Oxygen dynamics and plant-sediment interactions in isoetid ...

Paper 4Organic enrichment of sediments reduces arbuscular mycorrhizal fungicolonization of the aquatic macrophytes Lobelia dortmanna and Littorellauniflora.Claus Lindskov Møller 1 , Rasmus Kjøller 2 and Kaj Sand-Jensen 11 Freshwater Biological Laboratory, Biological Institute, University of Copenhagen, Helsingørsgade 51, DK-3400Hillerød, Denmark.2 Terrestrial Ecology, Biological Institute, University of Copenhagen, Øster Farimagsgade 2B.SummaryKey words: Isoetids, arbuscularmycorrhizal fungi, colonization, hyphaldensity, eutrophication, oxygen,anoxia, organic enrichment.● Arbuscular mycorrhizal fungi (AMF) commonly colonize isoetid speciesinhabiting the littoral zone in oligotrophic lakes but are absent in other aquaticplants. We hypothesize that organic enrichment of oligotrophic sedimentsreduces AMF colonization and hyphal growth because O 2 depletion, highernutrient availability and low carbon supply from stressed host plants.● We added organic matter to sediments inhabited by isoetids and measured O 2availability with mini O 2 electrodes, pore-water content of inorganic carbon,Fe 2+ and NH 4 + , colonization intensity of roots and hyphal density after exposure.We also report hyphal densities relative to root density and colonizationintensity.● Addition of organic matter reduced AMF colonization of roots of Lobelia andLittorella and high additions clearly stressed the plants. Even small additionsalmost stopped AMF colonization of initially un-colonized Littorella. Hyphaldensities in sediments were low in the upper 1 cm but otherwise high in the rootzone and positively related to root density. Hyphal surface area exceeded rootsurfaces 1.7-3.2 times.● We conclude that AMF efficiently colonize isoetids in oligotrophic sedimentsand mediate nutrients supply to plants by their extensive hyphal net-works.However, even small organic additions to sediments with no apparent plantstress reduce AMF colonization of roots.IntroductionMore than 80% of all plant species arecolonized by arbuscular mycorrhizal fungi(AMF; Smith & Read 2008). Although AMF isvery common on land, high AMF colonizationamong aquatic plants is only found in smallrosette species (so-called isoetids) inhabitingoxygenated sediments in oligotrophic freshwaterlakes (Søndergaard & Laegaard 1977, Beck-Nielsen & Madsen 2001, Nielsen et al. 2004). Ineutrophic lakes plants are rarely colonized andin streams only emergent species have beenfound to be mycorrhizal (Beck-Nielsen &Madsen 2001).71