Algae Can Carry A Silver Lining

Silver ions kill cells, which makes silver a useful antimicrobial but also means that it can cause harm in the environment. At least one type of algae, Coccomyxa actinabiotis, seems to have evolved to take up free-floating silver inside its cell walls and disarm the potent metal. Now researchers have figured out just how the algae traps silver (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b03306). They suggest the algae could make a cheap cleanup tool for silver in the environment and perhaps even offer a method of recycling it.

Humans have released concentrated amounts of silver into the environment through mining and industrial applications, jewelry making, and—in the days of film cameras—developing photographs. These days, silver, often in nanoparticle form, is widely used as an antimicrobial coating in products ranging from fabric and food packaging to refrigerators and washing machines, leading to increasing amounts in domestic wastewater. Millions of people wearing antimicrobial, nanosilver-threaded socks and washing them in silver-coated washing machines would put out tens of tons of silver per year in wastewater, according to some calculations, and that’s expected to grow.

C. actinabiotis seems to hold tightly to any silver it encounters. Corinne Rivasseau, a plant physiologist at the French Atomic Energy & Alternative Energies Commission, and her colleagues discovered this algae living in a nuclear reactor pool. It took up very high levels of radioactive silver, which is produced as a minor by-product of nuclear fission, and sequestered within its cells concentrations about 450,000 times those in the surrounding water. That led the team to examine how the algae might handle nonradioactive silver.

The team exposed the algae to varying concentrations of silver nitrate and used X-ray absorption spectroscopy, X-ray diffraction and transmission electron microscopy to examine just how the algae defanged it. At concentrations of less than 10 µM, the algae bound silver ions to sulfur-containing molecules within the cytosol, the liquid innards of the algal cell. Once those molecules were saturated, the metal ions spread to other parts of the cells. Reduced silver aggregated and formed crystals in the chloroplasts, mitochondria, membranes, and more. The cell stabilized these bits of silver by capping them with sulfur-containing molecules or other molecules at the site of crystal growth. Above 100 µM of ionic silver, the cells died.

“Algae have very efficient mechanisms for sequestration,” says Rivasseau. “We show that even if the amount of metal is so big that the algae dies, the silver will remain inside, concentrated so that it’s not released into the environment.” The team suggests that remediation with algae would be cheaper than chemical methods and easier to apply to large volumes of waste like mine tailings, to a stream contaminated with silver, or in controlled bioreactors to treat industrial wastewater. Plus, any trapped silver could be recycled, Rivasseau and her colleagues suggest, though they have yet to figure out how to extract the precious metal from the cells.

Phytoremediation is not a new idea, comments Samuel N. Luoma, a research ecologist at the University of California, Davis. He expresses skepticism that such silver bioremediation can be made to work at a large scale, especially for mining wastes, which are complex mixtures that may be very acidic or otherwise toxic to the algae. Silver recovery could also prove difficult because of the complexity of waste streams and the costs of separating the pure metal from cells. Still, Luoma says he could see such systems on a small scale for treating wastewater, such as in a closed lagoon.