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Rock Not Always a Hard Place

Many of us grew up playing Twenty Questions. Is it animal, vegetable, or mineral? Now, a youngster is supposed to ask: is it bacteria, protoctist, fungus, plant, animal, or mineral? Furthermore, is it an old-style mineral (a mineral produced by chemical and physical processes alone) or a biomineral fabricated in living cells? Distinguishing "ordinary minerals" from products made by members of life's kingdoms has become curiously difficult.

Over time, more and more inert matter has literally come to life, returned to rock, and come back through life once again. Vladimir I. Vernadsky (1863–1945), the great Russian scientist who founded biogeochemistry, asserted that life is the greatest geological force—it both makes and dissolves minerals. Living creatures incorporate mineral elements from the sea into their bodies as protection or support, in the form of shells and bones. Our own skeletons form from calcium phosphate, a sea salt that was initially a nuisance or a hazard to our remote ancestors. Ancient marine protists eventually found ways to cleanse their tissues by putting these minerals to use.

Microbial biomineralization—the bioengineering and manufacture of hard stuff with internal crystal structures by bacteria and protoctists—has altered all Earth scientists' view of planetary cycling and evolution. Certain huge geologic features owe their existence to living organisms. Photosynthetic microbial communities, for instance, created (and continue to create) the extensive deposits called stromatolites. These reefs of layered carbonate rocks trap, bind, and precipitate sediment particles. In addition, the layered iron ore deposits from which we manufacture our steel and cars formed when the iron in solution was concentrated by bacteria and oxidized by other bacteria over a period of 3,000 million summers. The planetary distribution of many other Earth minerals, including lead, zinc, silver, and gold, has been rearranged by bacterial activity. Professor Betsey Dyer of Wheaton College, Massachusetts, describes the history of the world's biggest deposit of gold in South Africa's Witwatersrand:

Gold, weathered from the Barberton Mountain area, washed down a huge paleoriver system into a gigantic delta area loaded with photosynthetic and other bacteria. This occurred 2–2.5 billion years ago, at a time when there was very little oxygen in our atmosphere. Some gold was probably particulate, and would have been physically trapped in the delta's oxygen-producing cyanobacterial filaments. Other gold was either dissolved or in suspension. The relative lack of oxygen would have encouraged the production of gold-sulfur compounds and other dissolvable and transportable forms of gold. At the delta, cyanobacteria would be producing oxygen (a product of photosynthesis) and the sudden increase in oxygen would cause all that gold to come out or precipitate. Something like this happens with iron too. Thus oxygen-generating photosynthesizers, at one time exclusively bacterial, gave us some of our major metal deposits.

Some of the beauty that opal has brought to the planet depends on biomineralization by diatoms. These ubiquitous protists, today 10,000-species strong, construct elaborate mineralized shells, or "valves," of silica. Some of the shimmer and sparkle of opal, mined and traded by Homo Sapiens , is due to fossil diatom valves. Often opaline spheres have diameters precisely the size of the wavelength of visible light. Microbial mineral-works are a crucial part of the story of how humans came to harvest iron, lead, gold, and opal.

Bacterial Bar Magnets

In the 1960s Cal Tech's Heinz Lowenstam (1912–1993) claimed that sedimentary magnetite was made inside living cells. His work was ignored. At that time, other scientists admitted calcium carbonate in seashells and silica in sponge spicules as of undoubted biological origin. Then, in 1975, it was discovered that the navigation of certain bacteria swimming in a Massachusetts marsh oriented to the geomagnetic field of the Earth, not to food. The bacteria, in fact, precisely engineer tiny magnetosomes, perfect wee chains of ferrimagnetic crystals which they use not only to discern north from south, but to distinguish up from down. "Down" leads them to sediments rich in decaying matter.

Many scientists now agree with Lowenstam's hunch that virtually all sedimentary magnetite derives from a bacterial invention. For more than 2,000 million years, "magnetotactic" bacteria have sought sediments in oxygen-poor sea- and lake-bottoms using these nano-compasses.

Sockeye salmon and many other fish, honeybees, green turtles, and various birds (including pigeons) all have internal bar magnets. How much magnetite is sequestered from the food chain and how much modified or manufactured within each animal's tissues remains one of the great biomineralization mysteries.

Today, more than sixty different inorganic crystals are recognized as products of life, and the count grows.

