Surface sorption and nanoparticle production as a silver detoxification mechanism of the freshwater alga Parachlorella kessleri

SEM, EDS, TEM, FTIR and UV-vis analysis were used to investigate the biosorption, bioaccumulation and bioreduction of silver by the freshwater green alga Parachlorella kessleri. The dead algal biomass showed high potential for silver removal; 75% of silver was removed within 2 min. Surface sorption was the main mechanism; bioreduction contributed to the biosorption only to a small extent. In the presence of living P. kessleri cells a 68% decrease of silver concentration was observed within 24 h, but subsequently the majority of silver was released back into the solution within the next 14 days. According to UV-vis spectrometry, silver nanoparticles were formed in that time. The nanoparticles produced by the alga exhibited a lower toxicity against algal cells than silver ions at the same silver concentrations. The study demonstrated that living algal cells used a combination of two main mechanisms (sorption and reduction) for silver detoxification in their environment.