Lachnit, Tim (2010) Epibacterial colonization of marine macroalgae : from communities to compounds. (PhD/ Doctoral thesis), Carl von Ossietzky Universität Oldenburg, Oldenburg, Germany, 134 pp.

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Abstract

Surfaces of marine macroalgae are exposed to a complex mixture of planktic colonisers in the surrounding seawater. In contrast to artificial substrates surfaces of marine macroalgae are relatively free from settlement of higher eukaryotic organisms, but they are colonised by bacteria. These epibacterial communities alter the algal interface and affect the host alga and further colonisers, respectively. Depending on the composition epiphytic bacteria can be neutral, beneficial or detrimental for its host alga. So far, most data available on bacterial algal interaction was based on cultivable bacterial strains. Thus, only a minor part of bacteria are cultivable, the ecological significance of bacterial algal association was unclear because most of the involved bacteria were not even identified. Therefore it was of substantial importance to fully analyse the composition of the associated epibacterial communities of different macroalgal species Fucus vesiculosus (Phaeophyta), Gracilaria vermiculophylla (Rhodophyta) and Ulva intestinalis (Chlorophyta) by using molecular fingerprinting methods in this thesis. Furthermore, the stability and specificity of algal bacterial associations was evaluated on a seasonal and annual scale. Due to this comprehensive research I could show that only a minor part of the associated bacteria was host specific and seasonally independent, whereas all other presumably commensal bacteria underlies strong shifts in their appearance. I could identify several of these specific bacteria as protective for its host alga, with regard to antibacterial or antilarval activities. Moreover, bacterial classes that are responsible for pathogenesis or to induce further colonisation of higher eukaryotic organisms were underrepresented on algal surfaces, irrespective of the time of year. In parallel I investigated the bacterial colonizer pool that algal surfaces were exposed to and I could demonstrate that Gammaproteobacteria, one of these potential negative bacterial classes, were the most dominate and seasonal independent biofilm forming bacteria on artificial substrates. The observation that algal surfaces are selectively colonised by bacteria suggests a colonisation control at the algal surface. In this thesis I focused on the chemical control of bacterial settlement at the algal surface of the brown alga Fucus vesiculosus. Due to the development of an algal surface extraction method and the investigation of an extract test system in this thesis I was able to mimic the algal surface chemistry on an artificial substrate. Using these techniques I could demonstrate that the surface chemistry of Fucus vesiculosus was responsible for the selective recruitment of bacteria and to shape an algal specific bacterial community even on an artificial substrate that were exposed to the natural bacterial coloniser pool in the marine environment. Finally, I could identify two different compounds within the complex mixture of algal surface metabolites that were responsible to deter the settlement of Gammaproteobacteria from the surface of Fucus vesicu/osus.

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