Growth inhibition of unicellular and colonial Microcystis strains (Cyanophyceae) by compounds isolated from rice (Oryza sativa) hulls

The algicidal effects of crude and pure rice hull extracts on the growth of Microcystis aeruginosa were investigated using cultured unicellular and colonial strains. Upon treatment with rice hull crude extract (RHE), growth inhibition of unicellular M. aeruginosa was much higher than that of colonial M. aeruginosa. However, purified compounds from the crude extract, β-sitosterol-β-d-glucoside and dicyclohexanyl orizane, powerfully inhibited the growth of colonial M. aeruginosa cells. At the same concentrations, the two compounds were almost equipotent (66% and 80% growth inhibition for colonial M. aeruginosa, respectively; P < 0.05). As rice hulls are readily obtainable, and as extracts show high algicidal activity (targeting colonial algae rather than unicellular organisms) at low concentrations, the results suggest that some pure compounds extracted from rice hulls, such as β-sitosterol-β-d-glucoside and dicyclohexanyl orizane, may serve as environmentally friendly agents for controlling the growth of toxic colonial M. aeruginosa in eutrophic waters.