Elisabeth M. Gross

Allelopathy of Aquatic Autotrophs

Allelopathy in aquatic environments may provide a competitive advantage to angiosperms, algae, or cyanobacteria in their interaction with other primary producers. Allelopathy can influence the competition between different photoautotrophs for resources and change the succession of species, for example, in phytoplankton communities. Field evidence and laboratory studies indicate that allelopathy occurs in all aquatic habitats (marine and freshwater), and that all primary producing organisms (cyanobacteria, micro- and macroalgae as well as angiosperms) are capable of producing and releasing allelopathically active compounds. Although allelopathy also includes positive (stimulating) interactions, the majority of studies describe the inhibitory activity of allelopathically active compounds. Different mechanisms operate depending on whether allelopathy takes place in the open water (pelagic zone) or is substrate associated (benthic habitats). Allelopathical interactions are especially common in fully aquatic species, such as submersed macrophytes or benthic algae and cyanobacteria. The prevention of shading by epiphytic and planktonic primary producers and the competition for space may be the ultimate cause for allelopathical interactions. Aquatic allelochemicals often target multiple physiological processes. The inhibition of photosynthesis of competing primary producers seems to be a frequent mode of action. Multiple biotic and abiotic factors determine the strength of allelopathic interactions. Bacteria associated with the donor or target organism can metabolize excreted allelochemicals. Frequently, the impact of surplus or limiting nutrients has been shown to affect the overall production of allelochemicals and their effect on target species. Similarities and differences of allelopathic interactions in marine and freshwater habitats as well as between the different types of producing organisms are discussed. Referee: Dr. Friedrich Jüttner, Universität Zürich-Limnologische Station, Institut für Pflanzen biologie, Universität Zürich, Seestrasse 187, Ch-8802 Klichberg ZH, Switzerland

Release and ecological impact of algicidal hydrolysable polyphenols in Myriophyllum spicatum

Abstract Aqueous acetone extracts of shoots of Eurasian watermilfoil (Myriophyllum spicatum) exhibit a strong inhibitory action against various coccoid and filamentous cyanobacteria and to a slightly less extent against chlorophytes and diatoms. Bioassay-directed fractionation led to the isolation of a hydrolysable polyphenol, tellimagrandin II, which turned out to be the main inhibitory substance. Myriophyllum spicatum contains large amounts of this compound (1.5% of dry wt). Part of the inhibitory activity is due to complexation and inactivation of algal extracellular enzymes (e.g. alkaline phosphatase) by hydrolysable polyphenols from M. spicatum.

Allelopathic activity of Ceratophyllum demersum L. and Najas marina ssp. intermedia (Wolfgang) Casper

We investigated the allelopathic activity of two submersed macrophytes with different growth forms and nutrient uptake modes, Ceratophyllum demersum and Najas marina ssp. intermedia. A bioassay-directed method development revealed optimal extraction solvents for allelochemicals from both macrophytes. For Najas, 50% methanol and for Ceratophyllum 50% acetone yielded the strongest inhibition in the agar-diffusion assay with various filamentous or chroococcal cyanobacteria as target species. Further fractionation by liquid–liquid extraction (LLE) and solid phase extraction (SPE) procedures showed that both aquatic plants appear to have more than one active fraction, one being hydrophilic and one moderately lipophilic. The water-soluble allelochemicals may inhibit phytoplankton whereas the lipophilic allelochemicals may act through direct cell–cell contact, e.g., against epiphytes. Both macrophytes also exuded allelopathically active compounds into the surrounding medium as shown by SPE of their incubation water.

