Ecotoxicity of silver nanoparticles on estuarine and coastal bacterial communities
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The increasing use of silver nanoparticles (AgNPs) as a biocidal agent and their potential accumulation in coastal environments may threaten non-target natural environmental bacterial communities. For this reason the main aim of this PhD project was to examine the effects of AgNPs on the functioning of natural bacterial assemblages that inhabit estuaries and coastal areas including the mechanisms behind the recovery and resistance to AgNPs. The susceptibility of pure bacterial cultures to three different AgNP types, two standard reference materials (Sigma Aldrich AgNPs and the Organisation for Economic Co-operation and Development (OECD) NM-300 AgNPs) and a cleaning product purchased from Mesosilver containing AgNPs was examined. The Mesosilver AgNPs product exhibited the highest antibacterial activity followed by the NM-300 AgNPs. The higher toxicity exhibited by the Mesosilver AgNPs was associated with their smaller particle size and initially higher concentration of silver in ionic form. For all the AgNPs types tested, the toxicity was bacterial species-specific, Gram negative bacteria being more resistant than the Gram positive species. This initial work informed the design of the microcosm experiments established with sediments and water samples collected from the estuary to develop the exposure to AgNPs under more realistic environmental conditions. The results showed that a single pulse of NM-300 AgNPs (1 mg L-1) that led to sediment concentrations below 6 mg Ag kg dry weight-1 decreased the bacterial carbon utilization rate of environmentally relevant carbon substrates. Following a 24 hr exposure the functional diversity changed, but recovered after 120 hr. This recovery may be explained by a number of possible factors, such as the formation of compounds less toxic than AgNPs, or by the complexation of AgNPs with natural organic matter and sediments reducing their bioavailability, or also due to the presence of silver resistance genes or groups of organisms more resistant to silver. AgNPs did not affect the bacterial community structure based on the phospholipid fatty acids (PLFAs) analysis. The microcosm experiments suggested that AgNPs under environmentally relevant conditions can negatively affect bacterial function and provides an insight into the understanding of the bacterial community response and resilience to AgNPs. The results of this research project have improved the current knowledge about the toxicity of different silver nanoparticles and the bacterial response under more realistic environmental conditions and will support future risk assessments and regulation process of products containing silver nanoparticles.