Control of algal growth by macrophytes and macrophyte-extracted bioactive compounds

Algae ( Selenastrum capricornutum and Microcystis aeruginosa ) were respectively assayed in a mixed-culture system using various concentrations of macrophytes obtained from Egeria densa, Cabomba caroliniana , and Myriophyllum spicatum in order to evaluate their inhibitory effects on algal growth due to possible secretion of anti-algal bioactive compounds under coexistence conditions. Based on this methodology, we also evaluated the methanol extract from macrophyte tissues, their extract fractions from a polarity-based fractionation process, and the biodegraded filtrate from the most inhibitory fraction of each macrophyte at an initial concentration which was quantitatively confirmed to produce strong inhibition of algal growth, although in this case, heterotrophic bacteria was also added to the assays. Algal assays investigating coexistence showed clear inhibitory effects which confirm the secretion theory, and also indicated the occurrence of a species-specific reaction. In these assays, and in those using the methanol extract fractions, M. spicatum demonstrated the strongest inhibitory effects on both algae. The extract fraction assays showed that in all cases the fraction containing the highest polarized compounds was the most inhibitory one, while the examined filtrates showed rapid biodegradation of all their anti-algal bioactive compounds in parallel with a substantial increase in biomass of heterotrophic bacteria.