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Award-winning gold recovery with bacteria

A team of students from Heidelberg University that took part in the international iGEM competition in 2013 have developed a method for the recycling of gold from electronic waste by way of biomineralisation assisted with a bacterial peptide. In order to achieve this, the entire peptide synthesis pathway was transferred into E. coli bacteria. The Heidelberg team’s method, which is on a par with classical chemical gold recovery, won them the grand prize at the 2013 iGEM World Championship Jamboree.

Gold-containing contact pins from an old processor. © iGEM Heidelberg 2013

No other chemical element has fascinated people over so many millennia as gold – from the ancient Egyptian pharaohs to the Incas and the Spanish conquistadors to the gold rush in California and Australia or the current gold speculation on the market. In the 1920s, the celebrated chemist Fritz Haber spent many years looking for ways to isolate gold from seawater which would have been used by Germany to pay the reparations imposed by the Treaty of Versailles following World War I. The Nobel Laureate, inventor of the synthesis of ammonia from atmospheric nitrogen and hydrogen, gave up “looking for the dubious needle in the haystack” as he wrote in his 1926 research report after discovering that the gold content of seawater was a thousand times lower than initially assumed. 

Since then, there have been countless attempts to extract gold from places other than normal deposits. Today, gold is not only used by central banks for national reserves as well as in jewelry and art as always, but increasingly in the electronics industry, for example as an essential component of modern computers and mobile phones. The dramatic rise of the price of gold along with new technical possibilities have in recent years further intensified the efforts to produce gold from unconventional sources.

World champion in the 2013 iGEM competition

Gold-containing solution made from pins of an old processor. © iGEM Heidelberg 2013

A team of 13 students from the University of Heidelberg and the German Cancer Research Center (DKFZ) have developed a method to recover pure gold which they entered in the renowned iGEM competition. The method involves producing gold from electronic waste using the bacterium Delftia acidovorans. Now into its ninth year, the Cambridge-based Massachusetts Institute of Technology’s (MIT) iGEM (international Genetically Engineered Machines) competition has become an international hallmark.

In 2013, 204 teams from all over the world participated in the competition, including eleven from Germany. Of the six teams that qualified for the iGEM finals at the MIT at the beginning of November, five were from Europe and three from Germany. The overall victory was achieved by a team from Heidelberg led by mathematician and systems biologist Prof. Dr. Roland Eils and the head of the synthetic biology group of the BioQuant in Heidelberg, Dr. Barbara Di Ventura. Moreover, the team from Heidelberg also won a prize for the “Best Foundational Advance Project”, a special award for projects that have made the greatest advances in the basics of synthetic biology. This prize was also awarded to the Freiburg iGEM team.

The award-winning project

Solid gold particles precipitated by Delftia acidovorans in Petri dishes. © iGEM Heidelberg 2013

The project that the up-and-coming scientists from Heidelberg entered in the iGEM competition involves the use of a compound secreted by the gold-ion-tolerant bacterium Delftia acidovorans for extracting pure gold from solutions containing gold ions. This extremophile bacterium is found in places such as gold mines, where it grows relatively slowly like any other bacteria that live in extreme environments.

Delftia acidovorans has the ability to withstand toxic quantities of gold ions (gold(III) chloride). This incredible ability is due to a molecule known as delftibactin, a small peptide consisting of ten amino acids that is able to precipitate gold efficiently. Delftibactin is a non-ribosomal peptide (NRP), i.e. a peptide synthesised by non-ribosomal peptide synthetases that, unlike the ribosomes, are independent of messenger RNA. Instead, delftibactin production involves the polyketide pathway where polyketide synthases form a kind of assembly line for incorporating amino acids into the growing peptide chain.

A team of American researchers recently found that the enzymes responsible for synthesising delftibactin are encoded on a single gene cluster consisting of 21 genes. Aiming at recombinant delftibactin expression, the iGEM team isolated the large cluster from Delftia and introduced it into the commonly used, easily cultured model organism Escherichia coli. The researchers also introduced two enzymes from Bacillus subtilis; one that is able to activate the polyketide synthase enzyme and another that is required for the production of a substrate (methylmalonyl CoA), which is not normally metabolised by E. coli.

The researchers from Heidelberg were able to show that their recombinant E. coli strain produces delftibactin in quantities suitable for industrial applications. In order to improve the visibility and purification of NRPs, the students also developed a method to label NRPs with the pigment indigoidine (another NRP). The group has filed a patent for this method. Using recombinant delftibactin, the researchers succeeded in recovering pure gold from solutions with gold concentrations that are as low as those contained in electronic waste.

The Philosopher’s Stone

Has the team from the University of Heidelberg and the German Cancer Research Center found the Philosopher’s Stone, that mysterious substance sought by alchemists for many centuries in their quest to turn common metals into precious ones such as gold? Whatever the answer, the 2013 iGEM World Championship Jamboree jury loved the researchers’ elegant synthetic biology approach, the quality of the project in terms of procedural methods and presentation and the results of a feasibility study showing the potential of delftibactin for the industrial recycling of gold. So the competition’s grand prize went to the team from Heidelberg.

Millions of tons of electronic waste containing tons of gold and other rare metals accumulate each year. According to a study carried out by the Technical University of Berlin, more than two tons of gold, worth $92 million, were discarded within electronic waste in Germany in 2007. The most common chemical method used for the extraction of gold from electronic waste is based on electrolysis, which, although it does not involve the use of strong toxic acids such as cyanide that are still frequently used in gold mines, consumes a lot of energy and is relatively inefficient. Only a minor fraction of 10 to 15% of the gold is currently recycled; most of it ends up on waste disposal sites. 

The biomineralisation of gold using delftibactin only costs around 180 euros per mol, which is about the same as for the chemical recovery of gold. “However, the innovative method produces much less environmental pollution,” said Roland Eils, coach of the iGEM team from Heidelberg, head of Theoretical Physics at the DKFZ, professor of bioinformatics and functional genomics as well as managing director of the BioQuant systems biology centre at the University of Heidelberg. Eils has also previously supervised other teams of students that won iGEM prizes. But the gold recycling project that won the grand prize at the 2013 World Championship Jamboree has surpassed all previous prizes. Maybe it is the needle that Fritz Haber never found in the haystack.

The award-winning iGEM team from Heidelberg. © University of Heidelberg
Website address: https://www.biooekonomie-bw.de/en/articles/news/award-winning-gold-recovery-with-bacteria