‘Silver’ Poses Environmental Threat for Marine Biofilms

A review of: Fabrega, J., Zhang, R., Renshaw, J. C., Lui, W., and Lead, J. R. (2011) Impact of silver nanoparticles on natural marine biofilm bacteria. Chemosphere 85: 961-966

There has been considerable research looking into the removal of biofilms, particularly for medical and domestic applications. Materials such as silver nanoparticles (Ag NPs), is low cost, widely used, and recently, more increasingly in medical and consumer products such as plastics, clothing and cosmetics due to the bactericidal properties of silver and therefore of silver nanoparticles.

Recent studies have suggested that the increased use of these materials have resulted in an increased environmental risk, however, research into this is still limited. This study looks at the bioaccumulation of silver nanoparticles in biofilms in more depth. This is important due to the environmental importance of bacteria and bacterial communities in many vital marine processes (e.g. biochemical cycling), as the antimicrobial properties of the silver nanoparticles could pose a risk to the biofilms in which bacteria are predominantly found.

This study aimed to investigate the impact of silver nanoparticles on biofilms through an experimental process which involved growing marine biofilms in situ for three days, followed by the exposure of these biofilms to silver nanoparticles of varying concentrations (20, 200 and 2000 µg L-1) for 24 hours. The uptake of Ag NPs by the biofilms and changes in the bacterial species and biomass were measured.

The study found that biofilms that had been exposed had a significantly lower biomass and volume than the control whilst the biofilm thickness was also reduced in the exposed biofilms (from 6µm, to 2.5µm in exposed). Terminal restriction fragment length polymorphisms (T-RFLP) and phylogenetic studies found that there were some unique peaks in both the control and the exposed biofilms indicating the presence of different taxonomic bacterial groups in the exposed and unexposed biofilm samples. Variations in the T-RF peaks also indicated that the Ag NPs affected the relative abundance of major bacterial groups (α, γ- Proteobacteria, Flavobacteria and Cyanobacteria) in the biofilms exposed to the highest concentration of silver nanoparticles. However, all exposed biofilms showed the presence of these major bacterial groups. Additionally, it was found that Ag NPs prevented settlement or colonisation of new bacterial groups onto the biofilm.

These results suggest that exposure to silver nanoparticles reduced normal biofilm development and impeded succession. This could be due to a toxic effect on potential colonisers or the accumulation of silver nanoparticles on the biofilm, affecting the settling and cell attachment of these potential colonisers.

This study effectively addressed its aims and looked into the impact of silver nanoparticles on natural marine biofilms. However, its aims were limited in their usefulness as the antimicrobial action of silver nanoparticles has been well known, hence their wide use. The study is also restricted, stating that Ag NPs affect marine biofilms and suggesting areas that require further study, such as how Ag NPs affect the settling of potential colonisers but does not expand on this to suggest how the environmental threat may be prevented or controlled. Despite this, the study was important in highlighting that the increased use of such materials for their antimicrobial action has a subsequent environmental impact and potentially risks biofilm diversity in marine environments, impacting community structure and function and, hopefully, encouraging further research in this area.