Allelopathic effects of submerged macrophytes on phytoplankton: In situ evidence, bacterial influence and new modes of action
Zusammenfassung der Projektergebnisse
Allelochemicals released by submerged macrophytes are thought to inhibit the phytoplankton growth and might therefore stabilize the clear-water regime and potentially prevent toxic cyanobacteria blooms. The results gained from this project significantly improve our understanding of the mechanisms involved in allelopathic effects of macrophytes on phytoplankton. Consequently, they contribute to a better understanding of the relevance of these organismic interactions at ecosystem level. Significant progress could be obtained on four different aspects: Firstly, the relevance of bacteria for differences in the sensitivity of different phytoplankton species towards allelochemicals has been further clarified. Using a newly developed in situ approach by exposing axenic and non-axenic algae inside and outside of allelopathically active macrophyte stands, it was shown that species-specific interactions between algae and bacteria strongly overrule the effects of environmental conditions and source communities. Although bacterial communities attached to algae included potential polyphenol degraders, their significant contribution to species-specific differences in the sensitivities of algae towards allelochemicals could not be shown. Secondly, new modes of action of phytoplankton growth inhibition by a polyphenolic allelochemical have been discovered using flow cytometry with specific fluorescent markers. A significant inhibition of esterase activity and a significant increase in the production of reactive oxygen species were detected for three algal species. Changes in esterase activity were most sensitive, and could be detected after short exposure periods and at naturally occurring concentrations. Thus, esterase activity may be a suitable variable for future investigations on allelopathic effects of submerged macrophytes on phytoplankton. Thirdly, we discovered that interactions between phytoplankton species may alter their susceptibility towards macrophyte allelochemicals. The ratio between a cyanobacterium and a green alga became significantly higher in treatments with allelochemicals as compared to controls. When initial growth rates were similar, this resulted in an increasing abundance of the cyanobacterium under allelochemical presence. Future investigations on the potential of macrophytes to suppress phytoplankton, especially cyanobacteria blooms, thus should not be based on single-species tests but consider interactions between phytoplankton species. Finally, strain-specific differences in the sensitivity of phytoplankton towards macrophytereleased allelochemicals were shown for the first time. Differences of one order of magnitude were found for the inhibition of the growth rates and the photosynthetic yields of 23 strains of a green alga exposed to polyphenolic allelochemicals added as tannic acid or released from Myriophyllum spicatum. The hypothesized local adaptation towards plant-released allelochemicals could not be shown. Due to a postponed start, the planned identification and test of effects of allelochemicals extracted from Myriophyllum verticillatum were realized already before the project started. Several isomers of HHDP-di- and -tri-galloylglucose were found to be responsible for the allelopathic effects on phytoplankton. Also before project start, we found that in situ measurements were influenced by adverse growth conditions in the lake and adapted plans for our field studies.
Projektbezogene Publikationen (Auswahl)
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(2009). Seasonal and interannual dynamics of polyphenols in Myriophyllum verticillatum L. and its allelopathic activity on phytoplankton. Aquatic Botany, 91, 110-116
Bauer, N., Blaschke, U., Beutler, E., Gross, E.M., Jenett-Siems, K., Siems, K. & Hilt, S.
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(2011). Bacterial colonization of Desmodesmus armatus under changing environmental conditions. 12th Symposium on Aquatic Microbial Ecology, Rostock
Eigemann, F., Salka, I., Hilt, S. & Grossart, H.P.
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(2012) Comparison of methods to prove allelopathic effects of submerged macrophytes on green algae. Journal of Phycology, 48, 40-44
Hilt, S., Beutler, E. & Bauer, N.
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(2012) Do macrophytes support harmful cyanobacteria? Interactions with a green alga reverse the inhibiting effects of macrophyte allelochemicals on Microcystis aeruginosa. Harmful Algae, 19, 76-84
Chang, X., Eigemann, F. & Hilt, S.
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(2013) Bacterial community composition associated with freshwater algae: species specificity vs. dependency on environmental conditions and source community. FEMS Microbiology Ecology, 83, 650-663
Eigemann, F., Hilt, S., Salka, I. & Grossart, H.P.
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(2013) New modes of action of polyphenolic allelochemicals on phytoplankton. Aquatic Botany, 104, 5-14
Eigemann, F., Hilt, S. & Schmitt-Jansen, M.
