Controlling silver nanoparticle exposure in algal toxicity testing - A matter of timing

The aquatic ecotoxicity testing of nanoparticles is complicated by unstable exposure conditions resulting from various transformation processes of nanoparticles in aqueous suspensions. In this study, we investigated the influence of exposure timing on the algal test response to silver nanoparticles (AgNPs), by reducing the incubation time and by aging the AgNPs in algal medium prior to testing. The freshwater green algae Pseudokirchneriella subcapitata were exposed to AgNO(3), NM-300 K (a representative AgNP) and citrate stabilized AgNPs from two different manufacturers (AgNP1 and AgNP2) in a standard algal growth inhibition test (ISO 8692:2004) for 48 h and a short-term (2 h) (14)C-assimilation test. For AgNO(3), similar responses were obtained in the two tests, whereas freshly prepared suspensions of citrate stabilized AgNPs were less toxic in the 2-h tests compared to the 48-h tests. The 2-h test was found applicable for dissolved silver, but yielded non-monotonous concentration–response relationships and poor reproducibility for freshly prepared AgNP suspensions. However, when aging AgNPs in algal medium 24 h prior to testing, clear concentration–response patterns emerged and reproducibility increased. Prolonged aging to 48 h increased toxicity in the 2-h tests whereas aging beyond 48 h reduced toxicity. Our results demonstrate that the outcome of algal toxicity testing of AgNPs is highly influenced not only by the test duration, but also by the time passed from the moment AgNPs are added to the test medium. This time-dependency should be considered when nanomaterial dispersion protocols for ecotoxicity testing are developed

Securing the Future: 'Survival Strategies' amongst Somerset Dairy Farmers

This paper examines the way in which farmers use survival strategies to ensure the reproduction of the family farm. These survival strategies develop from farmers' individual motivations and their attitudes towards social processes - the way they read history. However, farmers are constrained by their structural relationships. I argue that what is important is the way farmers understand their structural relationships and the meaning they give to them. This is so especially in the case of family farming, where there is an intersection between family-centred motivations and vocation-centred motivations. Initially I explore these issues by introducing the theoretical literature which attempts to understand the structural relationships in which farmers live and act. Then I describe farmers' family-centred and vocational-centred motivations (or privatisms) and the way in which they understand social processes. These form the foundation for farmers' survival philosophy and practice. I conclude by arguing that farmers are (unintentionally) reproducing the structural relationships that are threatening the survival of the family farm in its present form