The Insecticide Imidacloprid Promotes Algal Growth in Absence of Zooplankton

Abstract

Imidacloprid, a systemic neonicotinoid insecticide, exerts its neurotoxic effects by binding to nicotinic acetylcholine receptors in the central nervous system. In this study, we examined the effects of commercial imidacloprid formulations on the growth of Chlorella vulgaris and other algal species, comparing these responses with those induced by plant hormones. Our results demonstrate that formulated imidacloprid stimulates C. vulgaris growth at concentrations as low as 7.82 μM, with a more pronounced effect than certain phytohormones. We observed similar growth-enhancing effects in other algal species exposed to imidacloprid. Notably, pure imidacloprid induced equivalent growth responses in C. vulgaris, confirming that the observed stimulation results from the active ingredient itself rather than formulation adjuvants. Given its insecticidal mode of action, potential worst-case aquatic contamination scenarios with imidacloprid may lead to significant increases in algal biomass through both direct (growth stimulation) and indirect (reduction of zooplankton grazing pressure) mechanisms.

Keywords: algae; growth promotion; hormones; imidacloprid.

Figure 1

Molecular structures of imidacloprid and the plant hormones used in this work. The types of plant hormones selected highlight that we covered the most important modes of action.

Figure 2

C. vulgaris growth curves under different concentrations of imidacloprid Gleba^® followed for 10 days.

Figure 3

C. vulgaris growth curves under 78.2 μM of imidacloprid Glacoxan and Gleba commercial formulations, gibberellic acid, thidiazuron, and indolacetic acid. Cultures were followed up for 21 days. The naked eye showed more aggregation of cells under Gleba^® than under Glacoxan^® (Figure 4).

Figure 4

Inverted microscopy (630×) and sedimentation camaras (0×) at final time (21 days) for plant hormones and imidacloprid treatments at 78.2 μM.

Figure 5

Haematocccus pluvialis (A), Tetradesmus sp. (B), and Monoraphidium obtusa (C) growth curves under 78.2 μM of imidacloprid Gleba^® commercial formulation for control (C) and treated (T) experimental units. Cultures were followed up for 10 days. Below, the inverted microscopy (630×) and sedimentation cameras (0×) photography are shown for the final time (10 days).

Figure 6

(A) Growth curve of C. vulgaris under control (C) and technical-grade imidacloprid treatment (T) over 10 days and (B) inverted microscopy (630×) and sedimentation chamber (0×) images at the final time point (10 days).

Figure 7

Absorbance of C. vulgaris at 675 nm after 10 days for control, analytical-grade imidacloprid (78.2 µM), imidacloprid Gleba^® (78.2 µM), and imidacloprid Glacoxan^® (78.2 µM). Different letters indicate significant differences in Tukey’s multiple comparison test (p < 0.05).