Inhibitory Effect of Aqueous Extracts from Egeria densa Planch. on Cyanobacteria Microcystis aeruginosa (Kützing) Lemmermann Growth

This study aimed to investigate the allelopathic effect of live Egeria densa Planch. and its aqueous extracts in inhibiting the cyanobacterium Microcystis aeruginosa (Kützing) Lemmermann through a possible growth inhibition pathway. Under coexistence, the presence of live E. densa reduced the growth of M. aeruginosa by 48% compared to when M. aeruginosa was alone. Consequently, we prepared two separate aqueous extracts with distilled water: one from E. densa plants collected from monocultures, and the other from E. densa plants collected from co-cultivation with M. aeruginosa. At a concentration of 0.5 g/L, both extracts successfully suppressed the growth of M. aeruginosa throughout the 5-day exposure period. The extracts obtained from E. densa plants grown in a combined culture with M. aeruginosa showed significant growth-inhibiting capabilities compared to the extracts obtained from E. densa monoculture (p < 0.05). They showed 22% more growth inhibition compared to cultures exposed to monoculture extracts at day 5, indicating that the production of allelochemicals in E. densa was induced during co-existence with cyanobacteria. However, the higher concentrations (2 and 4 g/L) of both extracts did not effectively exhibit a successful inhibitive ability, possibly due to the presence of high nutrient concentrations, specifically PO43−, which may be potentially suppressing the activity of allelochemicals. Further studies are recommended in identifying the specific allelochemicals and exploring their practical implementation in the field.