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Minerals, molecules, and microbes

The study of minerals is the most fundamental aspect of the Earth and environmental sciences. Minerals existed long before any forms of life. They have played an important role in the origin and evolution of life and interact with biological systems in ways we are only now beginning to understand.

One of the most rapidly developing areas in what is now called ‘geobiology’ concerns the role of microbes in processes both of mineral formation and destruction. For example, the ‘geobacter’ bacteria, shown in the accompanying picture taken in an electron microscope, are not just sitting on an iron oxide mineral surface but interacting with it because it is their method of ‘respiration’ (just as breathing oxygen is ours).

A transfer of electrons between the microbe and the mineral in this case brings about a change in the chemical state of the iron (its ‘reduction’) which also causes the mineral to dissolve. Interactions of this type are now known to play important roles in the release and movement of metals and other elements, including pollutants such as arsenic, at the Earth’s surface.

A very different story linking minerals and the living world concerns the ways that many organisms form minerals to fulfil a particular function, such as providing an external skeleton (shell).

For example, the chalk rock responsible for the ‘White Cliffs of Dover’ in the south of England are almost entirely composed of the remains of microscopic plates of calcite derived from a protective armour around unicellular planktonic algae (‘coccolithophorids’). In many cases the products of such processes of ‘biomineralisation’ are delicate structures of great beauty.

A good example is provided by the (as illustrated) ‘radiolaria’, free-floating single celled organisms found in the upper regions of the water column in the oceans, and  which have skeletons of poorly crystalline (‘opaline’) silica.

Growth of Geobacter sulfurreducens on Poorly Crystalline Fe (III) Oxyhydroxide Coatings, used with permission from David Vaughan.
Growth of Geobacter sulfurreducens on Poorly Crystalline Fe (III) Oxyhydroxide Coatings. Used with permission from David Vaughan.

Amongst the most remarkable examples of organisms producing a mineral to serve a specific function are the ‘magnetotactic bacteria’. Here the bacterium concerned produces a chain of perfect crystals (see illustration of magnetite crystals), most commonly of magnetite, which make use of the magnetic properties of that mineral. It seems that these organisms use magnetite to become aligned in relation to the Earth’s magnetic field and therefore in the most advantageous position in relation to the sediment-water interface.

One of the most challenging questions in all of science is: ‘How did life on Earth originate’? It is now widely believed that minerals played a key role as catalysts for biochemical reactions and templates for the emergence of the complex biomolecules needed for life. Many different routes have been proposed for the emergence of the first living organisms, almost all have major roles for minerals. These roles may have been in providing catalysts through biomolecule sized cavities in their crystal structures or weathered surfaces. Other routes involve clay minerals as substrates aiding in the formation of the first self-replicating genetic molecules, or look to the environments at, or near, mid-ocean ridges where hot fluids emerge releasing a stream of metal sulphide mineral particles. At the present day, both micro- and macro-organisms utilise chemical energy available in these environments for their metabolisms. Iron sulphide minerals are suggested as the key catalysts in these models.

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A double chain of magnetite crystals in a magnetotactic bacterium. Courtesy of the Mineralogical Society of America. Used with permission.

There are challenging questions in all of these areas, whether it be understanding the electron transfer processes involved when bacteria interact with minerals, the mechanisms involved in biomineral formation, or the complex roles probably played by minerals in the emergence of life on Earth.

In these and many other cases, it is the processes at mineral surfaces which are critically important. Only in recent years has it been possible to study mineral surfaces at a molecular scale. Today, we are at the threshold of a new understanding of the processes taking place at the surface of the Earth which integrates the mineralogical, geochemical and biological realms at the molecular scale. Understanding what happens at surfaces and interfaces at scales from global to molecular is key to that understanding. Here, the emergent field of ‘molecular environmental science’ should provide new insights into the way our planet ‘works’ comparable to the revolutionary advances seen in human biology associated with the genetic code.

Featured image credit: Didimocrytus tetrathalamus, by Tim Evanson. CC-BY-SA-2.0 via Wikimedia Commons.

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