FISCHERELLIN, A NEW ALLELOCHEMICAL FROM THE FRESHWATER CYANOBACTERIUM FISCHERELLA MUSCICOLA1

The benthic cyanobacterium Fischerella muscicola (Thur.) Gom. UTEX 1829 produces a secondary metabolite, fischerellin, that strongly inhibits other cyanobacteria and to a lesser extent members of the Chlorophyceae. Eubacteria are not affected. The major active compound is lipophilic and exhibits a molecular ion at m/z 408. It is heat‐ and acid‐stable but decomposes in 1 M sodium hydroxide (80° C. 1 h). Fischerellin inhibits the photosynthetic but not the respiratory electron transport of cyanobacteria and chlorophytes. Its site of action is located in PS II. Two other species of Fischerella also produce fischerellin, indicating that the synthesis of such allelochemicals might be characteristic of the genus.

Searching for allelopathic effects of submerged macrophytes on phytoplankton—state of the art and open questions

Allelopathy, here defined as biochemical interactions between aquatic primary producers, has always been intriguing as a process explaining the dominance of certain plant or algal species over others. Negative chemical interference has been invoked as one of the steering mechanisms behind mutual dominance of either submerged macrophytes or phytoplankton in shallow eutrophic lakes. Yet, despite much effort, convincing evidence for allelopathic interactions in situ is still missing. Also, laboratory approaches often lack reality. Inspired by a series of talks at the Shallow Lakes 2005 meeting in Dalfsen, the Netherlands, we argue that there is circumstantial but strong evidence that allelopathic interference between submerged macrophytes and phytoplankton may indeed exist in aquatic ecosystems despite the problems associated with research in this field. We first discuss experimental approaches combining laboratory and field studies, based on examples presented at this meeting. We then discuss the impact of nutrient status of both producing and target organism and biotic factors such as herbivory or pathogens that might affect allelopathy. Further topics are the potential seasonality of effects and the species-specificity of certain allelochemicals. We conclude with some thoughts why a final proof for allelopathy in situ might remain difficult or even inaccessible in some cases, and why we nevertheless should not abandon this idea.

Epiphytic bacterial community composition on two common submerged macrophytes in brackish water and freshwater

BackgroundPlants and their heterotrophic bacterial biofilm communities possibly strongly interact, especially in aquatic systems. We aimed to ascertain whether different macrophytes or their habitats determine bacterial community composition. We compared the composition of epiphytic bacteria on two common aquatic macrophytes, the macroalga Chara aspera Willd. and the angiosperm Myriophyllum spicatum L., in two habitats, freshwater (Lake Constance) and brackish water (Schaproder Bodden), using fluorescence in situ hybridization. The bacterial community composition was analysed based on habitat, plant species, and plant part.ResultsThe bacterial abundance was higher on plants from brackish water [5.3 × 107 cells (g dry mass)-1] than on plants from freshwater [1.3 × 107 cells (g dry mass)-1], with older shoots having a higher abundance. The organic content of freshwater plants was lower than that of brackish water plants (35 vs. 58%), and lower in C. aspera than in M. spicatum (41 vs. 52%). The content of nutrients, chlorophyll, total phenolic compounds, and anthocyanin differed in the plants and habitats. Especially the content of total phenolic compounds and anthocyanin was higher in M. spicatum, and in general higher in the freshwater than in the brackish water habitat. Members of the Cytophaga-Flavobacteria-Bacteroidetes group were abundant in all samples (5–35% of the total cell counts) and were especially dominant in M. spicatum samples. Alphaproteobacteria were the second major group (3–17% of the total cell counts). Betaproteobacteria, gammaproteobacteria, and actinomycetes were present in all samples (5 or 10% of the total cell counts). Planctomycetes were almost absent on M. spicatum in freshwater, but present on C. aspera in freshwater and on both plants in brackish water.ConclusionBacterial biofilm communities on the surface of aquatic plants might be influenced by the host plant and environmental factors. Distinct plant species, plant part and habitat specific differences in total cell counts and two bacterial groups (CFB, planctomycetes) support the combined impact of substrate (plant) and habitat on epiphytic bacterial community composition. The presence of polyphenols might explain the distinct bacterial community on freshwater M. spicatum compared to that of M. spicatum in brackish water and of C. aspera in both habitats.

