12th International Conference on Harmful Algae

INTERNATIONAL SOCIETY FOR THE STUDY OF HARMFUL ALGAE12 th <strong>International</strong> <strong>Conference</strong> on Harmful Algae, Copenhagen, Denmark, 4-8 September 2006marine ecosystems, especiallyamong protists. In the marineplankton, temporal and spatialchanges of biomass and speciescomposition have traditionally beenthought to be mainly regulated byresource availability and abioticfactors. However, there isincreasing evidence that interspecificinteractions in the planktonplay a major role in succession,food web structure and bloomdevelopment. Many HAB speciesare regarded as rather poorexploitation competitors in terms ofgrowth rate and/or resource uptakecapabilities. There is someevidence for the hypothesis that anumber of HAB species may gaindominance by the production ofbioactive compounds, secondarymetabolites that affect growth orelicit other physiological responsesin other organisms. Suchallelochemicals may be targeted toexclude competitors from exploitinglimited resources (interferencecompetition), as well as toavoid/reduce predation.HABspecies with allelopathicpotency include variouscyanobacteria, haptophytes anddinoflagellates. Although thecausative substances are poorlydescribed, such allelochemicalsseem to be distinct from thecommonly known phycotoxins.O.09-01Marine phytoplantonallelochemicals affect growthand composition ofbacterioplanktonSession: O.09 - AllelopathyPresentation time: 14:55 - 15:15C Legrand 1 , T Bouvier 21 University of Kalmar, KALMAR, Sweden2 University of Montpellier II,MONTPELLIER, FranceChemical interactions amongmarine microbes are an adaptationto avoid predation and to eliminatecompetitors. Recently, theimportance of allelopathicinteractions among phytoplanktonand heterotrophic protists has beendemonstrated. Potentially,phytoplankton allelochemicals couldalso affect other microbes. Thetraditional view is thatbacterioplankton benefit from DOMproduced by phytoplankton.However, during high density algalblooms, bacterial biomass is oftenlower than expected in relation toalgal DOM. We tested thehypotheses that phytoplanktonallelochemicals would affectbacterioplankton composition andgrowth. The allelopathic effect of theprymnesiophyte Prymnesiumparvum was tested on a wide rangeof bacterial isolates belonging to 5different groups and on naturalpopulations. Results showed thatalgal allelochemicals selectivelyinhibited different bacterial targetswith different efficiency, rangingfrom 5-10% for Actinobacteria to>60% Proteobacteria. Bacteroideteswere not affected byallelochemicals. The allelopathiceffect on natural bacterialcommunities was proportional to theconcentration of allelochemicals,indicating that the inhibition ofbacterioplankton may increaseduring bloom formation. Thecomposition of naturalbacterioplankton was significantlyaffected by allelochemicals. Weconclude that algal allelochemicalsmust be considered, along withresources, bacterivory and virallysis, as a fundamental controlfactor of marine bacterial growth,48