The sea floor is strewn with raw materials that could be very important in the future: Manganese and iron, but also rarer and more precious elements such as cobalt, copper, zinc and nickel, are present in great quantities in the form of deep-sea nodules and crusts. The depositions of such materials from seawater and sediment is the result of a process known as biomineralization.
Microorganisms such as bacteria and algae contribute to this process of nodule and crust accretion and catalyze the accumulation of metals, as has been shown by new research at the Institute of Physiological Chemistry and Pathobiochemistry at Johannes Gutenberg University Mainz. The new findings could, the scientists believe, contribute to an environment-friendly and sustainable use of valuable marine natural resources.
Competition for the resources on the seabed has already begun; the industrialized countries have already staked their claims and marked off regions with large re-serves of raw materials. "This is a potential source of international conflict," believes Professor Werner Müller of the University of Mainz. Once we understand exactly how the deep-sea nodules and crusts are created, we might perhaps in the not too distant future be in the position to develop strains of microorganisms that could very specifically "grow" important raw materials for us.
Müller has been investigating the submarine world for over 30 years and is regarded as a pioneer of sponge research in Germany. But the interests of the qualified molecular biologist are not restricted to sponges, which he considers to offer a virtually inexhaustible source of raw materials, starting with bioactive substances for medical use to silicates for optic pathways. In his eyes, bacteria and algae are also genuine little magicians.
Manganese nodules are formed on the sea floor at depths of 4,000 to 5,000 meters. In the last 10 million years or so, an estimated 300 billion tonnes of manganese has accumulated in the form of nodules. "This is quite astonishing when you consider that the concentration of manganese in seawater is vanishingly small," says Müller. Besides manganese, the nodules (which resemble potato tubers) also contain iron and non-ferrous heavy metals which accumulate in layers. Once a tiny bio-seed has formed, metal ions attach themselves continuously to the outer layer.
Working in cooperation with Chinese scientists, mainly Professor Dr X. H. Wang, Müller has now discovered what triggers this process. According to their findings, the bio-seeds are bacteria that have an additional protein layer, known as the S-layer, on their outer membrane. "The outermost stratum of the S-layer is an ideal organic matrix that not only protects microorganisms against harmful environmental effects but also facilitates the deposition of minerals." Müller and his research partners have found complete chains of bacteria with S-layers in manganese nodules that provided the basis for the synthesis of the biomaterials. "Once the primary layer is present, autocatalysis takes over and the material completes the process itself."
In the case of deep-sea crusts, a unicellular alga rather than a bacterium provides the bio-seed. The deep-sea crusts – also known as manganese or cobalt crusts – are found at depths of 800 to 2,400 meters and also contain significant quantities of valuable raw materials. They are created by coccolithophorides, a form of armoured algae that are completely encased in a protective shell of calcium carbonate. These algae live at a depth of around 100 metres. When they die, their protective shells fall to deeper levels where bonds with manganese ions are formed by means of chemi-cal transformation.
"Perhaps we can use nature as our model, so that in future we will also be able to exploit algae and bacteria to extract manganese and other metals from a seawater environment," explains Müller. This could help to defuse potential future conflict for resources and contribute to sustainable production, without damaging the deep-sea environment.
- Xiaohong Wang, Werner E.G. Müller. Marine biominerals: perspectives and challenges for polymetallic nodules and crusts. Trends in Biotechnology, 2009; 27 (6): 375 DOI: 10.1016/j.tibtech.2009.03.004
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