Original Research ArticleFirst record of allelopathic activity of the picocyanobacterium Synechococcus sp. on a natural plankton community

Picocyanobacteria of the genera Synechococcus have been recognized in the last few years as important components of the phytoplankton in most of the world's oceans. Despite the ecological importance of marine Synechococcus, very little is known about their allelopathic effects on phytoplankton organisms. In this study, the influence of allelopathic compounds on the growth, expressed as chlorophyll a (Chl a) and chlorophyll c (Chl c), total abundance and structure of phytoplankton community was investigated by single and multiple addition of cell-free filtrate of picocyanobacterium Synechococcus sp. Our results indicated that picocyanobacterium Synechococcus sp. filtrate had generally an inhibitory effect on all phytoplankton community except the cyanobacteria N. spumigena and Gloeocapsa sp., which increased in the single and multiple filtrate treatment. This study also showed that the degree of inhibition was different for each species, causing a change in the phytoplankton abundance and dominance during the experiment. It was found that diatoms of the genera Navicula, Chaetoceros, Amphora, Coscinodiscus, Grammatophora and Nitzschia were the most affected organisms. This work demonstrated that the allelopathic activity exhibited by the picocyanobacterium Synechococcus sp. is probably one of the major competitive strategies affecting some of the coexisting phytoplankton species in aquatic ecosystems.