No toxicity to the tropical marine microalgae Tisochrysis lutea from gamma radiation

Environmental risk assessments for contaminants require toxicological data to evaluate the potential effect on organisms and ecosystems of interest. As human activities in marine environments continue to intensify, so does the presence of contaminants such as Naturally Occurring Radionuclides and Radioactive Materials (NOR and NORM, respectively), and the risk of NORM contamination has risen globally through resource extraction. The FASSET radiation effects database (FRED) has compiled radiation toxicity data to collate the existing ionising radiation effect data for non-human species, leading to environmental thresholds such as the 10 μGy/h international screening level to protect 95 % of organisms in all ecosystems. However, approximately 6 % of the data used to derive this, and other, guideline levels pertain to marine organisms and may not accurately represent the sensitivities of marine species. NORM can accumulate in subsea infrastructure during oil and gas extraction and may persist in the environment during production and after decommissioning. If NORM contaminated infrastructure is left in situ, or repurposed as an artificial reef, local marine organisms may receive elevated radiation doses prior to infrastructure degradation due to the potential presence of gamma emitting radionuclides. As the impacts of radiation are not well understood for marine organisms, more relevant toxicity data is required to develop appropriate guidelines and risk assessments to ensure ecosystem protection during and after decommissioning. In this study, tropical marine microalga Tisochrysis lutea were exposed to gamma radiation from a sealed source of Cesium-137 in chronic growth inhibition toxicity assays at dose rates ranging from 0.01 to 7.9 mGy/h. Results indicated a slight hormesis response and minimal impact on the population growth of T. lutea up to the maximum tested dose rate, and the extrapolated No (Significant) Effect Concentration (N(S)EC) was 8.8 mGy/h. This is higher than environmentally relevant exposure levels and international dose rate screening levels for non-human biota, suggesting that this species is resilient to radiation exposure at these levels. Further investigation is recommended to identify more sensitive species and endpoints for radiation toxicity and that standard toxicity tests are conducted for a wider range of marine species to derive a water quality guideline for radiation.