Effects of neonicotinoid insecticides and a metabolite on phototaxis and growth of two diatoms,Mayamaea pseudoterrestris and Nitzschia palea

Emerging contaminants in aquatic environments, such as neonicotinoids, are of great concern because of their adverse effects on non-target organisms. This study examined the effects of the neonicotinoid insecticides imidacloprid, dinotefuran, thiacloprid, and thiacloprid-amide on the phototaxis and growth of two diatoms, Mayamaea pseudoterrestris and Nitzschia palea. Neonicotinoids affect phototaxis positively and negatively, depending on the diatom and chemical species. Thiacloprid at 160 mg L^-1 significantly reduced phototaxis by approximately 72 % in M. pseudoterrestris, whereas thiacloprid-amide did not. Thiacloprid-amide significantly decreased the growth rate of M. pseudoterrestris, even at a relatively low dosage of 16 mg L^-1. However, phototaxis and growth rate were unlikely to decrease at the concentrations detected in the environment. A significant correlation (p < 0.05) was found between mean phototaxis inhibition rates and growth, particularly in N. palea. Our findings suggested that additional research on phototaxis could provide useful insights into subsequent growth inhibition in diatoms, contributing to a better understanding of the ecological effects of neonicotinoid insecticides.