Antisettlement defence of Fucus vesiculosus Chemistry and Ecology.

Saha, Mahasweta (2011) Antisettlement defence of Fucus vesiculosus Chemistry and Ecology. (Doctoral thesis/PhD), Christian-Albrechts-Universität, Kiel, Germany, 140 pp.

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Summary Bacteria are omnipresent in the marine environment. For example, in the Kiel Fjord (Baltic Sea) 0.7 to 2.24 x 106 bacterial cells are present in one ml of seawater. Marine organisms are thus continuously exposed to high densities of bacteria, some of which tend to settle and colonise living surfaces. Some associations between host and bacteria could be neutral or mutualistic, for example algae associated bacteria may protect their host from fouling. But mostly, bacterial epibionts have a number of negative impacts on their hosts, such as providing positive settlement cues to macrofoulers. Although macroalgae constitute a potential substratum for surface colonisation, many of them remain largely free from heavy fouling, presumably due to surface properties or release of antifouling chemicals. In the Baltic Sea this has also been observed in the ecologically important alga Fucus vesiculosus (Phaeophyceae), which remains free from heavy fouling during most of the year. So far there was no indication of a mechanical or structural defence against epibionts in this alga, which suggested that one or several chemical defence mechanisms may exist. The chemical defence of macroalgae has been a theme of literature over two decades. But so far, most of these effects have been demonstrated in studies investigating total tissue extracts instead of testing ecologically relevant surface extracts or surface metabolites at natural concentrations, which actually affect the fouling organisms in nature. Further, only very few among the studies which so far tested surface based compounds at ecologically relevant concentrations targeted the first phase of fouling, i.e. bacterial settlement. It was a main aim of my work to study the effect of surface-associated metabolites on microbial settlement, using the rockweed Fucus vesiculosus as a model organism. Since the control of bacterial fouling on macroalgae is of substantial ecological importance, it is highly relevant to characterize the nature and dynamics of potential bacterial settlement inhibitors - the first line of defensive compounds against microfoulers. In F. vesiculosus, polyphenols have been suggested as fouling deterrents. However, a bioassay-guided structure elucidation of the alga’s metabolites deployed in defence against bacterial settlement has been realized for the first time in my study, and a role of polyphenols in antimicrobial defence of F. vesiculosus was not detected. Using different chromatographic techniques followed by bioassays, I was instead able to isolate and identify an active lipophilic metabolite: fucoxanthin (Chapter I). Subsequently, I proved its surface presence. This was followed by an investigation of the metabolic provenance through segregation of fucoxanthin originating from Fucus and fucoxanthin originating from surface associated diatoms. Further I quantified the compound on algal tips and on whole individuals, in order to know whether it is sufficiently concentrated on the algal surface to warrant an inhibitory effect on bacterial settlers. With the help of this comprehensive study, I was able to report a novel defence strategy of Fucus. In chapter II, I proceeded to investigate the active polar metabolites on Fucus surfaces and reported DMSP and proline to be the hydrophilic metabolites contributing to an inhibition of bacterial settlement. This was the first study reporting the role of DMSP and of the amino acid proline as algal antifoulants. The combined results of chapter I and II provides a picture of multiple chemical defence strategies of Fucus vesiculosus in an ecological context. In chapter III, I show how the algal defence based on these inhibitors may be affected by environmental factors/ shifts such as high temperature and low light. Based on the surface concentrations of the three settlement inhibitors the defence capacity of F. vesiculosus appears to be only moderately affected by potential stressors. Finally, I report on the seasonal and geographical variation of antibacterial defence of the alga (Chapter IV). The anti-settlement defence showed a temporal variation with a peak activity in late summer/ autumn and also showed a strong and consistent difference between sites throughout the year. In summary this thesis highlights the capability of the Baltic foundation species Fucus vesiculosus for chemical defence against microfoulers, as well as the natural defence variability with site and season and the alga’s responses toward simulated environmental conditions. The study thus provides numerous new insights into algae-bacteria interactions and their dynamics and it helps in better understanding of the alga’s reaction towards bacterial epibiosis in response to potential abiotic stressors.

Document Type: Thesis (Doctoral thesis/PhD)
Thesis Advisors: Wahl, Martin and Pohnert, Georg
Keywords: Benthic Ecology; Fucus vesiculosus; Fucus, antifouling, defence variation, stressors, DMSP, fucoxanthin, proline
Research affiliation: OceanRep > GEOMAR > FB3 Marine Ecology > FB3-EOE-B Experimental Ecology - Benthic Ecology
Open Access Journal?: Yes
Projects: GAME
Date Deposited: 30 Nov 2011 12:42
Last Modified: 15 Jan 2013 09:07

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