The ability of living organisms to form minerals is the fundamental tenet of biomineralogy. Among plants and animals, this process involves the production of cystolith inclusions in leaves and hard mineralized body parts like bones, teeth, and shells. This process, biological mineral precipitation, is not exclusive to higher eukaryotic organisms. Prokaryotic microorganisms, or bacteria, are remarkably potent agents of biomineralization, too. These small wonders manage to form an enormous variety of minerals¸ carbonates, phosphates, oxides, sulfides, and silicates as well as silver and gold.
Microbial biomineralogy has extremely broad and deep biogeochemical roots. Bacteria are the most abundant and metabolically diverse forms of life on Earth. They grow under a wide range of geochemical conditions in an unparalleled variety of habitats. Basically, microbial life exists wherever there is liquid water at temperatures from -7oC to about 120oC. Even the most extreme environments¸ from Antarctica to the ocean bottom and deep underground¸ play host to thriving microbial populations. Fossil and isotopic evidence also reveal that microbial life is as old as the rock records, stretching back at least 3.8 billion years.
How minerals develop
Microorganisms produce minerals in two distinct ways, passive growth and as a result of metabolic activity. The first process involves the nucleation and growth of crystals from an oversaturated solution on the outside surface of individual cells. This happens because the cell walls and external sheaths of bacterial cells have an abundance of chemically reactive sites that bind dissolved mineral-forming elements. When this adsorption occurs, the activation energy barrier that normally inhibits spontaneous nucleation and crystal growth is greatly reduced. Epicellular mineral precipitation follows, often leading to the complete encrustation of cells. Bacterial precipitation of amorphous silica in hot springs provide good examples of this type of microbial biomineralization. Some forms of authigenic iron oxides, phosphates, carbonates, and clays develop in the way.
Microbial mineral precipitation also results from metabolic activities of microorganisms. The process can occur inside or outside the cells, or even some distance away. Often, bacterial activity simply triggers a change in solution chemistry that leads to oversaturation and mineral precipitation. For example, the growth of photosynthetic cyanobacteria in natural alkaline waters tends to promote an increase in pH. This supports the precipitation of carbonate minerals like calcite and strontianite. Similarly, sulfide production of mackinawite, pyrite, and other sulfide minerals occurs, particularly in marine sediments where these bacteria flourish.
Other mineral phases precipitate directly from bacterial enzyme action. Enzymes are proteins that catalyze chemical reactions and drive cellular metabolism. For example, diverse forms of iron and manganese oxides are deposited by bacteria that actively oxidize soluble, reduced forms of the metals to generate energy for growth. The formation of tiny magnetite particles inside magneto-tactic bacteria, and the reductive precipitation of uraninite by some metal-reducing bacteria, are further examples of enzymatically formed minerals. Bacterial formation of metallic gold and silver might be related, too, as it must stem from some kind of reductive precipitation. But with these metals, it isn't clear if enzymes are involved.
Small grain sizes
Regardless of how they are formed, mineral precipitates produced by microorganisms usually have an extremely small grain size and often exist in a poorly ordered, near amorphous state. This may be related to high rates of nucleation and precipitation. In some cases, mineral precipitates are fine enough to preserve microbial cell structure. Silica is a good example, forming small crystallites that cause complete silicification of structurally intact cells. Paleont-ologists study the mineralization of modern microbial cells by silica to gain insight into how ancient microorganisms were preserved as fossils billions of years ago in silicified carbonates and cherts.
The small grain size of microbial mineral precipitates also confers high surface reactivity; these minerals act as secondary adsorbents of dissolved inorganic cations and anions, and possibly even organic compounds. Whether this process benefits or harms microorganisms is not yet known. But we do know that the chemical composition of natural bodies of water is strongly influenced by the adsorption of dissolved substances to suspended particulate materials. These particulates are often made of living and dead bacteria encrusted with fine-grain minerals, including manganese and iron oxides. The implication is that microbial mineral precipitation helps regulate the chemistry of aquatic systems.
To paraphrase Louis Pasteur, the great 19th century microbiologist: If microbial biomineralogy were a disease, then one could speak of epidemics of mineral formation!
On a global environmental scale, microbial biomineralogy plays a major role in the geochemical cycling of mineral-forming elements. The transfer of dissolved iron and sulfur into marine sediments, for example, is driven mainly by microbial pyrite precipitation. Microbiological precipitation of phosphate minerals also contributes to the incorporation of phosphorus into sediments, particularly in oceanic upwelling zones along the west coasts of South America and Africa. Even the chemical weathering of continental rocks and the atmosphere are influenced by microbial biomineralogy. In this instance, the precipitation of carbonate minerals by microorganisms is especially relevant because these minerals serve as a sink for atmospheric carbon dioxide and as end members in the weathering of silicate minerals, such as feldspars or olivines of igneous rocks.
Future technological uses
Synthesis of single-domain, nanometer-size magnetic particles by magnetotactic bacteria may prove useful in electronics and medicine. Similarly, researchers are studying microbial metal adsorption and mineral precipitation processes to develop passive clean-up procedures for waters contaminated by toxic metals from mine wastes and other industrial activities. It may even be possible to use microbial biomineralogy for the secondary recovery of precious metals, such as gold and silver, from dilute waste-water streams.
In the petroleum industry, microbial mineral precipitation can be used to control the invasion of water into oil reservoirs, thus extending production and enhancing oil recovery. This is done by employing natural or injected microorganisms to precipitate a cementing mineral phase, such as calcite, to plug porous high-permeability water-bearing zones. The same strategy might be used to prevent contaminants from polluting natural aquifers.
Not all microbial-biomineral interactions are beneficial. In humans, some microorganisms produce struvite kidney stones. And, surface fouling by iron oxide-precipitating bacteria can lead to severe operational problems inside water-cooling towers and heat exchangers. The same microorganisms can plug water wells with rusty slime. Sulfate-reducing bacteria are notorious for causing corrosion problems by producing iron sulfides, which attack oil and gas pipelines, as well as refinery storage tanks and steel-reinforcement bars within concrete.
Like it or not, we're surrounded by bacteria ... which makes microbial biomineralogy an extremely broad, dynamic, and ever-growing discipline. It's a good thing mineral formation isn't infectious. Or is it?
Grant F. Ferris is assistant professor at the University of Toronto, where his research is supported by the Natural Sciences and Engineering Research Council of Canada. He obtained his Ph.D. in microbiology from the University of Guelph in 1985. He was a postdoctoral fellow in geology at the University of Western Ontario, and a research scientist with NOVA Corporation of Alberta.