THESIS
2003
144 leaves : ill. (some col.) ; 30 cm
Abstract
Larval settlement in the polychaete Hydroides elagans is mediated by marine biofilms, complex agglomerates of bacteria, diatoms, fungi and protozoa. The induction of larval settlement by marine biofilms has been mainly attributed to bacteria and diatoms in these matrices. In contrast to bacteria-derived settlement cues, the nature and origin of diatom-derived settlement cues has not been well studied so far. In this study, I present the first investigation on the nature and origin of larval settlement cues produced by marine diatoms. Diatoms with inductive (i.e. Achnanthes sp. and Nitzschia constricta) and non-inductive (i.e. Amphora tenerrima and Nitzschia frustulum) effects on larval settlement of H. elegans were selected as model strains in this investigation. Larval settlement bioas...[
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Larval settlement in the polychaete Hydroides elagans is mediated by marine biofilms, complex agglomerates of bacteria, diatoms, fungi and protozoa. The induction of larval settlement by marine biofilms has been mainly attributed to bacteria and diatoms in these matrices. In contrast to bacteria-derived settlement cues, the nature and origin of diatom-derived settlement cues has not been well studied so far. In this study, I present the first investigation on the nature and origin of larval settlement cues produced by marine diatoms. Diatoms with inductive (i.e. Achnanthes sp. and Nitzschia constricta) and non-inductive (i.e. Amphora tenerrima and Nitzschia frustulum) effects on larval settlement of H. elegans were selected as model strains in this investigation. Larval settlement bioassays with choice option between monospecies diatom films and unfilmed substratum revealed that the diatom-derived settlement cue was associated with the film surface. There was a clear correlation between the surface occupation of diatoms in films and their inductive effect on larval settlement. In the case of Achnanthes sp., even the lowest surface occupation (1.8 % of total surface area) induced significantly higher larval settlement than the control (filtered seawater). The inductive effect of diatom films on larval settlement prevailed even after heat treatment (121
oC for 1 hr) that completely killed diatoms in the films, which was showed by the vital stain fluorescein diacetate. These results suggested that the induction of larval settlement by diatoms was not related to their viability. Extracellular polysaccharides (EPS ) of the four diatom species were then isolated and characterized. EPS from all the diatoms did not induced larval settlement when it was in solution, but when EPS was loaded in phytagel, the EPS from the inductive diatoms induced a significant higher percentage of larval settlement than that from the non-inductive diatoms. The results showed that each EPS sample contained more carbohydrates than proteins. The molecular size of the bioactive fraction in the EPS sample was larger than 100 kD. Contrary to marine bacteria, these results demonstrated that diatomaceous larval settlement cues may be composed of rather heat-stable surface components, such as capsular extracellular polysaccharides. These results indicate that not only different biofilm components play different inductive/inhibitive roles in larval settlement but also their relative space occupation in biofilms determine the attractiveness of the biofilm.
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