Species-specific toxic responses of marine phytoplankton to metals leached from cobalt-rich ferromanganese crusts

Cobalt-rich ferromanganese (Fe-Mn) crusts are minerals distributed on the slope of a seabed mountain from a depth of 400 to 7000 m. These crusts are considered a next-generation deep-sea mineral resource with abundant industrially useful metals and rare earth elements. However, the ecological impact of deep-sea mining activity on the marine ecosystem has not been fully elucidated. In this study, the effects of Fe-Mn crust leachate were assessed using three different marine phytoplankton as experimental species. Overall, Chlorella sp. showed a dose-dependent increase of growth, while Thalassiosira weissflogii showed a tendency toward dose-dependent growth inhibition in response to the leachate samples. No significant effects were observed on Synechococcus elongatus growth rate. These results imply a species-specific sensitivity to the leachates, potentially associated with variations in accumulated metal compositions and antioxidant responses to the leachate. Although species-specific, our results indicate that mining activity on Fe-Mn crusts could have adverse ecological consequences. These experimental species have the potential to serve as biomarkers for Fe-Mn crust mining activities.