Humans have been using silver for millennia – as currency, jewellery, or fine dining cutlery. In these forms, the lustrous metal is harmless. But when silver is allowed to dissolve into solution – as it does in photographic processing applications or in mining operations – the innocuous metal becomes highly toxic. That’s because the free silver ions (Ag+) present in such solutions are far more reactive than those in the solid metal.
A team of Swiss researchers has undertaken a comprehensive study into the molecular reactions of algae to silver to uncover how the pollutant kills them, and how the cells fight back to save themselves. Their research was published this week in the Proceedings of the National Academy of Sciences, USA.
Silver has been gaining popularity over recent years for its anti-microbial properties. It has been added – in nano-particle form – to everything from air sanitisers to cleansing face creams to odourless socks. In the environment, silver is known to bio-accumulate, just as mercury does, and it is especially toxic in aquatic ecosystems. The increasing use of silver in consumer products could place these aquatic ecosystems at increased risk of silver contamination.
The Swiss researchers set out to discover exactly how silver ions exert their toxic effects on the aquatic environment by analyzing how the single-celled green alga, Chlamydomonas reinhardtii, responds to an onslaught of silver ions. The scientists took a so-called ‘systems-based’ approach, meaning that instead of just looking to see whether the cells gave up the ghost or not, they also measured the abundance of thousands of gene transcripts and proteins during silver exposure. This gave them a picture of what genes were turned on or off, switched up or down, and how the cells tried to cope with toxicity by modulating the protein machinery present.
Silver’s sneaky tactics
The team found that the silver ions hijack the alga’s own molecular machinery to gain entry into the cell and wreak havoc within. A transporter protein located on the algal cell surface that usually transports essential copper ions across the membrane is the first apparatus to be hoodwinked by the silver.
Once inside, a copper chaperone protein unwittingly ferries the silver ions around the cell, where the real damage begins. The silver ions bind to proteins within the cell, preventing them from folding correctly and rendering them dysfunctional. In the powerhouse organs of the cell – the mitochondria and the chloroplasts – the silver once again masquerades as a useful copper ion, taking the place of copper in protein complexes necessary for energy generation and photosynthesis.
When silver ions are at a low concentration, the algal cells have some mechanisms to combat the intruder. Using copper pumps in the cell membrane, the cells are able to pump silver ions back out into the external environment. The researchers believe that the cells could also be releasing molecules to bind with the silver outside of the cell, thereby preventing it from once again gaining entry.
In addition to directly binding to proteins in the cell, silver also causes damage by generating reactive oxygen species – powerful and highly reactive free radicals that further damage proteins, fats and DNA that they encounter. The algal cells respond to this oxidative stress by mounting an antioxidant response, synthesising the antioxidants glutathione and thioredoxin among others.
Damaging marine life
Many of the adaptive responses the researchers detected occurred in algae that did not appear to be affected by the silver, indicating that although growth was normal, the cells were not impassive to the effects of the contaminant. This could suggest that continuous exposure of marine organisms to sub-lethal doses of silver ions may be having subtle toxic effects over long periods of exposure.
Further studies will be needed to understand how these findings translate into other aquatic organisms, but a thorough understanding of the mechanisms affecting C. reinhardtii may help to elucidate common mechanisms of silver toxicity. And perhaps those odourless socks won’t seem so important if we know we are placing our aquatic ecosystems in peril.
Photo: Flickr, birdsey7
Pillai S, Behra R, Nestler H, Suter MJF, Sigg L & Schirmer K (2014). Linking toxicity and adaptive responses across the transcriptome, proteome, and phenotype ofChlamydomonas reinhardtii exposed to silver Proceedings of the National Academy of Sciences
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