long cylinders of 80-million-year-old rock, some of which are beautifully marbled in striking shades of maroon and green. In laboratory studies, Templeton has shown that these samples are teeming with bacteria, some of which are changing the composition of the Earth’s crust: they eat hydrogen and breathe in sulfates in the rocks, exhale hydrogen sulfide, and create new deposits of sulfide minerals that resemble pyrite, also known as fool’s gold.
Through related processes, microbes have helped form some of Earth’s reserves of gold, silver, iron, copper, lead and zinc, among other metals. As microbes beneath the surface break down rock, they often release the metals trapped within. Some of the chemicals microbes release, such as hydrogen sulfide, bind with free-floating metals to form new solid compounds. Other molecules produced by microbes grab soluble metals and bind them together. Some microbes store metal in their cells or grow a crust of microscopic metal flakes that continually attract more metal, potentially creating significant deposits over long periods of time.
Life, especially microbial life, has formed a large amount of minerals on Earth, which are naturally occurring inorganic solid compounds with highly organized atomic structures, or, to put it more plainly, very elegant rocks. Today, the Earth has more than 6,000 different mineral species, most of which are crystals such as diamond, quartz and graphite. However, in its early years, Earth did not have much mineral diversity. Over time, the continuous crumbling, melting and re-solidification of the planet’s early crust shifted and concentrated unusual elements. Life began to break down rock and recycle elements, creating entirely new chemical processes of mineralization. More than half of the planet’s minerals can only exist in a high-oxygen environment, which did not exist before microbes, algae and plants oxygenated the ocean and atmosphere.
Through the combination of tectonic activity and the ceaseless hustle and bustle of life, Earth developed a mineral repertoire unmatched by any other known planetary body. By comparison, the moon, Mercury and Mars are mineral-poor, with perhaps a few hundred mineral types at most. The variety of minerals on Earth depends not only on the existence of life, but also on its peculiarities. Robert Hazen, a mineralogist and astrobiologist at Carnegie Science, and the statistician Grethe Hystad have calculated that the chance of two planets having an identical set of mineral species is one in 10³²². Given that there are only an estimated 10²⁵ Earth-like planets in the cosmos, there is almost certainly no other planet with Earth’s exact complement of minerals. “The realization that Earth’s mineral evolution is so directly dependent on biological evolution is somewhat shocking,” Hazen writes in his book “Symphony in C.” “It represents a fundamental shift from the position of a few decades ago, when my mineralogy Ph.D. advisor told me, ‘Don’t take a biology course. You’ll never use it!’ ”
