Ecotoxicological Bioassay Using Marine Algae for Deep-Sea Mining

A new bioassay method using delayed fluorescence (DF) intensity in marine cyanobacterium has been developed. This method offers several advantages for marine environmental risk assessment in deep-sea mining areas: DF-based bioassay uses smaller amounts of a test substance or wastewater and takes less time and space than the standard bioassay method. We selected the marine cyanobacterium Cyanobium sp. (NIES-981) as our test algal species and demonstrated that use of this species was valid in standard growth inhibition testing based on OECD guideline criteria. Standard inhibition tests and shorter testing using DF were performed on NIES-981 by using five chemicals (3,5-DCP, simazine, diflufenican, K₂Cr₂O₇, and CuSO₄), and their EC₅₀ and low-toxic-effect values (EC₁₀, EC₅, and NOEC) were determined from dose-response curves. On the basis of comparisons of the two dose-response curves and the EC₅₀ values, we concluded that DF intensity was useful as an endpoint for rapid estimation of EC₅₀ in NIES-981. In addition, a delayed fluorescence-based bioassay using Cyanobium sp. NIES-981 was used to evaluate the toxicity of core samples obtained from drill holes at the Izena Hole, Middle Okinawa Trough, East China Sea. The results revealed that unexpected leakage of recovered minerals and mining wastewater from the mining plant could result in heavy metal contamination of the surface water. Moreover, on the basis of the results of microplate-based assay using various marine algae, we suggest using eukaryotic marine algae such as Emiliania huxleyi NIES-1310, Micromonas pusilla NIES-1411, and Bathycoccus prasinos NIES-2670 in addition to Cyanobium sp. NIES-981 for management of seawater quality at deep-sea mining sites because sensitivity to lead in eukaryotic marine algae are more sensitive than cyanobacteria.