Introduction

Aquatic macrophytes living in lakes and rivers face strong competition with other primary producers for light and nutrients. Strategies developed by submersed

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macrophytes to overcome shading by phytoplankton and epiphytes include fast growth, canopy formation and the production of growth inhibitors for algae and cyanobacteria. Several allelopathic active compounds have been isolated so far, and the ability of some natural products to inhibit the in vitro development of microalgae has been reported by our research group in recent years.16,13 We also have shown that bioactive products isolated from Pistia stratiotes are released into the environment.17

In pursuing our chemical investigation of aquatic plants distributed in Italy, as well as the assessment of antialgal properties of their components, we have focused on Potamogetonaceae, which grow in Volturno, the largest river of Southern Italy for its length. The first, Potamogeton natans, commonly known as water tongue, is a fresh water species, while Ruppia maritima, commonly known as sea hay, lives at the mouth of the river in brackish waters.

The plants were air dried and extracted with solvents of increasing polarity. Chromatographic processing of the extracts led to the isolation of twenty entlabdane diterpenes, identified on the basis of their spectroscopic properties and by chemical correlation.

The biological properties of labdane diterpenes as antimicrobials,32 insect antifeedants2 and their cytotoxic activity33 have been extensively reported, but little data have been given for their phytotoxicity.26 In this study, we determined the toxic potential of these metabolites on algae and even on aquatic species from various phylogenetic groups to provide a wider range of ecotoxicity information.

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