Long-term silver contamination in the ocean: Insights from brown algae on total Ag and nanoparticles

Silver nanoparticles (AgNPs) are increasingly released into the environment. However, their presence and behavior in marine biota remain poorly understood. Brown algae, known for their high metal concentration capacity, have never been directly analysed for AgNPs in natural conditions. Moreover, data on total silver (Ag) in marine organisms are scarce, despite Ag's high toxicity and potential for biomagnification. In this study, we analysed Fucus spp. samples collected from 173 sites along the Galician coast (NW Iberian Peninsula) between 1990 and 2021 to assess long-term trends in total Ag and AgNP concentrations. A total of 446 samples were analysed for Ag, and 30 samples from 10 representative sites (collected in 1990, 2005-2007, and 2021) were assessed for AgNPs. We also conducted a global review of Ag concentrations in brown algae worldwide. Total Ag concentrations in Fucus spp. declined significantly over the past three decades (-58.1%). Accordingly, a global downward trend was observed in brown algae since the 1980s. AgNP concentrations also decreased over time and showed a positive correlation with total Ag levels, despite the increasing release of engineered AgNPs. These results are more consistent with the hypothesis of in situ formation via algal-mediated reduction of dissolved Ag rather than direct accumulation from external sources. Our findings indicate that brown algae serve not only as sentinels of Ag pollution but could also play an active role in AgNP formation. Although AgNPs accounted for less than 5% of total Ag, their persistence and possible biomagnification potential highlight the need for continued monitoring. By integrating long-term data, nanoparticle detection, and a global perspective, this study provides new insights into the behavior and fate of silver in marine ecosystems.