A field application feasibility assessment of naphthoquinone derivatives for the mitigation of freshwater diatom Stephanodiscus blooms

Although many algicidal substances have been developed, their field application remains a serious challenge due to their high toxicity in the ecosystem and also due to economic reasons. Therefore, we evaluated the claim that naphthoquinone (NQ) derivatives can be used for the efficient mitigation of natural Stephanodiscus hantzschii blooms in freshwater. We tested a total of 23 algicidal NQ compounds to select the best based on the algicidal activity of S. hantzschii species. Among them, NQ 4–6 showed the highest algicidal activities (96 % at ≥0.2 μM). We conducted an acute toxicity assessment of the new algicide and found that NQ 4–6 had advanced solubility and lower toxicity, as represented by the survival rates of Selenastrum capricornutum , Daphnia magna , and Danio rerio , which are interconnected components of the freshwater ecosystem. None of these organisms seemed to be affected by the highest NQ concentration (2 μM), indicating a lack of significant toxic effects. These results indicate that the NQ 4–6 compound is promising for use as a selective control agent for S. hantzschii that does not cause negative side effects in the freshwater ecosystem. To evaluate the possibility of field application of NQ 4–6, we first performed microcosm tests. In the microcosm tests (60 L), S. hantzschii was completely eliminated 10 days after inoculation with the NQ 4–6 compound (0.2 μM). In the microcosm experiments, the dynamics of the biotic (except the phytoplankton) and abiotic factors showed similar trends in the control and treatment groups. Therefore, the NQ 4–6 compound has potential as an alternative algicidal substance to effectively mitigate natural S. hantzschii blooms.