Allelopathic effects of submerged macrophytes on phytoplankton: In situ evidence, bacterial influence and new modes of action

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Inhibition of phytoplankton by allelochemicals released by submerged macrophytes is thought to be one of the mechanisms that contribute to the stabilisation of clear-water states in shallow lakes. Despite numerous studies showing the inhibiting effect of plant extracts on phytoplankton, the relevance of this process at ecosystem level is still debated. In situ evidence is difficult to achieve and a number of important details of allelopathic interactions are unknown. The proposed project aims to provide in situ evidence of alielopathic effects of the macrophyte species Myriophyllum verticillatum, which is known to contain allelopathically active substances, on phytoplankton using dialysis bag experiments in a small lake. Field and laboratory experiments will be carried out to test whether specific algae-associated bacteria metabolise allelochemicals and thus contribute to the observed differential sensitivity of algal species. New modes of action of polyphenols, a common group of allelochemicals, will be investigated in different algae species using fluorescent dye staining flow cytometry. Purified compounds from M. verticillatum will be chemically identified and their effects on phytoplankton tested using PAM fluorometry and flow cytometry. We expect a significant contribution to the understanding of alielopathic interactions in the aquatic environment.