Polluted land can be slowly reclaimed by bioremediation — using microorganisms or plants to detoxify the pollution. Certain ‘hyperaccumulator’ plants can accumulate amazing concentrations of metal in their tissues, often as insoluble or biologically unavailable compounds such as histidine complexes. Plants have no way of excreting toxic metals, and have evolved this defence against them. Metal hyperaccumulators often occur naturally on the metal-rich terrain of mining districts. Daedalus is now taking the biochemistry a step further.

He notes that a bacterial enzyme, mercury reductase, can reduce mercury ions in solution to mercury metal. The gene for this enzyme has recently been transferred to a water-weed. The weed can then reduce deadly mercury ions in the water to the insoluble metal, which simply sits inertly in its tissues. The same enzyme can also reduce silver and cadmium ions. DREADCO biochemists are therefore looking for plant and bacterial enzymes that can work the trick on other metals. They will then insert them into plants which are hyperaccumulators for that metal. The resulting varieties will extract metallic residues from contaminated land, concentrate them, and smelt them internally to pure metal.

Daedalus is not sure what form the metal will take inside the plant. He would like it to occur as one or more big chunks, ideally in the nuts or fruit of the plant, so that it could be picked at harvest time. But it will probably be distributed widely in the cells as tiny single crystals. The plants will have to be reaped in their entirety, and comminuted in water. The suspension could then be centrifuged or sedimented to extract the metal.

This elegant and highly ‘green’ metal-smelting process will be widely applauded. Unlike conventional smelting, it uses no fuel and puts no CO2 into the atmosphere. It should be ideal for reclaiming regions despoiled by conventional mining, as well as tracts of low-grade ore too dilute to dig out in bulk. Easily reduced, high-value metals such as silver, copper and nickel are the most obvious targets. Iron looks feasible too — it certainly has a rich biochemistry. Titanium is a tantalizing hope. But the big target is undoubtedly aluminium. It occurs plentifully in almost all soils, and its conventional smelting needs vast amounts of costly electricity. Yet its biochemistry seems forbidding. Only around Chernobyl may plants have mutated enough to have invented the necessary enzymes.