Isolation, Identification and Determination of the Absolute Configuration of Fischerellin B : a New Algicide from the Freshwater Cyanobacterium Fischerella muscicola (Thuret)

A new 2-pyrrolidinone with a polyunsaturated side chain has been identified from the cyanobacterium Fischerella muscicola (Thuret). Its structure was elucidated by UV, NMR and mass spectroscopy. Derivatisation of the natural product and stereocontrolled synthesis of the derivative allowed the determination of the absolute configuration by means of chiral gas chromatography. The new compound, fischerellin B, shows algicidal properties.

In Situ Allelopathic Potential of Myriophyllum Verticillatum (Haloragaceae) against selected Phytoplankton Species

The potential allelopathic impact of Myriophyllum verticillatum L. under in situ conditions was determined in a series of field and laboratory experiments. Coexistence experiments were performed in a lake dominated by M. verticillatum (Van Goor) Meffert where we exposed three unialgal phytoplankton cultures in dialysis tubes to macrophyte exudates regularly during the vegetated period. Plant content and exudation of polyphenolic compounds were determined, and the inhibitory activity of polyphenol‐containing extracts was tested in bioassays with cyanobacteria. To account for possible resource interference, we monitored growth and photosynthesis of phosphorus‐limited and unlimited cyanobacterium Limnothrix redekei in dialysis tubes exposed to M. verticillatum in aquaria.

IMPACT OF POLYPHENOLS ON GROWTH OF THE AQUATIC HERBIVORE Acentria ephemerella

Larvae of Acentria ephemerella live fully submerged, feeding on submersed aquatic angiosperms such as pondweeds (Potamogeton spp.) and Myriophyllum spicatum. Only the latter contains high concentrations of hydrolyzable tannins known to interfere with the growth of insect herbivores. We tested whether larvae grow faster on Potamogeton perfoliatus or M. spicatum and whether this is due to polyphenols in their food source. Larvae originating from the same egg clutch grew faster and larger on P. perfoliatus than on M. spicatum. The same growth response was observed with larvae that spent winter diapause on either P. perfoliatus or M. spicatum. These larvae were fed either with their host plant or the other macrophyte. No prior feeding effect was found, but growth of larvae reared on M. spicatum was less than when grown on P. perfoliatus. Larvae from another egg-clutch reared on M. spicatum, either from lake or cultivated in aquaria, exhibited reduced growth on the lake plants. P. perfoliatus contained less than 1% and M. spicatum (aquarium or field material) between 5 and 9% phenolic compounds. No differences in nitrogen content of leaves were found, but apical shoot sections of M. spicatum exhibited a significantly higher nitrogen content than P. perfoliatus. Our results indicate that hydrolyzable tannins are responsible for the reduced growth of Acentria when fed with M. spicatum.

Center Stage: The Crucial Role of Macrophytes in Regulating Trophic Interactions in Shallow Lake Wetlands

Hydrophilic, or water-loving, macrophytes characterize wetland ecosystems, indicating prerequisite conditions of hydric soils and sufficient hydrology. The presence of such macrophytes is a key descriptor in multiple wetland def- initions (Lewis 2001a) and macrophytes may be further used to actually describe particular types of wetlands, such as cattail marshes. Macrophytes contribute significant biomass to wetland systems and represent a critical component of wetland biogeochemistry as primary producers and drivers of organic matter cycling within aquatic systems. In this chapter, we argue that macrophytes occupy the center of trophic interactions in shallow lakes, influ- encing outcomes through structural, behavioral and chemical interactions. We define shallow lakes as permanently flooded wetlands that often contain submerged or floating macrophytes and that may be surrounded by emergent vegetation (i.e.marshy habitat). Shallow remains a relative term in limnology circles, but typically is less than 3 m average depth, such that macrophytes can fill a substantial portion of the water column and stratification is neither pre- dictable nor long-term. Such systems may be termed lakes, ponds or wetlands, depending on their size and the ecological context. Macrophytes may regulate trophic interactions in ephemeral systems without permanent inundation.