The migratory patterns of salmon are legendary. Some salmon swim thousands of miles to return to their natal stream for reproduction. As they enter freshwater they stop feeding and their stomachs disintegrate to make more room for developing sperm and eggs. Salmon produce a lot of sperm. In a confined fishery, where the salmons’ annual migratory roam is prevented, superfluous sperm starts becoming a problem: in one year alone, 10,000 tons of sperm is discarded as industrial waste from fishery industries in Hokkaido, Japan. That’s the equivalent of throwing out a cruise liner worth a sperm each year.
However, Japanese scientist Yoshio Takahashi and his colleagues have recently discovered a way of using this unwanted sperm to extract and recycle rare earth elements (REEs). REEs have been used for over a decade in various advanced materials, such as catalysts, alloys, magnets and lasers. Neodymium in particular is used in a number of technologies including wind-turbines, hybrid cars and Apple iPhones - which contain as many as 8 different REEs.
The current method for separating REEs is by solvent extraction. This technique has been used since the 1930s, and like all chemical processes it has its limitations. These are chiefly concerned with the expense of the solvent and resulting environmental effects. It’s estimated that 75,000 litres of waste water (which may contain damaging chemicals like mercury and arsenic) is produced per ton of REE refined. All this is compounded by the fact that the global production of REEs is almost entirely dominated by China, who is steadily reducing REE exportation amidst increased domestic demand. It’s never been more important to recycle.
The idea to use salmon sperm as an inexpensive and greener alternative for REE extraction owes its origins to Takahashi’s earlier experiments, which used bacterial cell surfaces. Bacteria adsorb various toxic substances containing lead and cadmium and this led Takahashi to suspect that they might also be capable of adsorbing REEs. His hunch was borne out when he found that phosphate sites on bacterial surfaces were capable of binding to REEs dissolved in solution, so effectively that the bacteria were enriched 100,000-fold relative to the surrounding solution. Common bacteria, including those found in a typical kitchen sink, were capable of such formidable adsorption rates.
It was a logical leap from using bacteria to using DNA, which is much more easily stored. Phosphate is part of DNA’s backbone and it too can bind to dissolved REE’s. The problem with DNA lies in its solubility; normally it has to be attached to a plant-protein-carbohydrate matrix. Enter salmon sperm. It’s insoluble in its powdered form and of course it’s brimming with DNA. Just as with bacterial surfaces, the phosphate in salmon sperm-powder binds to dissolved REEs and after that it’s a simple process to extract the REEs using an acid bath and centrifugation (spinning).
There are worries that the salmon sperm process will be difficult to scale-up but it’s a promising discovery nonetheless. Especially seeing as Takahashi is confident that the process will also be useful in the large scale extraction of other elements. Suprisingly, this isn’t the first time fish sperm has proven itself handy. Salmon sperm provides the DNA template for making silver nanoparticles and DNA isolated from herring sperm, dissolved in water and then rubbed on garments can render them flame-proof! Again, it’s the phosphate-containing backbone of DNA that’s responsible. When heated, phosphoric acid is produced which strips water from the cloth fibre and leaves behind a carbon-rich and flame-resistant residue.
So if last night you were lying restlessly in bed thinking: "Why is fish sperm so useless?" then consider your consciousness raised.