Selective Bactericidal Potential of Rice (Oryza sativa L. var. japonica) Hull Extract on Microcystis Strains in Comparison with Green Algae and Zooplankton

We examined the selective inhibitory potential of rice hull extract on the toxic cyanobacterium Microcystis aeruginosa, in comparison with inhibitory effects on two green algae (Ankistrodesmus convolutus and Scenedesmus quadricauda) and a zooplankton (Daphnia magna) species. The inhibitory effect of rice hull extract, measured by algal growth or zooplankton survival using four different concentrations of extract (1, 10, 100 and 1000 μg L⁻¹), was highest on Microcystis strains (average: 98%, range: 95%–99%), followed by D. magna (average: 22%, range: 10%–47%), A. convolutus (average: 20%, range: 16%–24%), and S. quadricauda (average: 8%, range: 0%–15%). Rice hull extract had only a small effect on the growth of the green algae and Daphnia, particularly in the range 1–100 μg L⁻¹, and the inhibitory effect was somewhat diminished even at the 1,000 μg L⁻¹ level, at the end of the experimental period, especially for Daphnia. Our study indicates that rice hull extract has a strong specific algicide potential when used to combat M. aeruginosa.