Thinking Locally, Acting Globally

Great groups within the kingdom Protoctista (algae, slime molds, water molds, ciliates, and the like) have shown that microbial biomineralization is a planetary phenomenon, not limited to local habitat. The baroque buttons of the coccolith-makers and the multi-chambered tests (shells) of foraminifera (see illustrations, page 68) serve as a major reservoir for calcium carbonate in the Earth's ocean. Coccolithophorids (coccolith-bearing algae) also trap carbon dioxide as they photosynthesize and manufacture the calcium carbonate "buttons"; concomitantly they lower the planet's concentration of this greenhouse gas. The mean annual surface temperature of Earth, with an atmosphere 0.03 percent CO2, is 17° C. Without biomodulation of temperature, Earth's atmosphere would be far more like that of Venus (98 percent CO2), whose mean surface temperature is 482° C. Vast blooms of coccolithophorids help regulate climate through modulation of ocean cloud-cover. For instance, they generate "condensation particles" around which the water droplets of clouds form.

The gas emissions of Emiliania huxleyi and its many relatives are equally potent buffers of our atmosphere. They emit sulfur-containing gas that wafts into the atmosphere and is transformed by solar radiation to sulfuric acid. The acid droplets serve as nucleation sites for water condensation and the formation of ocean cloud cover. They indirectly help shade the planet's surface and, most importantly, help generate rain over the ocean.

Finally, coccoliths play a critical role in the planetary biogeochemical cycling of nitrogen and phosphorus.

The area of coccolith-covered seafloor is greater than the area of all the continents combined. Fix upon the great pyramids of Egypt or the white cliffs of Dover; you are gazing at the resting place of multitudes of once-living foraminifera or coccolithophorids.

"Biodemineralization"

In a recent discussion, James Lovelock honed his concept of Gaia:

Gaia is an evolutionary system made up of all the surface inorganic components of the Earth—the rocks, the atmosphere, the oceans—and all of the living organisms of the Earth. The two parts are tightly coupled together....Biology is controlling things, but it controls it subtly, by regulating the rate of rock weathering.

Today's rock weathering and erosion rates are increased 1,000 times from what they would be on a lifeless planet, because of the relentless activity of living beings. The low-lying crustose and other lichens, for instance, palpably alter the planet's surface. Lichen fungi produce fourteen different kinds of acids which eat at the rock face and greatly accelerate the rates and extent of rock weathering and the release of "plant food"—ingestible mineral compounds and elements. Dissolved in the water, such elements are ingested and become parts of microbes, plants, and animals which, in turn, precipitate and release them. These life-providing biochemical actions help regulate climate.

The Planetary Record

Bacterial proficiency followed by protoctist and animal expertise in mineral making have led to a multitude of hard parts and masterpieces in the fossil record: magnetite teeth of chiton mollusks, silica phytoliths in green plants, calcium phosphate shells of brachiopods, calcium oxalate "kidney stones" (which are normal deposits in tunicates), silica filigree baskets of chrysophyte algae, and hardened cell walls in red seaweed. The reefs of reef-building clams and green and red seaweeds, as well as the lepidocrocite (iron mineral) and apatite (calcium phosphorus mineral) of hard teeth, are but a few of the treasures first recognized by Heinz Lowenstam and his young Israeli colleague Stephen Weiner.

In the great Homo sapiens exploitation tradition, humans use technology to accelerate evolution's biomineralization. Biomimetics, a growing interdisciplinary field, attempts to understand and to imitate natural processes and to "microbufacture" little chain gangs of bacteria to do the work themselves. Mining engineers in the former Soviet Union added select communities of microorganisms into old gold mines to coax them to concentrate gold too diffuse to mine by usual means. Some researchers laud the potential commercial value of nanometer-size magnetic particles synthesized for electronic and medical uses. Others study microbial metal adsorption and mineral precipitation to enhance the rate of toxic cleanup.

In short, living creatures produce biominerals inside or induce biomineralization outside the cells of their bodies. These biominerals, in turn, travel in sediment, deposit, and lithify to become part of distinct geological formations that cover wide areas of the planet. By regulation, in part, of the construction and deconstruction of mineral complexes, living creatures modify climate and generate the richness of local soils. Over three billion years of microbial action led to the possibility of iron, gold, and lead mining, which deflected the course of human history.