Combined effects of gamma irradiation and cadmium on cellular and population-level endpoints of the micro-alga Pseudokirchneriella subcapitata

A major challenge in evaluating the risks of radiation to organisms is that radioactive substances often co-occur with other contaminants in the environment. The combined effects of multiple contaminants is poorly understood, particularly where radiation is involved, but mixture toxicity can give rise to synergistic, antagonistic or additive effects. The challenge of understanding mixture toxicity in a radiation context is the focus of one of the work packages of the STAR EU Network of Excellence in Radioecology, of which this study is a part. This paper presents results from an experiment where the green micro-alga Pseudokirchneriella subcapitata was exposed to both acute external gamma irradiation and the toxic metal cadmium (Cd) (over 72 hours); the experiment had a fully factorial design with 4 gamma doses and 4 Cd concentrations. The endpoints measured were chosen to reflect subcellular, cellular and population-level effects: antioxidant enzyme expression; membrane damage; protein, vitamin and pigment content of the cells; individual cell biomass and growth; population growth (biomass per ml and cells per ml). Preliminary results suggest effects of both Cd and gamma on some of the cellular and subcellular endpoints such as thiamine (vitamin B1) and chlorophyll concentrations in the cells, and individual cell biomass. In some cases interactive effects of the combined Cd and gamma treatments were seen, and these appeared to be dose level dependent. This lack of a consistent pattern of interactive mixture toxicity effects across the endpoints measured means that such effects would be very hard to predict in a risk assessment context. The lack of measurable effects at the population level was probably due to the short experimental duration (72 hours). Other experiments in our research group on the same micro-alga species that have looked at longer term effects (weeks) have shown that effects may not manifest themselves until at least a week after an acute gamma exposure. Thus we would expect the early indications of changes at the cellular level seen in this experiment to result in decreased population size after several days. Even if micro-algal populations themselves are not affected, cellular level effects can also affect their zooplankton grazers. micro-algal pigments and vitamins may be essential components that grazers cannot synthesise themselves but must obtain through their diet. Thus, changes in food quality in these primary producers may have knock-on effects for primary consumers. Document available in abstract form only. (authors)